Chapter 5 MySQL Server Administration

Table of Contents

5.1 The MySQL Server
5.1.1 Server Option and Variable Reference
5.1.2 Server Configuration Defaults
5.1.3 Server Command Options
5.1.4 Server System Variables
5.1.5 Using System Variables
5.1.6 Server Status Variables
5.1.7 Server SQL Modes
5.1.8 IPv6 Support
5.1.9 Server-Side Help
5.1.10 Server Response to Signals
5.1.11 The Server Shutdown Process
5.2 The MySQL Data Directory
5.3 The mysql System Database
5.4 MySQL Server Logs
5.4.1 Selecting General Query and Slow Query Log Output Destinations
5.4.2 The Error Log
5.4.3 The General Query Log
5.4.4 The Binary Log
5.4.5 The Slow Query Log
5.4.6 The DDL Log
5.4.7 Server Log Maintenance
5.5 MySQL Server Plugins
5.5.1 Server Plugins Available
5.5.2 Installing and Uninstalling Plugins
5.5.3 Obtaining Server Plugin Information
5.5.4 MySQL Enterprise Thread Pool
5.6 Running Multiple MySQL Instances on One Machine
5.6.1 Setting Up Multiple Data Directories
5.6.2 Running Multiple MySQL Instances on Windows
5.6.3 Running Multiple MySQL Instances on Unix
5.6.4 Using Client Programs in a Multiple-Server Environment
5.7 Tracing mysqld Using DTrace
5.7.1 mysqld DTrace Probe Reference

MySQL Server (mysqld) is the main program that does most of the work in a MySQL installation. This chapter provides an overview of MySQL Server and covers general server administration:

For additional information on administrative topics, see also:

5.1 The MySQL Server

mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:

  • Startup options that the server supports. You can specify these options on the command line, through configuration files, or both.

  • Server system variables. These variables reflect the current state and values of the startup options, some of which can be modified while the server is running.

  • Server status variables. These variables contain counters and statistics about runtime operation.

  • How to set the server SQL mode. This setting modifies certain aspects of SQL syntax and semantics, for example for compatibility with code from other database systems, or to control the error handling for particular situations.

  • The server shutdown process. There are performance and reliability considerations depending on the type of table (transactional or nontransactional) and whether you use replication.

Note

Not all storage engines are supported by all MySQL server binaries and configurations. To find out how to determine which storage engines your MySQL server installation supports, see Section 13.7.5.17, “SHOW ENGINES Syntax”.

5.1.1 Server Option and Variable Reference

The following table provides a list of all the command line options, server and status variables applicable within mysqld.

The table lists command-line options (Cmd-line), options valid in configuration files (Option file), server system variables (System Var), and status variables (Status var) in one unified list, with notification of where each option/variable is valid. If a server option set on the command line or in an option file differs from the name of the corresponding server system or status variable, the variable name is noted immediately below the corresponding option. For status variables, the scope of the variable is shown (Scope) as either global, session, or both. Please see the corresponding sections for details on setting and using the options and variables. Where appropriate, a direct link to further information on the item as available.

For a version of this table that is specific to MySQL Cluster, see Section 18.3.2.5, “MySQL Cluster mysqld Option and Variable Reference”.

Table 5.1 Option/Variable Summary

NameCmd-LineOption FileSystem VarStatus VarVar ScopeDynamic
abort-slave-event-countYesYes    
Aborted_clients   YesGlobalNo
Aborted_connects   YesGlobalNo
allow-suspicious-udfsYesYes    
ansiYesYes    
audit-logYesYes    
audit_log_buffer_sizeYesYesYes GlobalNo
audit_log_fileYesYesYes GlobalNo
audit_log_flush  Yes GlobalYes
audit_log_formatYesYesYes GlobalNo
audit_log_policyYesYesYes GlobalYes
audit_log_rotate_on_sizeYesYesYes GlobalYes
audit_log_strategyYesYesYes GlobalNo
authentication_windows_log_levelYesYes    
authentication_windows_use_principal_nameYesYes    
auto_increment_increment  Yes BothYes
auto_increment_offset  Yes BothYes
autocommitYesYesYes VariesYes
automatic_sp_privileges  Yes GlobalYes
back_log  Yes GlobalNo
basedirYesYesYes GlobalNo
big-tablesYesYes  VariesYes
- Variable: big_tables  Yes VariesYes
bind-addressYesYes    
Binlog_cache_disk_use   YesGlobalNo
binlog_cache_sizeYesYesYes GlobalYes
Binlog_cache_use   YesGlobalNo
binlog_direct_non_transactional_updatesYesYesYes BothYes
binlog-do-dbYesYes    
binlog-formatYesYes  BothYes
- Variable: binlog_format  Yes BothYes
binlog-ignore-dbYesYes    
binlog-row-event-max-sizeYesYes    
Binlog_stmt_cache_disk_use   YesGlobalNo
binlog_stmt_cache_sizeYesYesYes GlobalYes
Binlog_stmt_cache_use   YesGlobalNo
bootstrapYesYes    
bulk_insert_buffer_sizeYesYesYes BothYes
Bytes_received   YesBothNo
Bytes_sent   YesBothNo
character_set_client  Yes BothYes
character-set-client-handshakeYesYes    
character_set_connection  Yes BothYes
character_set_database[a]  Yes BothYes
character-set-filesystemYesYes  BothYes
- Variable: character_set_filesystem  Yes BothYes
character_set_results  Yes BothYes
character-set-serverYesYes  BothYes
- Variable: character_set_server  Yes BothYes
character_set_system  Yes GlobalNo
character-sets-dirYesYes  GlobalNo
- Variable: character_sets_dir  Yes GlobalNo
chrootYesYes    
collation_connection  Yes BothYes
collation_database[b]  Yes BothYes
collation-serverYesYes  BothYes
- Variable: collation_server  Yes BothYes
Com_admin_commands   YesBothNo
Com_alter_db   YesBothNo
Com_alter_db_upgrade   YesBothNo
Com_alter_event   YesBothNo
Com_alter_function   YesBothNo
Com_alter_procedure   YesBothNo
Com_alter_server   YesBothNo
Com_alter_table   YesBothNo
Com_alter_tablespace   YesBothNo
Com_analyze   YesBothNo
Com_assign_to_keycache   YesBothNo
Com_begin   YesBothNo
Com_binlog   YesBothNo
Com_call_procedure   YesBothNo
Com_change_db   YesBothNo
Com_change_master   YesBothNo
Com_check   YesBothNo
Com_checksum   YesBothNo
Com_commit   YesBothNo
Com_create_db   YesBothNo
Com_create_event   YesBothNo
Com_create_function   YesBothNo
Com_create_index   YesBothNo
Com_create_procedure   YesBothNo
Com_create_server   YesBothNo
Com_create_table   YesBothNo
Com_create_trigger   YesBothNo
Com_create_udf   YesBothNo
Com_create_user   YesBothNo
Com_create_view   YesBothNo
Com_dealloc_sql   YesBothNo
Com_delete   YesBothNo
Com_delete_multi   YesBothNo
Com_do   YesBothNo
Com_drop_db   YesBothNo
Com_drop_event   YesBothNo
Com_drop_function   YesBothNo
Com_drop_index   YesBothNo
Com_drop_procedure   YesBothNo
Com_drop_server   YesBothNo
Com_drop_table   YesBothNo
Com_drop_trigger   YesBothNo
Com_drop_user   YesBothNo
Com_drop_view   YesBothNo
Com_empty_query   YesBothNo
Com_execute_sql   YesBothNo
Com_flush   YesBothNo
Com_grant   YesBothNo
Com_ha_close   YesBothNo
Com_ha_open   YesBothNo
Com_ha_read   YesBothNo
Com_help   YesBothNo
Com_insert   YesBothNo
Com_insert_select   YesBothNo
Com_install_plugin   YesBothNo
Com_kill   YesBothNo
Com_load   YesBothNo
Com_lock_tables   YesBothNo
Com_optimize   YesBothNo
Com_preload_keys   YesBothNo
Com_prepare_sql   YesBothNo
Com_purge   YesBothNo
Com_purge_before_date   YesBothNo
Com_release_savepoint   YesBothNo
Com_rename_table   YesBothNo
Com_rename_user   YesBothNo
Com_repair   YesBothNo
Com_replace   YesBothNo
Com_replace_select   YesBothNo
Com_reset   YesBothNo
Com_resignal   YesBothNo
Com_revoke   YesBothNo
Com_revoke_all   YesBothNo
Com_rollback   YesBothNo
Com_rollback_to_savepoint   YesBothNo
Com_savepoint   YesBothNo
Com_select   YesBothNo
Com_set_option   YesBothNo
Com_show_authors   YesBothNo
Com_show_binlog_events   YesBothNo
Com_show_binlogs   YesBothNo
Com_show_charsets   YesBothNo
Com_show_collations   YesBothNo
Com_show_contributors   YesBothNo
Com_show_create_db   YesBothNo
Com_show_create_event   YesBothNo
Com_show_create_func   YesBothNo
Com_show_create_proc   YesBothNo
Com_show_create_table   YesBothNo
Com_show_create_trigger   YesBothNo
Com_show_databases   YesBothNo
Com_show_engine_logs   YesBothNo
Com_show_engine_mutex   YesBothNo
Com_show_engine_status   YesBothNo
Com_show_errors   YesBothNo
Com_show_events   YesBothNo
Com_show_fields   YesBothNo
Com_show_function_code   YesBothNo
Com_show_function_status   YesBothNo
Com_show_grants   YesBothNo
Com_show_keys   YesBothNo
Com_show_master_status   YesBothNo
Com_show_ndb_status   YesBothNo
Com_show_new_master   YesBothNo
Com_show_open_tables   YesBothNo
Com_show_plugins   YesBothNo
Com_show_privileges   YesBothNo
Com_show_procedure_code   YesBothNo
Com_show_procedure_status   YesBothNo
Com_show_processlist   YesBothNo
Com_show_profile   YesBothNo
Com_show_profiles   YesBothNo
Com_show_relaylog_events   YesBothNo
Com_show_slave_hosts   YesBothNo
Com_show_slave_status   YesBothNo
Com_show_status   YesBothNo
Com_show_storage_engines   YesBothNo
Com_show_table_status   YesBothNo
Com_show_tables   YesBothNo
Com_show_triggers   YesBothNo
Com_show_variables   YesBothNo
Com_show_warnings   YesBothNo
Com_signal   YesBothNo
Com_slave_start   YesBothNo
Com_slave_stop   YesBothNo
Com_stmt_close   YesBothNo
Com_stmt_execute   YesBothNo
Com_stmt_fetch   YesBothNo
Com_stmt_prepare   YesBothNo
Com_stmt_reprepare   YesBothNo
Com_stmt_reset   YesBothNo
Com_stmt_send_long_data   YesBothNo
Com_truncate   YesBothNo
Com_uninstall_plugin   YesBothNo
Com_unlock_tables   YesBothNo
Com_update   YesBothNo
Com_update_multi   YesBothNo
Com_xa_commit   YesBothNo
Com_xa_end   YesBothNo
Com_xa_prepare   YesBothNo
Com_xa_recover   YesBothNo
Com_xa_rollback   YesBothNo
Com_xa_start   YesBothNo
completion_typeYesYesYes BothYes
Compression   YesSessionNo
concurrent_insertYesYesYes GlobalYes
connect_timeoutYesYesYes GlobalYes
Connections   YesGlobalNo
consoleYesYes    
core-fileYesYes    
Created_tmp_disk_tables   YesBothNo
Created_tmp_files   YesGlobalNo
Created_tmp_tables   YesBothNo
datadirYesYesYes GlobalNo
date_format  Yes VariesNo
datetime_format  Yes VariesNo
debugYesYesYes BothYes
debug_sync  Yes VariesYes
debug-sync-timeoutYesYes    
default-character-setYesYes    
default-storage-engineYesYes  BothYes
- Variable: default_storage_engine  Yes BothYes
default-time-zoneYesYes    
default_week_formatYesYesYes BothYes
defaults-extra-fileYes     
defaults-fileYes     
defaults-group-suffixYes     
delay-key-writeYesYes  GlobalYes
- Variable: delay_key_write  Yes GlobalYes
Delayed_errors   YesGlobalNo
delayed_insert_limitYesYesYes GlobalYes
Delayed_insert_threads   YesGlobalNo
delayed_insert_timeoutYesYesYes GlobalYes
delayed_queue_sizeYesYesYes GlobalYes
Delayed_writes   YesGlobalNo
des-key-fileYesYes    
disconnect-slave-event-countYesYes    
div_precision_incrementYesYesYes BothYes
enable-lockingYesYes    
enable-named-pipeYesYes    
- Variable: named_pipe      
enable-pstackYesYes    
engine-condition-pushdownYesYes  BothYes
- Variable: engine_condition_pushdown  Yes BothYes
error_count  Yes SessionNo
event-schedulerYesYes  GlobalYes
- Variable: event_scheduler  Yes GlobalYes
exit-infoYesYes    
expire_logs_daysYesYesYes GlobalYes
external-lockingYesYes    
- Variable: skip_external_locking      
external_user  Yes SessionNo
federatedYesYes    
flushYesYesYes GlobalYes
Flush_commands   YesGlobalNo
flush_timeYesYesYes GlobalYes
foreign_key_checks  Yes VariesYes
ft_boolean_syntaxYesYesYes GlobalYes
ft_max_word_lenYesYesYes GlobalNo
ft_min_word_lenYesYesYes GlobalNo
ft_query_expansion_limitYesYesYes GlobalNo
ft_stopword_fileYesYesYes GlobalNo
gdbYesYes    
general-logYesYes  GlobalYes
- Variable: general_log  Yes GlobalYes
general_log_fileYesYesYes GlobalYes
group_concat_max_lenYesYesYes BothYes
Handler_commit   YesBothNo
Handler_delete   YesBothNo
Handler_discover   YesBothNo
Handler_prepare   YesBothNo
Handler_read_first   YesBothNo
Handler_read_key   YesBothNo
Handler_read_last   YesBothNo
Handler_read_next   YesBothNo
Handler_read_prev   YesBothNo
Handler_read_rnd   YesBothNo
Handler_read_rnd_next   YesBothNo
Handler_rollback   YesBothNo
Handler_savepoint   YesBothNo
Handler_savepoint_rollback   YesBothNo
Handler_update   YesBothNo
Handler_write   YesBothNo
have_compress  Yes GlobalNo
have_crypt  Yes GlobalNo
have_csv  Yes GlobalNo
have_dynamic_loading  Yes GlobalNo
have_geometry  Yes GlobalNo
have_innodb  Yes GlobalNo
have_ndbcluster  Yes GlobalNo
have_openssl  Yes GlobalNo
have_partitioning  Yes GlobalNo
have_profiling  Yes GlobalNo
have_query_cache  Yes GlobalNo
have_rtree_keys  Yes GlobalNo
have_ssl  Yes GlobalNo
have_symlink  Yes GlobalNo
helpYesYes    
hostname  Yes GlobalNo
identity  Yes SessionYes
ignore-builtin-innodbYesYes  GlobalNo
- Variable: ignore_builtin_innodb  Yes GlobalNo
init_connectYesYesYes GlobalYes
init-fileYesYes  GlobalNo
- Variable: init_file  Yes GlobalNo
init-rpl-roleYesYes    
init_slaveYesYesYes GlobalYes
innodbYesYes    
innodb_adaptive_flushingYesYesYes GlobalYes
innodb_adaptive_hash_indexYesYesYes GlobalYes
innodb_additional_mem_pool_sizeYesYesYes GlobalNo
innodb_autoextend_incrementYesYesYes GlobalYes
innodb_autoinc_lock_modeYesYesYes GlobalNo
Innodb_buffer_pool_bytes_data   YesGlobalNo
Innodb_buffer_pool_bytes_dirty   YesGlobalNo
innodb_buffer_pool_instancesYesYesYes GlobalNo
Innodb_buffer_pool_pages_data   YesGlobalNo
Innodb_buffer_pool_pages_dirty   YesGlobalNo
Innodb_buffer_pool_pages_flushed   YesGlobalNo
Innodb_buffer_pool_pages_free   YesGlobalNo
Innodb_buffer_pool_pages_latched   YesGlobalNo
Innodb_buffer_pool_pages_misc   YesGlobalNo
Innodb_buffer_pool_pages_total   YesGlobalNo
Innodb_buffer_pool_read_ahead   YesGlobalNo
Innodb_buffer_pool_read_ahead_evicted   YesGlobalNo
Innodb_buffer_pool_read_ahead_rnd   YesGlobalNo
Innodb_buffer_pool_read_requests   YesGlobalNo
Innodb_buffer_pool_reads   YesGlobalNo
innodb_buffer_pool_sizeYesYesYes GlobalNo
Innodb_buffer_pool_wait_free   YesGlobalNo
Innodb_buffer_pool_write_requests   YesGlobalNo
innodb_change_bufferingYesYesYes GlobalYes
innodb_change_buffering_debugYesYesYes GlobalYes
innodb_checksumsYesYesYes GlobalNo
innodb_commit_concurrencyYesYesYes GlobalYes
innodb_concurrency_ticketsYesYesYes GlobalYes
innodb_data_file_pathYesYesYes GlobalNo
Innodb_data_fsyncs   YesGlobalNo
innodb_data_home_dirYesYesYes GlobalNo
Innodb_data_pending_fsyncs   YesGlobalNo
Innodb_data_pending_reads   YesGlobalNo
Innodb_data_pending_writes   YesGlobalNo
Innodb_data_read   YesGlobalNo
Innodb_data_reads   YesGlobalNo
Innodb_data_writes   YesGlobalNo
Innodb_data_written   YesGlobalNo
Innodb_dblwr_pages_written   YesGlobalNo
Innodb_dblwr_writes   YesGlobalNo
innodb_doublewriteYesYesYes GlobalNo
innodb_fast_shutdownYesYesYes GlobalYes
innodb_file_formatYesYesYes GlobalYes
innodb_file_format_checkYesYesYes GlobalVaries
innodb_file_format_maxYesYesYes GlobalYes
innodb_file_per_tableYesYesYes GlobalYes
innodb_flush_log_at_trx_commitYesYesYes GlobalYes
innodb_flush_methodYesYesYes GlobalNo
innodb_force_load_corruptedYesYesYes GlobalNo
innodb_force_recoveryYesYesYes GlobalNo
Innodb_have_atomic_builtins   YesGlobalNo
innodb_io_capacityYesYesYes GlobalYes
innodb_large_prefixYesYesYes GlobalYes
innodb_limit_optimistic_insert_debugYesYesYes GlobalYes
innodb_lock_wait_timeoutYesYesYes BothYes
innodb_locks_unsafe_for_binlogYesYesYes GlobalNo
innodb_log_buffer_sizeYesYesYes GlobalNo
innodb_log_file_sizeYesYesYes GlobalNo
innodb_log_files_in_groupYesYesYes GlobalNo
innodb_log_group_home_dirYesYesYes GlobalNo
Innodb_log_waits   YesGlobalNo
Innodb_log_write_requests   YesGlobalNo
Innodb_log_writes   YesGlobalNo
innodb_max_dirty_pages_pctYesYesYes GlobalYes
innodb_max_purge_lagYesYesYes GlobalYes
innodb_mirrored_log_groupsYesYesYes GlobalNo
innodb_old_blocks_pctYesYesYes GlobalYes
innodb_old_blocks_timeYesYesYes GlobalYes
innodb_open_filesYesYesYes GlobalNo
Innodb_os_log_fsyncs   YesGlobalNo
Innodb_os_log_pending_fsyncs   YesGlobalNo
Innodb_os_log_pending_writes   YesGlobalNo
Innodb_os_log_written   YesGlobalNo
Innodb_page_size   YesGlobalNo
Innodb_pages_created   YesGlobalNo
Innodb_pages_read   YesGlobalNo
Innodb_pages_written   YesGlobalNo
innodb_print_all_deadlocksYesYesYes GlobalYes
innodb_purge_batch_sizeYesYesYes GlobalYes
innodb_purge_threadsYesYesYes GlobalNo
innodb_random_read_aheadYesYesYes GlobalYes
innodb_read_ahead_thresholdYesYesYes GlobalYes
innodb_read_io_threadsYesYesYes GlobalNo
innodb_replication_delayYesYesYes GlobalYes
innodb_rollback_on_timeoutYesYesYes GlobalNo
innodb_rollback_segmentsYesYesYes GlobalYes
Innodb_row_lock_current_waits   YesGlobalNo
Innodb_row_lock_time   YesGlobalNo
Innodb_row_lock_time_avg   YesGlobalNo
Innodb_row_lock_time_max   YesGlobalNo
Innodb_row_lock_waits   YesGlobalNo
Innodb_rows_deleted   YesGlobalNo
Innodb_rows_inserted   YesGlobalNo
Innodb_rows_read   YesGlobalNo
Innodb_rows_updated   YesGlobalNo
innodb_spin_wait_delayYesYesYes GlobalYes
innodb_stats_methodYesYesYes GlobalYes
innodb_stats_on_metadataYesYesYes GlobalYes
innodb_stats_sample_pagesYesYesYes GlobalYes
innodb-status-fileYesYes    
innodb_strict_modeYesYesYes BothYes
innodb_support_xaYesYesYes BothYes
innodb_sync_spin_loopsYesYesYes GlobalYes
innodb_table_locksYesYesYes BothYes
innodb_thread_concurrencyYesYesYes GlobalYes
innodb_thread_sleep_delayYesYesYes GlobalYes
Innodb_truncated_status_writes   YesGlobalNo
innodb_trx_purge_view_update_only_debugYesYesYes GlobalYes
innodb_trx_rseg_n_slots_debugYesYesYes GlobalYes
innodb_use_native_aioYesYesYes GlobalNo
innodb_use_sys_mallocYesYesYes GlobalNo
innodb_version  Yes GlobalNo
innodb_write_io_threadsYesYesYes GlobalNo
insert_id  Yes SessionYes
installYes     
install-manualYes     
interactive_timeoutYesYesYes BothYes
join_buffer_sizeYesYesYes BothYes
keep_files_on_createYesYesYes BothYes
Key_blocks_not_flushed   YesGlobalNo
Key_blocks_unused   YesGlobalNo
Key_blocks_used   YesGlobalNo
key_buffer_sizeYesYesYes GlobalYes
key_cache_age_thresholdYesYesYes GlobalYes
key_cache_block_sizeYesYesYes GlobalYes
key_cache_division_limitYesYesYes GlobalYes
Key_read_requests   YesGlobalNo
Key_reads   YesGlobalNo
Key_write_requests   YesGlobalNo
Key_writes   YesGlobalNo
languageYesYesYes GlobalNo
large_files_support  Yes GlobalNo
large_page_size  Yes GlobalNo
large-pagesYesYes  GlobalNo
- Variable: large_pages  Yes GlobalNo
last_insert_id  Yes SessionYes
Last_query_cost   YesSessionNo
lc-messagesYesYes  BothYes
- Variable: lc_messages  Yes BothYes
lc-messages-dirYesYes  GlobalNo
- Variable: lc_messages_dir  Yes GlobalNo
lc_time_names  Yes BothYes
license  Yes GlobalNo
local_infile  Yes GlobalYes
local-serviceYes     
lock_wait_timeoutYesYesYes BothYes
locked_in_memory  Yes GlobalNo
logYesYesYes GlobalYes
log-binYesYesYes GlobalNo
log_bin  Yes GlobalNo
log-bin-indexYesYes    
log-bin-trust-function-creatorsYesYes  GlobalYes
- Variable: log_bin_trust_function_creators  Yes GlobalYes
log-bin-trust-routine-creatorsYesYes  GlobalYes
- Variable: log_bin_trust_routine_creators  Yes GlobalYes
log-bin-use-v1-row-eventsYesYes  GlobalNo
- Variable: log_bin_use_v1_row_events  Yes GlobalNo
log_bin_use_v1_row_eventsYesYesYes GlobalNo
log-errorYesYes  GlobalNo
- Variable: log_error  Yes GlobalNo
log-isamYesYes    
log-outputYesYes  GlobalYes
- Variable: log_output  Yes GlobalYes
log-queries-not-using-indexesYesYes  GlobalYes
- Variable: log_queries_not_using_indexes  Yes GlobalYes
log-short-formatYesYes    
log-slave-updatesYesYes  GlobalNo
- Variable: log_slave_updates  Yes GlobalNo
log_slave_updatesYesYesYes GlobalNo
log-slow-admin-statementsYesYes    
log-slow-queriesYesYes  GlobalYes
- Variable: log_slow_queries  Yes GlobalYes
log-slow-slave-statementsYesYes    
log-tcYesYes    
log-tc-sizeYesYes    
log-warningsYesYes  BothYes
- Variable: log_warnings  Yes BothYes
long_query_timeYesYesYes BothYes
low-priority-updatesYesYes  BothYes
- Variable: low_priority_updates  Yes BothYes
lower_case_file_system  Yes GlobalNo
lower_case_table_namesYesYesYes GlobalNo
master-connect-retryYesYes    
master-hostYesYes    
master-info-fileYesYes    
master-passwordYesYes    
master-portYesYes    
master-retry-countYesYes    
master-sslYesYes    
master-ssl-caYesYes    
master-ssl-capathYesYes    
master-ssl-certYesYes    
master-ssl-cipherYesYes    
master-ssl-keyYesYes    
master-userYesYes    
max_allowed_packetYesYesYes BothYes
max_binlog_cache_sizeYesYesYes GlobalYes
max-binlog-dump-eventsYesYes    
max_binlog_sizeYesYesYes GlobalYes
max_binlog_stmt_cache_sizeYesYesYes GlobalYes
max_connect_errorsYesYesYes GlobalYes
max_connectionsYesYesYes GlobalYes
max_delayed_threadsYesYesYes BothYes
max_error_countYesYesYes BothYes
max_heap_table_sizeYesYesYes BothYes
max_insert_delayed_threads  Yes BothYes
max_join_sizeYesYesYes BothYes
max_length_for_sort_dataYesYesYes BothYes
max_long_data_sizeYesYesYes GlobalNo
max_prepared_stmt_countYesYesYes GlobalYes
max_relay_log_sizeYesYesYes GlobalYes
max_seeks_for_keyYesYesYes BothYes
max_sort_lengthYesYesYes BothYes
max_sp_recursion_depthYesYesYes BothYes
max_tmp_tables  Yes BothYes
Max_used_connections   YesGlobalNo
max_user_connectionsYesYesYes BothYes
max_write_lock_countYesYesYes GlobalYes
memlockYesYes    
- Variable: locked_in_memory      
metadata_locks_cache_size  Yes GlobalNo
min-examined-row-limitYesYesYes BothYes
multi_range_countYesYesYes BothYes
myisam-block-sizeYesYes    
myisam_data_pointer_sizeYesYesYes GlobalYes
myisam_max_sort_file_sizeYesYesYes GlobalYes
myisam_mmap_sizeYesYesYes GlobalNo
myisam-recoverYesYes    
- Variable: myisam_recover_options      
myisam-recover-optionsYesYes    
- Variable: myisam_recover_options      
myisam_recover_options  Yes GlobalNo
myisam_repair_threadsYesYesYes BothYes
myisam_sort_buffer_sizeYesYesYes BothYes
myisam_stats_methodYesYesYes BothYes
myisam_use_mmapYesYesYes GlobalYes
named_pipe  Yes GlobalNo
Ndb_api_bytes_received_count   YesGlobalNo
Ndb_api_bytes_received_count_session   YesSessionNo
Ndb_api_bytes_received_count_slave   YesGlobalNo
Ndb_api_bytes_sent_count   YesGlobalNo
Ndb_api_bytes_sent_count_session   YesSessionNo
Ndb_api_bytes_sent_count_slave   YesGlobalNo
Ndb_api_event_bytes_count   YesGlobalNo
Ndb_api_event_bytes_count_injector   YesGlobalNo
Ndb_api_event_data_count   YesGlobalNo
Ndb_api_event_data_count_injector   YesGlobalNo
Ndb_api_event_nondata_count   YesGlobalNo
Ndb_api_event_nondata_count_injector   YesGlobalNo
Ndb_api_pk_op_count   YesGlobalNo
Ndb_api_pk_op_count_session   YesSessionNo
Ndb_api_pk_op_count_slave   YesGlobalNo
Ndb_api_pruned_scan_count   YesGlobalNo
Ndb_api_pruned_scan_count_session   YesSessionNo
Ndb_api_pruned_scan_count_slave   YesGlobalNo
Ndb_api_range_scan_count   YesGlobalNo
Ndb_api_range_scan_count_session   YesSessionNo
Ndb_api_range_scan_count_slave   YesGlobalNo
Ndb_api_read_row_count   YesGlobalNo
Ndb_api_read_row_count_session   YesSessionNo
Ndb_api_read_row_count_slave   YesGlobalNo
Ndb_api_scan_batch_count   YesGlobalNo
Ndb_api_scan_batch_count_session   YesSessionNo
Ndb_api_scan_batch_count_slave   YesGlobalNo
Ndb_api_table_scan_count   YesGlobalNo
Ndb_api_table_scan_count_session   YesSessionNo
Ndb_api_table_scan_count_slave   YesGlobalNo
Ndb_api_trans_abort_count   YesGlobalNo
Ndb_api_trans_abort_count_session   YesSessionNo
Ndb_api_trans_abort_count_slave   YesGlobalNo
Ndb_api_trans_close_count   YesGlobalNo
Ndb_api_trans_close_count_session   YesSessionNo
Ndb_api_trans_close_count_slave   YesGlobalNo
Ndb_api_trans_commit_count   YesGlobalNo
Ndb_api_trans_commit_count_session   YesSessionNo
Ndb_api_trans_commit_count_slave   YesGlobalNo
Ndb_api_trans_local_read_row_count   YesGlobalNo
Ndb_api_trans_local_read_row_count_session   YesSessionNo
Ndb_api_trans_local_read_row_count_slave   YesGlobalNo
Ndb_api_trans_start_count   YesGlobalNo
Ndb_api_trans_start_count_session   YesSessionNo
Ndb_api_trans_start_count_slave   YesGlobalNo
Ndb_api_uk_op_count   YesGlobalNo
Ndb_api_uk_op_count_session   YesSessionNo
Ndb_api_uk_op_count_slave   YesGlobalNo
Ndb_api_wait_exec_complete_count   YesGlobalNo
Ndb_api_wait_exec_complete_count_session   YesSessionNo
Ndb_api_wait_exec_complete_count_slave   YesGlobalNo
Ndb_api_wait_meta_request_count   YesGlobalNo
Ndb_api_wait_meta_request_count_session   YesSessionNo
Ndb_api_wait_meta_request_count_slave   YesGlobalNo
Ndb_api_wait_nanos_count   YesGlobalNo
Ndb_api_wait_nanos_count_session   YesSessionNo
Ndb_api_wait_nanos_count_slave   YesGlobalNo
Ndb_api_wait_scan_result_count   YesGlobalNo
Ndb_api_wait_scan_result_count_session   YesSessionNo
Ndb_api_wait_scan_result_count_slave   YesGlobalNo
ndb_autoincrement_prefetch_szYesYesYes BothYes
ndb-batch-sizeYesYesYes GlobalNo
ndb-blob-read-batch-bytesYesYesYes BothYes
ndb-blob-write-batch-bytesYesYesYes BothYes
ndb_cache_check_timeYesYesYes GlobalYes
ndb-cluster-connection-poolYesYesYes GlobalNo
Ndb_cluster_node_id   YesBothNo
Ndb_config_from_host   YesBothNo
Ndb_config_from_port   YesBothNo
Ndb_conflict_fn_epoch   YesGlobalNo
Ndb_conflict_fn_epoch_trans   YesGlobalNo
Ndb_conflict_fn_max   YesGlobalNo
Ndb_conflict_fn_old   YesGlobalNo
Ndb_conflict_trans_conflict_commit_count   YesGlobalNo
Ndb_conflict_trans_detect_iter_count   YesGlobalNo
Ndb_conflict_trans_reject_count   YesGlobalNo
Ndb_conflict_trans_row_reject_count   YesGlobalNo
ndb-connectstringYesYes    
ndb-deferred-constraintsYesYes  BothYes
- Variable: ndb_deferred_constraints  Yes BothYes
ndb_deferred_constraintsYesYesYes BothYes
ndb-distributionYesYes  GlobalYes
- Variable: ndb_distribution  Yes GlobalYes
ndb_distributionYesYesYes GlobalYes
ndb_eventbuffer_max_allocYesYesYes GlobalYes
Ndb_execute_count   YesGlobalNo
ndb_extra_loggingYesYesYes GlobalYes
ndb_force_sendYesYesYes BothYes
ndb_index_stat_cache_entriesYesYesYes BothYes
ndb_index_stat_enableYesYesYes BothYes
ndb_index_stat_optionYesYesYes BothYes
ndb_index_stat_update_freqYesYesYes BothYes
ndb_join_pushdown  Yes BothYes
ndb-log-apply-statusYesYes  GlobalNo
- Variable: ndb_log_apply_status  Yes GlobalNo
ndb_log_apply_statusYesYesYes GlobalNo
ndb_log_binYes Yes BothYes
ndb_log_binlog_indexYes Yes GlobalYes
ndb-log-empty-epochsYesYesYes GlobalYes
ndb_log_empty_epochsYesYesYes GlobalYes
ndb-log-origYesYes  GlobalNo
- Variable: ndb_log_orig  Yes GlobalNo
ndb_log_origYesYesYes GlobalNo
ndb-log-transaction-idYesYes  GlobalNo
- Variable: ndb_log_transaction_id  Yes GlobalNo
ndb_log_transaction_id  Yes GlobalNo
ndb-log-update-as-writeYesYesYes GlobalYes
ndb_log_updated_onlyYesYesYes GlobalYes
ndb-mgmd-hostYesYes    
ndb-nodeidYesYes YesGlobalNo
Ndb_number_of_data_nodes   YesGlobalNo
ndb_optimization_delay  Yes GlobalYes
ndb_optimized_node_selectionYesYesYes GlobalNo
Ndb_pruned_scan_count   YesGlobalNo
Ndb_pushed_queries_defined   YesGlobalNo
Ndb_pushed_queries_dropped   YesGlobalNo
Ndb_pushed_queries_executed   YesGlobalNo
Ndb_pushed_reads   YesGlobalNo
ndb_report_thresh_binlog_epoch_slipYesYesYes GlobalYes
ndb_report_thresh_binlog_mem_usageYesYesYes GlobalYes
Ndb_scan_count   YesGlobalNo
ndb_table_no_logging  Yes SessionYes
ndb_table_temporary  Yes SessionYes
ndb-transid-mysql-connection-mapYes     
ndb_use_copying_alter_table  Yes BothNo
ndb_use_exact_count  Yes BothYes
ndb_use_transactionsYesYesYes BothYes
ndb_version  Yes GlobalNo
ndb_version_string  Yes GlobalNo
ndb-wait-connectedYesYesYes GlobalNo
ndb-wait-setupYesYesYes GlobalNo
ndbclusterYesYes    
- Variable: have_ndbcluster      
ndbinfo_database  Yes GlobalNo
ndbinfo_max_bytesYes Yes BothYes
ndbinfo_max_rowsYes Yes BothYes
ndbinfo_offline  Yes GlobalYes
ndbinfo_show_hiddenYes Yes BothYes
ndbinfo_table_prefixYes Yes BothYes
ndbinfo_version  Yes GlobalNo
net_buffer_lengthYesYesYes BothYes
net_read_timeoutYesYesYes BothYes
net_retry_countYesYesYes BothYes
net_write_timeoutYesYesYes BothYes
newYesYesYes BothYes
no-defaultsYes     
Not_flushed_delayed_rows   YesGlobalNo
oldYesYesYes GlobalNo
old-alter-tableYesYes  BothYes
- Variable: old_alter_table  Yes BothYes
old_passwords  Yes BothYes
old-style-user-limitsYesYes    
one-threadYesYes    
Open_files   YesGlobalNo
open-files-limitYesYes  GlobalNo
- Variable: open_files_limit  Yes GlobalNo
Open_streams   YesGlobalNo
Open_table_definitions   YesGlobalNo
Open_tables   YesBothNo
Opened_files   YesGlobalNo
Opened_table_definitions   YesBothNo
Opened_tables   YesBothNo
optimizer_prune_levelYesYesYes BothYes
optimizer_search_depthYesYesYes BothYes
optimizer_switchYesYesYes BothYes
partitionYesYes    
- Variable: have_partitioning      
performance_schemaYesYesYes GlobalNo
Performance_schema_cond_classes_lost   YesGlobalNo
Performance_schema_cond_instances_lost   YesGlobalNo
performance_schema_events_waits_history_long_sizeYesYesYes GlobalNo
performance_schema_events_waits_history_sizeYesYesYes GlobalNo
Performance_schema_file_classes_lost   YesGlobalNo
Performance_schema_file_handles_lost   YesGlobalNo
Performance_schema_file_instances_lost   YesGlobalNo
Performance_schema_locker_lost   YesGlobalNo
performance_schema_max_cond_classesYesYesYes GlobalNo
performance_schema_max_cond_instancesYesYesYes GlobalNo
performance_schema_max_file_classesYesYesYes GlobalNo
performance_schema_max_file_handlesYesYesYes GlobalNo
performance_schema_max_file_instancesYesYesYes GlobalNo
performance_schema_max_mutex_classesYesYesYes GlobalNo
performance_schema_max_mutex_instancesYesYesYes GlobalNo
performance_schema_max_rwlock_classesYesYesYes GlobalNo
performance_schema_max_rwlock_instancesYesYesYes GlobalNo
performance_schema_max_table_handlesYesYesYes GlobalNo
performance_schema_max_table_instancesYesYesYes GlobalNo
performance_schema_max_thread_classesYesYesYes GlobalNo
performance_schema_max_thread_instancesYesYesYes GlobalNo
Performance_schema_mutex_classes_lost   YesGlobalNo
Performance_schema_mutex_instances_lost   YesGlobalNo
Performance_schema_rwlock_classes_lost   YesGlobalNo
Performance_schema_rwlock_instances_lost   YesGlobalNo
Performance_schema_table_handles_lost   YesGlobalNo
Performance_schema_table_instances_lost   YesGlobalNo
Performance_schema_thread_classes_lost   YesGlobalNo
Performance_schema_thread_instances_lost   YesGlobalNo
pid-fileYesYes  GlobalNo
- Variable: pid_file  Yes GlobalNo
pluginYesYes    
plugin_dirYesYesYes GlobalNo
plugin-loadYesYes    
portYesYesYes GlobalNo
port-open-timeoutYesYes    
preload_buffer_sizeYesYesYes BothYes
Prepared_stmt_count   YesGlobalNo
print-defaultsYes     
profiling  Yes VariesYes
profiling_history_sizeYesYesYes BothYes
protocol_version  Yes GlobalNo
proxy_user  Yes SessionNo
pseudo_slave_mode  Yes SessionYes
pseudo_thread_id  Yes SessionYes
Qcache_free_blocks   YesGlobalNo
Qcache_free_memory   YesGlobalNo
Qcache_hits   YesGlobalNo
Qcache_inserts   YesGlobalNo
Qcache_lowmem_prunes   YesGlobalNo
Qcache_not_cached   YesGlobalNo
Qcache_queries_in_cache   YesGlobalNo
Qcache_total_blocks   YesGlobalNo
Queries   YesBothNo
query_alloc_block_sizeYesYesYes BothYes
query_cache_limitYesYesYes GlobalYes
query_cache_min_res_unitYesYesYes GlobalYes
query_cache_sizeYesYesYes GlobalYes
query_cache_typeYesYesYes BothYes
query_cache_wlock_invalidateYesYesYes BothYes
query_prealloc_sizeYesYesYes BothYes
Questions   YesBothNo
rand_seed1  Yes SessionYes
rand_seed2  Yes SessionYes
range_alloc_block_sizeYesYesYes BothYes
read_buffer_sizeYesYesYes BothYes
read_onlyYesYesYes GlobalYes
read_rnd_buffer_sizeYesYesYes BothYes
relay-logYesYes  GlobalNo
- Variable: relay_log  Yes GlobalNo
relay-log-indexYesYes  GlobalNo
- Variable: relay_log_index  Yes GlobalNo
relay_log_indexYesYesYes GlobalNo
relay-log-info-fileYesYes    
- Variable: relay_log_info_file      
relay_log_info_fileYesYesYes GlobalNo
relay_log_purgeYesYesYes GlobalYes
relay-log-recoveryYesYes    
- Variable: relay_log_recovery      
relay_log_recoveryYesYesYes GlobalYes
relay_log_space_limitYesYesYes GlobalNo
removeYes     
replicate-do-dbYesYes    
replicate-do-tableYesYes    
replicate-ignore-dbYesYes    
replicate-ignore-tableYesYes    
replicate-rewrite-dbYesYes    
replicate-same-server-idYesYes    
replicate-wild-do-tableYesYes    
replicate-wild-ignore-tableYesYes    
report-hostYesYes  GlobalNo
- Variable: report_host  Yes GlobalNo
report-passwordYesYes  GlobalNo
- Variable: report_password  Yes GlobalNo
report-portYesYes  GlobalNo
- Variable: report_port  Yes GlobalNo
report-userYesYes  GlobalNo
- Variable: report_user  Yes GlobalNo
rpl_recovery_rank  Yes GlobalYes
Rpl_semi_sync_master_clients   YesGlobalNo
rpl_semi_sync_master_enabled  Yes GlobalYes
Rpl_semi_sync_master_net_avg_wait_time   YesGlobalNo
Rpl_semi_sync_master_net_wait_time   YesGlobalNo
Rpl_semi_sync_master_net_waits   YesGlobalNo
Rpl_semi_sync_master_no_times   YesGlobalNo
Rpl_semi_sync_master_no_tx   YesGlobalNo
Rpl_semi_sync_master_status   YesGlobalNo
Rpl_semi_sync_master_timefunc_failures   YesGlobalNo
rpl_semi_sync_master_timeout  Yes GlobalYes
rpl_semi_sync_master_trace_level  Yes GlobalYes
Rpl_semi_sync_master_tx_avg_wait_time   YesGlobalNo
Rpl_semi_sync_master_tx_wait_time   YesGlobalNo
Rpl_semi_sync_master_tx_waits   YesGlobalNo
rpl_semi_sync_master_wait_no_slave  Yes GlobalYes
Rpl_semi_sync_master_wait_pos_backtraverse   YesGlobalNo
Rpl_semi_sync_master_wait_sessions   YesGlobalNo
Rpl_semi_sync_master_yes_tx   YesGlobalNo
rpl_semi_sync_slave_enabled  Yes GlobalYes
Rpl_semi_sync_slave_status   YesGlobalNo
rpl_semi_sync_slave_trace_level  Yes GlobalYes
Rpl_status   YesGlobalNo
safe-modeYesYes    
safe-show-databaseYesYes    
safe-user-createYesYes    
safemalloc-mem-limitYesYes    
secure-authYesYes  GlobalYes
- Variable: secure_auth  Yes GlobalYes
secure-file-privYesYes  GlobalNo
- Variable: secure_file_priv  Yes GlobalNo
Select_full_join   YesBothNo
Select_full_range_join   YesBothNo
Select_range   YesBothNo
Select_range_check   YesBothNo
Select_scan   YesBothNo
server-idYesYes  GlobalYes
- Variable: server_id  Yes GlobalYes
server-id-bitsYesYes  GlobalNo
- Variable: server_id_bits  Yes GlobalNo
server_id_bitsYesYesYes GlobalNo
set-variableYesYes    
shared_memoryYesYesYes GlobalNo
shared_memory_base_nameYesYesYes GlobalNo
show-slave-auth-infoYesYes    
skip-character-set-client-handshakeYesYes    
skip-concurrent-insertYesYes    
- Variable: concurrent_insert      
skip-event-schedulerYesYes    
skip_external_lockingYesYesYes GlobalNo
skip-grant-tablesYesYes    
skip-host-cacheYesYes    
skip-lockingYesYes    
skip-name-resolveYesYes  GlobalNo
- Variable: skip_name_resolve  Yes GlobalNo
skip-ndbclusterYesYes    
skip-networkingYesYes  GlobalNo
- Variable: skip_networking  Yes GlobalNo
skip-newYesYes    
skip-partitionYesYes    
skip-safemallocYesYes    
skip-show-databaseYesYes  GlobalNo
- Variable: skip_show_database  Yes GlobalNo
skip-slave-startYesYes    
skip-sslYesYes    
skip-stack-traceYesYes    
skip-symbolic-linksYes     
skip-symlinkYesYes    
skip-thread-priorityYesYes    
slave_allow_batchingYesYesYes GlobalYes
slave_compressed_protocolYesYesYes GlobalYes
slave_exec_modeYesYesYes GlobalYes
Slave_heartbeat_period   YesGlobalNo
slave-load-tmpdirYesYes  GlobalNo
- Variable: slave_load_tmpdir  Yes GlobalNo
slave-max-allowed-packetYesYes    
- Variable: slave_max_allowed_packet      
slave_max_allowed_packet  Yes GlobalYes
slave-net-timeoutYesYes  GlobalYes
- Variable: slave_net_timeout  Yes GlobalYes
Slave_open_temp_tables   YesGlobalNo
Slave_received_heartbeats   YesGlobalNo
Slave_retried_transactions   YesGlobalNo
Slave_running   YesGlobalNo
slave-skip-errorsYesYes  GlobalNo
- Variable: slave_skip_errors  Yes GlobalNo
slave_transaction_retriesYesYesYes GlobalYes
slave_type_conversionsYesYesYes GlobalNo
Slow_launch_threads   YesBothNo
slow_launch_timeYesYesYes GlobalYes
Slow_queries   YesBothNo
slow-query-logYesYes  GlobalYes
- Variable: slow_query_log  Yes GlobalYes
slow_query_log_fileYesYesYes GlobalYes
slow-start-timeoutYesYes    
socketYesYesYes GlobalNo
sort_buffer_sizeYesYesYes BothYes
Sort_merge_passes   YesBothNo
Sort_range   YesBothNo
Sort_rows   YesBothNo
Sort_scan   YesBothNo
sporadic-binlog-dump-failYesYes    
sql_auto_is_null  Yes VariesYes
sql_big_selects  Yes VariesYes
sql_big_tables  Yes VariesYes
sql_buffer_result  Yes VariesYes
sql_log_bin  Yes VariesYes
sql_log_off  Yes VariesYes
sql_log_update  Yes SessionYes
sql_low_priority_updates  Yes BothYes
sql_max_join_size  Yes BothYes
sql-modeYesYes  BothYes
- Variable: sql_mode  Yes BothYes
sql_notes  Yes VariesYes
sql_quote_show_create  Yes VariesYes
sql_safe_updates  Yes VariesYes
sql_select_limit  Yes BothYes
sql_slave_skip_counter  Yes GlobalYes
sql_warnings  Yes VariesYes
sslYesYes    
Ssl_accept_renegotiates   YesGlobalNo
Ssl_accepts   YesGlobalNo
ssl-caYesYes  GlobalNo
- Variable: ssl_ca  Yes GlobalNo
Ssl_callback_cache_hits   YesGlobalNo
ssl-capathYesYes  GlobalNo
- Variable: ssl_capath  Yes GlobalNo
ssl-certYesYes  GlobalNo
- Variable: ssl_cert  Yes GlobalNo
Ssl_cipher   YesBothNo
ssl-cipherYesYes  GlobalNo
- Variable: ssl_cipher  Yes GlobalNo
Ssl_cipher_list   YesBothNo
Ssl_client_connects   YesGlobalNo
Ssl_connect_renegotiates   YesGlobalNo
Ssl_ctx_verify_depth   YesGlobalNo
Ssl_ctx_verify_mode   YesGlobalNo
Ssl_default_timeout   YesBothNo
Ssl_finished_accepts   YesGlobalNo
Ssl_finished_connects   YesGlobalNo
ssl-keyYesYes  GlobalNo
- Variable: ssl_key  Yes GlobalNo
Ssl_session_cache_hits   YesGlobalNo
Ssl_session_cache_misses   YesGlobalNo
Ssl_session_cache_mode   YesGlobalNo
Ssl_session_cache_overflows   YesGlobalNo
Ssl_session_cache_size   YesGlobalNo
Ssl_session_cache_timeouts   YesGlobalNo
Ssl_sessions_reused   YesBothNo
Ssl_used_session_cache_entries   YesGlobalNo
Ssl_verify_depth   YesBothNo
Ssl_verify_mode   YesBothNo
Ssl_version   YesBothNo
standaloneYesYes    
storage_engine  Yes BothYes
stored_program_cacheYesYesYes GlobalYes
super-large-pagesYesYes    
symbolic-linksYesYes    
sync_binlogYesYesYes GlobalYes
sync_frmYesYesYes GlobalYes
sync_master_infoYesYesYes GlobalYes
sync_relay_logYesYesYes GlobalYes
sync_relay_log_infoYesYesYes GlobalYes
sysdate-is-nowYesYes    
system_time_zone  Yes GlobalNo
table_definition_cache  Yes GlobalYes
table_lock_wait_timeoutYesYesYes GlobalYes
Table_locks_immediate   YesGlobalNo
Table_locks_waited   YesGlobalNo
table_open_cache  Yes GlobalYes
table_type  Yes BothYes
tc-heuristic-recoverYesYes    
Tc_log_max_pages_used   YesGlobalNo
Tc_log_page_size   YesGlobalNo
Tc_log_page_waits   YesGlobalNo
temp-poolYesYes    
thread_cache_sizeYesYesYes GlobalYes
thread_concurrencyYesYesYes GlobalNo
thread_handlingYesYesYes GlobalNo
thread_pool_algorithmYesYesYes GlobalNo
thread_pool_high_priority_connectionYesYesYes BothYes
thread_pool_max_unused_threadsYesYesYes GlobalYes
thread_pool_prio_kickup_timerYesYesYes BothYes
thread_pool_sizeYesYesYes GlobalNo
thread_pool_stall_limitYesYesYes GlobalYes
thread_stackYesYesYes GlobalNo
Threads_cached   YesGlobalNo
Threads_connected   YesGlobalNo
Threads_created   YesGlobalNo
Threads_running   YesGlobalNo
time_format  Yes VariesNo
time_zone  Yes BothYes
timed_mutexesYesYesYes GlobalYes
timestamp  Yes SessionYes
tmp_table_sizeYesYesYes BothYes
tmpdirYesYesYes GlobalNo
transaction_alloc_block_sizeYesYesYes BothYes
transaction_allow_batching  Yes SessionYes
transaction-isolationYesYes    
- Variable: tx_isolation      
transaction_prealloc_sizeYesYesYes BothYes
tx_isolation  Yes BothYes
unique_checks  Yes VariesYes
updatable_views_with_limitYesYesYes BothYes
Uptime   YesGlobalNo
Uptime_since_flush_status   YesGlobalNo
userYesYes    
verboseYesYes    
version  Yes GlobalNo
version_comment  Yes GlobalNo
version_compile_machine  Yes GlobalNo
version_compile_os  Yes GlobalNo
wait_timeoutYesYesYes BothYes
warning_count  Yes SessionNo

[a] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

[b] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.


5.1.2 Server Configuration Defaults

The MySQL server has many operating parameters, which you can change at server startup using command-line options or configuration files (option files). It is also possible to change many parameters at runtime. For general instructions on setting parameters at startup or runtime, see Section 5.1.3, “Server Command Options”, and Section 5.1.4, “Server System Variables”.

MySQL provides a number of preconfigured option files that can be used as a basis for tuning the MySQL server. Look for files named my-small.cnf, my-medium.cnf, my-large.cnf, and my-huge.cnf, which are sample option files for small, medium, large, and very large systems. On Windows, the extension is .ini rather than .cnf.

Note

On Windows, the .ini or .cnf option file extension might not be displayed.

For a binary distribution, look for the sample files in or under your installation directory. If you have a source distribution, look in the support-files directory. To use a sample file as a base configuration file, rename a copy of it and place the copy in the appropriate location. Regarding names and appropriate location, see the general information provided in Section 4.2.6, “Using Option Files”. That section also describes option file format and syntax.

5.1.3 Server Command Options

When you start the mysqld server, you can specify program options using any of the methods described in Section 4.2.3, “Specifying Program Options”. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 4.2.6, “Using Option Files”. That section also describes option file format and syntax.

mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. mysql.server reads options from the [mysqld] and [mysql.server] groups.

An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.

mysqld accepts many command options. For a brief summary, execute mysqld --help. To see the full list, use mysqld --verbose --help.

The following list shows some of the most common server options. Additional options are described in other sections:

Some options control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to an option that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to an option for which the minimal value is 1024, the server will set the value to 1024.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Some options take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued option is given as a relative path name, it will be located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.

You can also set the values of server system variables at server startup by using variable names as options. To assign a value to a server system variable, use an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.

When you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest permissible value if only certain values are permitted.

To restrict the maximum value to which a system variable can be set at runtime with the SET statement, specify this maximum by using an option of the form --maximum-var_name=value at server startup.

You can change the values of most system variables for a running server with the SET statement. See Section 13.7.4.1, “SET Syntax for Variable Assignment”.

Section 5.1.4, “Server System Variables”, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 8.12.2, “Tuning Server Parameters”, includes information on optimizing the server by tuning system variables.

  • --help, -?

    Command-Line Format--help

    Display a short help message and exit. Use both the --verbose and --help options to see the full message.

  • --allow-suspicious-udfs

    Command-Line Format--allow-suspicious-udfs
    Permitted ValuesTypeboolean
    DefaultFALSE

    This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. See Section 24.4.2.6, “UDF Security Precautions”.

  • --ansi

    Command-Line Format--ansi

    Use standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the --sql-mode option instead. See Section 1.7, “MySQL Standards Compliance”, and Section 5.1.7, “Server SQL Modes”.

  • --basedir=dir_name, -b dir_name

    Command-Line Format--basedir=dir_name
    System VariableNamebasedir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The path to the MySQL installation directory. All paths are usually resolved relative to this directory.

  • --big-tables

    Command-Line Format--big-tables
    System Variable (<= 5.5.2)Namebig_tables
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namebig_tables
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Enable large result sets by saving all temporary sets in files. This option prevents most table full errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.

  • --bind-address=addr

    Command-Line Format--bind-address=addr
    Permitted ValuesTypestring
    Default0.0.0.0

    The MySQL server listens on a single network socket for TCP/IP connections. This socket is bound to a single address, but it is possible for an address to map onto multiple network interfaces. The default address is 0.0.0.0. To specify an address explicitly, use the --bind-address=addr option at server startup, where addr is an IPv4 or IPv6 address or a host name. (IPv6 addresses are not supported before MySQL 5.5.3.) If addr is a host name, the server resolves the name to an IP address and binds to that address.

    The server treats different types of addresses as follows:

    • If the address is 0.0.0.0, the server accepts TCP/IP connections on all server host IPv4 interfaces.

    • If the address is ::, the server accepts TCP/IP connections on all server host IPv4 and IPv6 interfaces. Use this address to permit both IPv4 and IPv6 connections on all server interfaces.

    • If the address is an IPv4-mapped address, the server accepts TCP/IP connections for that address, in either IPv4 or IPv6 format. For example, if the server is bound to ::ffff:127.0.0.1, clients can connect using --host=127.0.0.1 or --host=::ffff:127.0.0.1.

    • If the address is a regular IPv4 or IPv6 address (such as 127.0.0.1 or ::1), the server accepts TCP/IP connections only for that IPv4 or IPv6 address.

    If you intend to bind the server to a specific address, be sure that the mysql.user grant table contains an account with administrative privileges that you can use to connect to that address. Otherwise, you will not be able to shut down the server. For example, if you bind the server to ::, you can connect to it using all existing accounts. But if you bind the server to ::1, it accepts connections only on that address. In that case, first make sure that the 'root'@'::1' account is present in the mysql.user table so you can still connect to the server to shut it down.

  • --binlog-format={ROW|STATEMENT|MIXED}

    Command-Line Format--binlog-format=format
    System VariableNamebinlog_format
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    DefaultSTATEMENT
    Valid ValuesROW
    STATEMENT
    MIXED
    Permitted Values (>= 5.5.15-ndb-7.2.1, <= 5.5.30-ndb-7.2.12)Typeenumeration
    DefaultSTATEMENT
    Valid ValuesROW
    STATEMENT
    MIXED
    Permitted Values (>= 5.5.31-ndb-7.2.13)Typeenumeration
    DefaultMIXED
    Valid ValuesROW
    STATEMENT
    MIXED

    Specify whether to use row-based, statement-based, or mixed replication. Statement-based is the default in MySQL 5.5. This is also true for MySQL Cluster NDB 7.2.1 and later. See Section 17.1.2, “Replication Formats”.

    Under some conditions, changing this variable at runtime is not possible, or causes replication to fail. See Section 5.4.4.2, “Setting The Binary Log Format”, for more information.

    Prior to MySQL 5.5, setting the binary logging format without enabling binary logging prevented the MySQL server from starting. In MySQL 5.5, the server starts in such cases, the binlog_format global system variable is set, and a warning is logged instead of an error. (Bug #42928)

  • --bootstrap

    Command-Line Format--bootstrap

    This option is used by the mysql_install_db program to create the MySQL privilege tables without having to start a full MySQL server.

    When the server operates in bootstap mode, some functionality is unavailable that limits the statements permitted in any file named by the --init-file option. For more information, see the description of that option.

  • --character-sets-dir=dir_name

    Command-Line Format--character-sets-dir=dir_name
    System VariableNamecharacter_sets_dir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The directory where character sets are installed. See Section 10.5, “Character Set Configuration”.

  • --character-set-client-handshake

    Command-Line Format--character-set-client-handshake
    Permitted ValuesTypeboolean
    DefaultTRUE

    Do not ignore character set information sent by the client. To ignore client information and use the default server character set, use --skip-character-set-client-handshake; this makes MySQL behave like MySQL 4.0.

  • --character-set-filesystem=charset_name

    Command-Line Format--character-set-filesystem=name
    System VariableNamecharacter_set_filesystem
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaultbinary

    The file system character set. This option sets the character_set_filesystem system variable.

  • --character-set-server=charset_name, -C charset_name

    Command-Line Format--character-set-server
    System VariableNamecharacter_set_server
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaultlatin1

    Use charset_name as the default server character set. See Section 10.5, “Character Set Configuration”. If you use this option to specify a nondefault character set, you should also use --collation-server to specify the collation.

  • --chroot=dir_name, -r dir_name

    Command-Line Format--chroot=dir_name
    Permitted ValuesTypedirectory name

    Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure. Use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.

  • --collation-server=collation_name

    Command-Line Format--collation-server
    System VariableNamecollation_server
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaultlatin1_swedish_ci

    Use collation_name as the default server collation. See Section 10.5, “Character Set Configuration”.

  • --console

    Command-Line Format--console
    Platform SpecificWindows

    (Windows only.) Write error log messages to stderr and stdout. mysqld does not close the console window if this option is used.

    As of MySQL 5.5.3, if both --log-error and --console are specified, --log-error takes precedence. The server writes to the log file, but not to the console. Before 5.5.3, whichever option is given last takes precedence.

  • --core-file

    Command-Line Format--core-file
    Permitted ValuesTypeboolean
    DefaultOFF

    Write a core file if mysqld dies. The name and location of the core file is system dependent. On Linux, a core file named core.pid is written to the current working directory of the process, which for mysqld is the data directory. pid represents the process ID of the server process. On OS X, a core file named core.pid is written to the /cores directory. On Solaris, use the coreadm command to specify where to write the core file and how to name it.

    For some systems, to get a core file you must also specify the --core-file-size option to mysqld_safe. See Section 4.3.2, “mysqld_safe — MySQL Server Startup Script”. On some systems, such as Solaris, you do not get a core file if you are also using the --user option. There might be additional restrictions or limitations. For example, it might be necessary to execute ulimit -c unlimited before starting the server. Consult your system documentation.

  • --datadir=dir_name, -h dir_name

    Command-Line Format--datadir=dir_name
    System VariableNamedatadir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The path to the data directory.

  • --debug[=debug_options], -# [debug_options]

    Command-Line Format--debug[=debug_options]
    System VariableNamedebug
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (Unix)Typestring
    Defaultd:t:i:o,/tmp/mysqld.trace
    Permitted Values (Windows)Typestring
    Defaultd:t:i:O,\mysqld.trace

    If MySQL is configured with the -DWITH_DEBUG=1 CMake option, you can use this option to get a trace file of what mysqld is doing. A typical debug_options string is d:t:o,file_name. The default is d:t:i:o,/tmp/mysqld.trace on Unix and d:t:i:O,\mysqld.trace on Windows.

    Using -DWITH_DEBUG=1 to configure MySQL with debugging support enables you to use the --debug="d,parser_debug" option when you start the server. This causes the Bison parser that is used to process SQL statements to dump a parser trace to the server's standard error output. Typically, this output is written to the error log.

    This option may be given multiple times. Values that begin with + or - are added to or subtracted from the previous value. For example, --debug=T --debug=+P sets the value to P:T.

    For more information, see Section 24.5.3, “The DBUG Package”.

  • --debug-sync-timeout[=N]

    Command-Line Format--debug-sync-timeout[=#]
    Permitted ValuesTypeinteger

    Controls whether the Debug Sync facility for testing and debugging is enabled. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 CMake option (see Section 2.9.4, “MySQL Source-Configuration Options”). If Debug Sync is not compiled in, this option is not available. The option value is a timeout in seconds. The default value is 0, which disables Debug Sync. To enable it, specify a value greater than 0; this value also becomes the default timeout for individual synchronization points. If the option is given without a value, the timeout is set to 300 seconds.

    For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.

  • --default-character-set=charset_name

    Deprecated5.0.0
    Removed5.5.3
    Command-Line Format--default-character-set=name
    Permitted ValuesTypestring

    Use charset_name as the default character set. This option is deprecated in favor of --character-set-server. See Section 10.5, “Character Set Configuration”. --default-character-set was removed in MySQL 5.5.3.

  • --default-collation=collation_name

    Deprecated4.1.3
    Command-Line Format--default-collation=name
    Permitted ValuesTypestring

    Use collation_name as the default collation. This option is deprecated in favor of --collation-server. See Section 10.5, “Character Set Configuration”. --default-collation was removed in MySQL 5.5.3.

  • --default-storage-engine=type

    Command-Line Format--default-storage-engine=name
    System Variable (<= 5.5.2)Namestorage_engine
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namedefault_storage_engine
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.4)Typeenumeration
    DefaultMyISAM
    Permitted Values (>= 5.5.5)Typeenumeration
    DefaultInnoDB

    Set the default storage engine (table type) for tables. See Chapter 15, Alternative Storage Engines.

    If you disable the default storage engine at server startup, you must set the default engine to a different engine or the server will not start.

  • --default-time-zone=timezone

    Command-Line Format--default-time-zone=name
    Permitted ValuesTypestring

    Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone is the same as the system time zone (given by the value of the system_time_zone system variable.

  • --defaults-extra-file=file_name

    Read this option file after the global option file but (on Unix) before the user option file. If the file does not exist or is otherwise inaccessible, an error occurs. Before MySQL 5.5.8, file_name must be the full path name to the file. As of MySQL 5.5.8, the name is interpreted relative to the current directory if given as a relative path name.

  • --defaults-file=file_name

    Use only the given option file. If the file does not exist or is otherwise inaccessible, an error occurs. Before MySQL 5.5.8, file_name must be the full path name to the file. As of MySQL 5.5.8, the name is interpreted relative to the current directory if given as a relative path name.

  • --defaults-group-suffix=str

    Read not only the usual option groups, but also groups with the usual names and a suffix of str. For example, mysqld normally reads the [mysqld] group. If the --defaults-group-suffix=_other option is given, mysqld also reads the [mysqld_other] group.

  • --delay-key-write[={OFF|ON|ALL}]

    Command-Line Format--delay-key-write[=name]
    System VariableNamedelay_key_write
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    DefaultON
    Valid ValuesON
    OFF
    ALL

    Specify how to use delayed key writes. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAY_KEY_WRITE option. ALL delays key writes for all MyISAM tables. See Section 8.12.2, “Tuning Server Parameters”, and Section 15.3.1, “MyISAM Startup Options”.

    Note

    If you set this variable to ALL, you should not use MyISAM tables from within another program (such as another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.

  • --des-key-file=file_name

    Command-Line Format--des-key-file=file_name

    Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.

  • --enable-locking

    This option is deprecated and was removed in MySQL 5.5.3. Use --external-locking instead.

  • --enable-named-pipe

    Command-Line Format--enable-named-pipe
    Platform SpecificWindows

    Enable support for named pipes. This option applies only on Windows.

  • --enable-pstack

    Deprecated5.1.54
    Removed5.5.7
    Command-Line Format--enable-pstack
    Permitted ValuesTypeboolean
    DefaultFALSE

    This option is nonfunctional before MySQL 5.5.7 and removed in 5.5.7.

  • --engine-condition-pushdown={ON|OFF}

    Deprecated5.5.3, by optimizer_switch
    Command-Line Format--engine-condition-pushdown
    System VariableNameengine_condition_pushdown
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultON

    Sets the engine_condition_pushdown system variable. For more information, see Section 8.2.1.5, “Engine Condition Pushdown Optimization”.

  • --event-scheduler[=value]

    Command-Line Format--event-scheduler[=value]
    System VariableNameevent_scheduler
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    DefaultOFF
    Valid ValuesON
    OFF
    DISABLED

    Enable or disable, and start or stop, the event scheduler.

    For detailed information, see The --event-scheduler Option.

  • --exit-info[=flags], -T [flags]

    Command-Line Format--exit-info[=flags]
    Permitted ValuesTypeinteger

    This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!

  • --external-locking

    Command-Line Format--external-locking
    Permitted ValuesTypeboolean
    DefaultFALSE

    Enable external locking (system locking), which is disabled by default. If you use this option on a system on which lockd does not fully work (such as Linux), it is easy for mysqld to deadlock.

    To disable external locking explicitly, use --skip-external-locking.

    External locking affects only MyISAM table access. For more information, including conditions under which it can and cannot be used, see Section 8.11.5, “External Locking”.

  • --flush

    Command-Line Format--flush
    System VariableNameflush
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.5.3.3, “What to Do If MySQL Keeps Crashing”.

  • --gdb

    Command-Line Format--gdb
    Permitted ValuesTypeboolean
    DefaultFALSE

    Install an interrupt handler for SIGINT (needed to stop mysqld with ^C to set breakpoints) and disable stack tracing and core file handling. See Section 24.5, “Debugging and Porting MySQL”.

  • --general-log[={0|1}]

    Command-Line Format--general-log
    System VariableNamegeneral_log
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Specify the initial general query log state. With no argument or an argument of 1, the --general-log option enables the log. If omitted or given with an argument of 0, the option disables the log.

  • --init-file=file_name

    Command-Line Format--init-file=file_name
    System VariableNameinit_file
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.

    If the server is started with the --bootstrap option, it operates in bootstap mode and some functionality is unavailable that limits the statements permitted in the file. These include statements that relate to account management (such as CREATE USER or GRANT).

  • --innodb-xxx

    Set an option for the InnoDB storage engine. The InnoDB options are listed in Section 14.17, “InnoDB Startup Options and System Variables”.

  • --install [service_name]

    Command-Line Format--install [service_name]
    Platform SpecificWindows

    (Windows only) Install the server as a Windows service that starts automatically during Windows startup. The default service name is MySQL if no service_name value is given. For more information, see Section 2.3.7.7, “Starting MySQL as a Windows Service”.

    Note

    If the server is started with the --defaults-file and --install options, --install must be first.

  • --install-manual [service_name]

    Command-Line Format--install-manual [service_name]
    Platform SpecificWindows

    (Windows only) Install the server as a Windows service that must be started manually. It does not start automatically during Windows startup. The default service name is MySQL if no service_name value is given. For more information, see Section 2.3.7.7, “Starting MySQL as a Windows Service”.

    Note

    If the server is started with the --defaults-file and --install-manual options, --install-manual must be first.

  • --language=lang_name, -L lang_name

    Deprecated5.5.0, by lc-messages-dir
    Command-Line Format--language=name
    System VariableNamelanguage
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name
    Default/usr/local/mysql/share/mysql/english/

    The language to use for error messages. lang_name can be given as the language name or as the full path name to the directory where the language files are installed. See Section 10.2, “Setting the Error Message Language”.

    As of MySQL 5.5, --lc-messages-dir and --lc-messages should be used rather than --language, which is deprecated and handled as an alias for --lc-messages-dir.

  • --large-pages

    Command-Line Format--large-pages
    System VariableNamelarge_pages
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificLinux
    Permitted Values (Linux)Typeboolean
    DefaultFALSE

    Some hardware/operating system architectures support memory pages greater than the default (usually 4KB). The actual implementation of this support depends on the underlying hardware and operating system. Applications that perform a lot of memory accesses may obtain performance improvements by using large pages due to reduced Translation Lookaside Buffer (TLB) misses.

    MySQL supports the Linux implementation of large page support (which is called HugeTLB in Linux). See Section 8.12.5.2, “Enabling Large Page Support”. For Solaris support of large pages, see the description of the --super-large-pages option.

    --large-pages is disabled by default.

  • --lc-messages=locale_name

    Command-Line Format--lc-messages=name
    System VariableNamelc_messages
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaulten_US

    The locale to use for error messages. The default is en_US. The server converts the argument to a language name and combines it with the value of --lc-messages-dir to produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”.

  • --lc-messages-dir=dir_name

    Command-Line Format--lc-messages-dir=dir_name
    System VariableNamelc_messages_dir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The directory where error messages are located. The server uses the value together with the value of --lc-messages to produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”.

  • --local-service

    Command-Line Format--local-service

    (Windows only) A --local-service option following the service name causes the server to run using the LocalService Windows account that has limited system privileges. This account is available only for Windows XP or newer. If both --defaults-file and --local-service are given following the service name, they can be in any order. See Section 2.3.7.7, “Starting MySQL as a Windows Service”.

  • --log[=file_name], -l [file_name]

    Deprecated5.1.29, by general-log
    Command-Line Format--log[=file_name]
    System VariableNamelog
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypefile name

    This option enables logging to the general query log, which contains entries that record client connections and SQL statements received from clients. The log output destination can be selected with the --log-output option. If you omit the file name, MySQL uses host_name.log as the file name. See Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”, and Section 5.4.3, “The General Query Log”.

    The --log option is deprecated and is removed (along with the log system variable) in MySQL 5.6. Instead, use the --general_log option to enable the general query log and the --general_log_file=file_name option to set the general query log file name.

  • --log-error[=file_name]

    Command-Line Format--log-error[=file_name]
    System VariableNamelog_error
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    Log errors and startup messages to this file. See Section 5.4.2, “The Error Log”. If the file name has no extension, the server adds an extension of .err. If you omit the file name, the default log file on Unix and Unix-like systems is host_name.err in the data directory. The default on Windows is host_name.err in the data directory, unless the --pid-file option is specified. In that case, the default name is the PID file base name with a suffix of .err in the data directory.

  • --log-isam[=file_name]

    Command-Line Format--log-isam[=file_name]
    Permitted ValuesTypefile name

    Log all MyISAM changes to this file (used only when debugging MyISAM).

  • --log-long-format

    Deprecated4.1.0
    Command-Line Format--log-long-format

    Log extra information to the binary log and slow query log, if they have been activated. For example, the user name and timestamp are logged for all queries. This option is deprecated, as it now represents the default logging behavior. (See the description for --log-short-format.) The --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log. --log-long-format was removed in MySQL 5.5.3.

  • --log-output=value,...

    Command-Line Format--log-output=name
    System VariableNamelog_output
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeset
    DefaultFILE
    Valid ValuesTABLE
    FILE
    NONE

    This option determines the destination for general query log and slow query log output. The option value can be given as one or more of the words TABLE, FILE, or NONE. TABLE select logging to the general_log and slow_log tables in the mysql database as a destination. FILE selects logging to log files as a destination. NONE disables logging. If NONE is present in the option value, it takes precedence over any other words that are present. TABLE and FILE can both be given to select to both log output destinations.

    This option selects log output destinations, but does not enable log output. To do that, use the --general_log and --slow_query_log options. For FILE logging, the --general_log_file and -slow_query_log_file options determine the log file location. For more information, see Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”.

  • --log-queries-not-using-indexes

    Command-Line Format--log-queries-not-using-indexes
    System VariableNamelog_queries_not_using_indexes
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    If you are using this option with the slow query log enabled, queries that are expected to retrieve all rows are logged. See Section 5.4.5, “The Slow Query Log”. This option does not necessarily mean that no index is used. For example, a query that uses a full index scan uses an index but would be logged because the index would not limit the number of rows.

  • --log-short-format

    Command-Line Format--log-short-format
    Permitted ValuesTypeboolean
    DefaultFALSE

    Log less information to the slow query log, if it has been activated.

  • --log-slow-admin-statements

    Command-Line Format--log-slow-admin-statements
    Permitted ValuesTypeboolean
    DefaultOFF

    Include slow administrative statements in the statements written to the slow query log. Administrative statements include ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE.

  • --log-slow-queries[=file_name]

    Deprecated5.1.29, by slow-query-log
    Command-Line Format--log-slow-queries[=name]
    System VariableNamelog_slow_queries
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean

    This option enables logging to the slow query log, which contains entries for all queries that have taken more than long_query_time seconds to execute. See the descriptions of the --log-long-format and --log-short-format options for details. The log output destination can be selected with the --log-output option. If you omit the file name, MySQL uses host_name-slow.log as the file name. See Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”, and Section 5.4.5, “The Slow Query Log”.

    The --log-slow-queries option is deprecated and is removed (along with the log_slow_queries system variable) in MySQL 5.6. Instead, use the --slow_query_log option to enable the slow query log and the --slow_query_log_file=file_name option to set the slow query log file name.

  • --log-tc=file_name

    Command-Line Format--log-tc=file_name
    Permitted ValuesTypefile name
    Defaulttc.log

    The name of the memory-mapped transaction coordinator log file (for XA transactions that affect multiple storage engines when the binary log is disabled). The default name is tc.log. The file is created under the data directory if not given as a full path name. This option is unused.

  • --log-tc-size=size

    Command-Line Format--log-tc-size=#
    Permitted Values (32-bit platforms)Typeinteger
    Default24576
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default24576
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default24576
    Max Value18446744073709551615

    The size in bytes of the memory-mapped transaction coordinator log. The default size is 24KB.

  • --log-warnings[=level], -W [level]

    Command-Line Format--log-warnings[=#]
    System VariableNamelog_warnings
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default1
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default1
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default1
    Min Value0
    Max Value18446744073709551615

    Print out warnings such as Aborted connection... to the error log. This option is enabled (1) by default. To disable it, use --log-warnings=0. Specifying the option without a level value increments the current value by 1. Enabling this option by setting it greater than 0 is recommended, for example, if you use replication (you get more information about what is happening, such as messages about network failures and reconnections). If the value is greater than 1, aborted connections are written to the error log, and access-denied errors for new connection attempts are written. See Section B.5.2.11, “Communication Errors and Aborted Connections”.

    If a slave server was started with --log-warnings enabled, the slave prints messages to the error log to provide information about its status, such as the binary log and relay log coordinates where it starts its job, when it is switching to another relay log, when it reconnects after a disconnect, and so forth. The server logs messages about statements that are unsafe for statement-based logging if --log-warnings is greater than 0.

  • --low-priority-updates

    Command-Line Format--low-priority-updates
    System VariableNamelow_priority_updates
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultFALSE

    Give table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) lower priority than selects. This can also be done using {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. This affects only storage engines that use only table-level locking (MyISAM, MEMORY, MERGE). See Section 8.11.2, “Table Locking Issues”.

  • --min-examined-row-limit=number

    Command-Line Format--min-examined-row-limit=#
    System VariableNamemin_examined_row_limit
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default0
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709551615

    When this option is set, queries which examine fewer than number rows are not written to the slow query log. The default is 0.

  • --memlock

    Command-Line Format--memlock
    Permitted ValuesTypeboolean
    DefaultFALSE

    Lock the mysqld process in memory. This option might help if you have a problem where the operating system is causing mysqld to swap to disk.

    --memlock works on systems that support the mlockall() system call; this includes Solaris, most Linux distributions that use a 2.4 or higher kernel, and perhaps other Unix systems. On Linux systems, you can tell whether or not mlockall() (and thus this option) is supported by checking to see whether or not it is defined in the system mman.h file, like this:

    shell> grep mlockall /usr/include/sys/mman.h
    

    If mlockall() is supported, you should see in the output of the previous command something like the following:

    extern int mlockall (int __flags) __THROW;
    
    Important

    Use of this option may require you to run the server as root, which, for reasons of security, is normally not a good idea. See Section 6.1.5, “How to Run MySQL as a Normal User”.

    On Linux and perhaps other systems, you can avoid the need to run the server as root by changing the limits.conf file. See the notes regarding the memlock limit in Section 8.12.5.2, “Enabling Large Page Support”.

    You must not try to use this option on a system that does not support the mlockall() system call; if you do so, mysqld will very likely crash as soon as you try to start it.

  • --myisam-block-size=N

    Command-Line Format--myisam-block-size=#
    Permitted ValuesTypeinteger
    Default1024
    Min Value1024
    Max Value16384

    The block size to be used for MyISAM index pages.

  • --myisam-recover[=option[,option]...]]

    This option is renamed as of MySQL 5.5.3 to --myisam-recover-options. See the description of that option for more information.

  • --myisam-recover-options[=option[,option]...]]

    Introduced5.5.3
    Command-Line Format--myisam-recover-options[=name]
    Permitted ValuesTypeenumeration
    DefaultOFF
    Valid ValuesOFF
    DEFAULT
    BACKUP
    FORCE
    QUICK

    Set the MyISAM storage engine recovery mode. The option value is any combination of the values of OFF, DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. Specifying the option with no argument is the same as specifying DEFAULT, and specifying with an explicit value of "" disables recovery (same as a value of OFF). If recovery is enabled, each time mysqld opens a MyISAM table, it checks whether the table is marked as crashed or was not closed properly. (The last option works only if you are running with external locking disabled.) If this is the case, mysqld runs a check on the table. If the table was corrupted, mysqld attempts to repair it.

    The following options affect how the repair works.

    OptionDescription
    OFFNo recovery.
    DEFAULTRecovery without backup, forcing, or quick checking.
    BACKUPIf the data file was changed during recovery, save a backup of the tbl_name.MYD file as tbl_name-datetime.BAK.
    FORCERun recovery even if we would lose more than one row from the .MYD file.
    QUICKDo not check the rows in the table if there are not any delete blocks.

    Before the server automatically repairs a table, it writes a note about the repair to the error log. If you want to be able to recover from most problems without user intervention, you should use the options BACKUP,FORCE. This forces a repair of a table even if some rows would be deleted, but it keeps the old data file as a backup so that you can later examine what happened.

    This option was named --myisam-recover, before MySQL 5.5.3. The old option name still works because it is recognized as an unambiguous prefix of the new name, --myisam-recover-options. (Option prefix recognition occurs as described in Section 4.2.3, “Specifying Program Options”.)

    The option value OFF is available as of MySQL 5.5.3.

    See Section 15.3.1, “MyISAM Startup Options”.

  • --no-defaults

    Do not read any option files. If program startup fails due to reading unknown options from an option file, --no-defaults can be used to prevent them from being read.

  • --old-alter-table

    Command-Line Format--old-alter-table
    System VariableNameold_alter_table
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    When this option is given, the server does not use the optimized method of processing an ALTER TABLE operation. It reverts to using a temporary table, copying over the data, and then renaming the temporary table to the original, as used by MySQL 5.0 and earlier. For more information on the operation of ALTER TABLE, see Section 13.1.7, “ALTER TABLE Syntax”.

  • --old-style-user-limits

    Command-Line Format--old-style-user-limits
    Permitted ValuesTypeboolean
    DefaultFALSE

    Enable old-style user limits. (Before MySQL 5.0.3, account resource limits were counted separately for each host from which a user connected rather than per account row in the user table.) See Section 6.3.4, “Setting Account Resource Limits”.

  • --one-thread

    Command-Line Format--one-thread

    Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See Section 24.5, “Debugging and Porting MySQL”.

    This option is deprecated and is removed in MySQL 5.6. Use --thread_handling=no-threads instead.

  • --open-files-limit=count

    Command-Line Format--open-files-limit=#
    System VariableNameopen_files_limit
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Valueplatform dependent

    Changes the number of file descriptors available to mysqld. You should try increasing the value of this option if mysqld gives you the error Too many open files. mysqld uses the option value to reserve descriptors with setrlimit(). Internally, the maximum value for this option is the maximum unsigned integer value, but the actual maximum is platform dependent. If the requested number of file descriptors cannot be allocated, mysqld writes a warning to the error log.

    mysqld may attempt to allocate more than the requested number of descriptors (if they are available), using the values of max_connections and table_open_cache to estimate whether more descriptors will be needed.

    On Unix, the value cannot be set less than ulimit -n.

  • --partition[=value]

    Command-Line Format--partition
    Disabled byskip-partition
    Permitted ValuesTypeboolean
    DefaultON

    Enables or disables user-defined partitioning support in the MySQL Server.

  • --pid-file=file_name

    Command-Line Format--pid-file=file_name
    System VariableNamepid_file
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The path name of the process ID file. The server creates the file in the data directory unless an absolute path name is given to specify a different directory. This file is used by other programs such as mysqld_safe to determine the server's process ID.

  • --plugin-xxx

    Specifies an option that pertains to a server plugin. For example, many storage engines can be built as plugins, and for such engines, options for them can be specified with a --plugin prefix. Thus, the --innodb_file_per_table option for InnoDB can be specified as --plugin-innodb_file_per_table.

    For boolean options that can be enabled or disabled, the --skip prefix and other alternative formats are supported as well (see Section 4.2.5, “Program Option Modifiers”). For example, --skip-plugin-innodb_file_per_table disables innodb_file_per_table.

    The rationale for the --plugin prefix is that it enables plugin options to be specified unambiguously if there is a name conflict with a built-in server option. For example, were a plugin writer to name a plugin sql and implement a mode option, the option name might be --sql-mode, which would conflict with the built-in option of the same name. In such cases, references to the conflicting name are resolved in favor of the built-in option. To avoid the ambiguity, users can specify the plugin option as --plugin-sql-mode. Use of the --plugin prefix for plugin options is recommended to avoid any question of ambiguity.

  • --plugin-load=plugin_list

    Command-Line Format--plugin-load=plugin_list
    Permitted ValuesTypestring

    This option tells the server to load the named plugins at startup. All plugins to load must be named in the same --plugin-load option. If multiple --plugin-load options are given, only the last one is used.

    The option value is a semicolon-separated list of name=plugin_library and plugin_library values. Each name is the name of a plugin to load, and plugin_library is the name of the library file that contains the plugin code. If a plugin library is named without any preceding plugin name, the server loads all plugins in the library. The server looks for plugin library files in the directory named by the plugin_dir system variable.

    For example, if plugins named myplug1 and myplug2 have library files myplug1.so and myplug2.so, use this option to perform an early plugin load:

    shell> mysqld --plugin-load="myplug1=myplug1.so;myplug2=myplug2.so"
    

    Quotes are used around the argument value here because otherwise semicolon (;) is interpreted as a special character by some command interpreters. (Unix shells treat it as a command terminator, for example.)

    Each named plugin is loaded for a single invocation of mysqld only. After a restart, the plugin is not loaded unless --plugin-load is used again. This is in contrast to INSTALL PLUGIN, which adds an entry to the mysql.plugins table to cause the plugin to be loaded for every normal server startup.

    Under normal startup, the server determines which plugins to load by reading the mysql.plugins system table. If the server is started with the --skip-grant-tables option, it does not consult the mysql.plugins table and does not load plugins listed there. --plugin-load enables plugins to be loaded even when --skip-grant-tables is given. --plugin-load also enables plugins to be loaded at startup under configurations when plugins cannot be loaded at runtime.

    For additional information about plugin loading, see Section 5.5.2, “Installing and Uninstalling Plugins”.

  • --port=port_num, -P port_num

    Command-Line Format--port=#
    System VariableNameport
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default3306
    Min Value0
    Max Value65535

    The port number to use when listening for TCP/IP connections. On Unix and Unix-like systems, the port number must be 1024 or higher unless the server is started by the root system user.

  • --port-open-timeout=num

    Command-Line Format--port-open-timeout=#
    Permitted ValuesTypeinteger
    Default0

    On some systems, when the server is stopped, the TCP/IP port might not become available immediately. If the server is restarted quickly afterward, its attempt to reopen the port can fail. This option indicates how many seconds the server should wait for the TCP/IP port to become free if it cannot be opened. The default is not to wait.

  • --print-defaults

    Print the program name and all options that it gets from option files.

  • --remove [service_name]

    Command-Line Format--remove [service_name]
    Platform SpecificWindows

    (Windows only) Remove a MySQL Windows service. The default service name is MySQL if no service_name value is given. For more information, see Section 2.3.7.7, “Starting MySQL as a Windows Service”.

  • --safe-mode

    Deprecated5.5.26
    Command-Line Format--safe-mode

    Skip some optimization stages. This option is deprecated and is removed in MySQL 5.6.

  • --safe-show-database

    Deprecated4.0.2
    Removed5.5.3
    Command-Line Format--safe-show-database
    Permitted ValuesTypeboolean

    This option is deprecated and does not do anything because there is a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See Section 6.2.1, “Privileges Provided by MySQL”. --safe-show-database was removed in MySQL 5.5.3.

  • --safe-user-create

    Command-Line Format--safe-user-create
    Permitted ValuesTypeboolean
    DefaultFALSE

    If this option is enabled, a user cannot create new MySQL users by using the GRANT statement unless the user has the INSERT privilege for the mysql.user table or any column in the table. If you want a user to have the ability to create new users that have those privileges that the user has the right to grant, you should grant the user the following privilege:

    GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
    

    This ensures that the user cannot change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.

  • --secure-auth

    Command-Line Format--secure-auth
    System VariableNamesecure_auth
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    This option causes the server to block connections by clients that attempt to use accounts that have passwords stored in the old (pre-4.1) format. Use it to prevent all use of passwords employing the old format (and hence insecure communication over the network).

    Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format. See Section B.5.2.4, “Client does not support authentication protocol”.

    The mysql client also has a --secure-auth option, which prevents connections to a server if the server requires a password in old format for the client account.

    Note

    Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.

  • --secure-file-priv=dir_name

    Command-Line Format--secure-file-priv=dir_name
    System VariableNamesecure_file_priv
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring
    Defaultempty
    Valid Valuesempty
    dirname

    This option limits the effect of the LOAD DATA and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function to work only with files in the specified directory.

  • --shared-memory

    Command-Line Format--shared_memory[={0,1}]
    System VariableNameshared_memory
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificWindows
    Permitted ValuesTypeboolean
    DefaultFALSE

    Enable shared-memory connections by local clients. This option is available only on Windows.

  • --shared-memory-base-name=name

    Command-Line Format--shared_memory_base_name=name
    System VariableNameshared_memory_base_name
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificWindows
    Permitted ValuesTypestring
    DefaultMYSQL

    The name of shared memory to use for shared-memory connections. This option is available only on Windows. The default name is MYSQL. The name is case sensitive.

  • --skip-concurrent-insert

    Turn off the ability to select and insert at the same time on MyISAM tables. (This is to be used only if you think you have found a bug in this feature.) See Section 8.11.3, “Concurrent Inserts”.

  • --skip-event-scheduler

    Command-Line Format--skip-event-scheduler
     --disable-event-scheduler

    Turns the Event Scheduler OFF. This is not the same as disabling the Event Scheduler, which requires setting --event-scheduler=DISABLED; see The --event-scheduler Option, for more information.

  • --skip-grant-tables

    This option causes the server to start without using the privilege system at all, which gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL FLUSH PRIVILEGES statement after connecting to the server.

    This option also causes the server to suppress during its startup sequence the loading of user-defined functions (UDFs), scheduled events, and plugins that were installed with the INSTALL PLUGIN statement. To cause plugins to be loaded anyway, use the --plugin-load option.

    FLUSH PRIVILEGES might be executed implicitly by other actions performed after startup. For example, mysql_upgrade flushes the privileges during the upgrade procedure.

  • --skip-host-cache

    Disable use of the internal host cache for faster name-to-IP resolution. In this case, the server performs a DNS lookup every time a client connects. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.

  • --skip-innodb

    Disable the InnoDB storage engine. In this case, if the default storage engine is InnoDB, the server will not start unless you also use --default-storage-engine to set the default to some other engine.

  • --skip-name-resolve

    Do not resolve host names when checking client connections. Use only IP addresses. If you use this option, all Host column values in the grant tables must be IP addresses or localhost. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.

    Depending on the network configuration of your system and the Host values for your accounts, clients may need to connect using an explicit --host option, such as --host=localhost, --host=127.0.0.1, or --host=::1.

    An attempt to connect to the host 127.0.0.1 normally resolves to the localhost account. However, this fails if the server is run with the --skip-name-resolve option, so make sure that an account exists that can accept a connection. For example, to be able to connect as root using --host=127.0.0.1 or --host=::1, create these accounts:

    CREATE USER 'root'@'127.0.0.1' IDENTIFIED BY 'root-password';
    CREATE USER 'root'@'::1' IDENTIFIED BY 'root-password';
    
  • --skip-networking

    Do not listen for TCP/IP connections at all. All interaction with mysqld must be made using named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are permitted. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.

  • --skip-partition

    Command-Line Format--skip-partition
     --disable-partition

    Disables user-defined partitioning. Partitioned tables can be seen using SHOW TABLES or by querying the INFORMATION_SCHEMA.TABLES table, but cannot be created or modified, nor can data in such tables be accessed. All partition-specific columns in the INFORMATION_SCHEMA.PARTITIONS table display NULL.

    Since DROP TABLE removes table definition (.frm) files, this statement works on partitioned tables even when partitioning is disabled using the option. The statement, however, does not remove .par files associated with partitioned tables in such cases. For this reason, you should avoid dropping partitioned tables with partitioning disabled, or take action to remove the orphaned .par files manually.

  • --ssl*

    Options that begin with --ssl specify whether to permit clients to connect using SSL and indicate where to find SSL keys and certificates. See Section 6.4.5, “Command Options for Secure Connections”.

  • --standalone

    Command-Line Format--standalone
    Platform SpecificWindows

    Available on Windows only; instructs the MySQL server not to run as a service.

  • --super-large-pages

    Introduced5.5.3
    Command-Line Format--super-large-pages
    Platform SpecificSolaris
    Permitted Values (Solaris)Typeboolean
    DefaultFALSE

    Standard use of large pages in MySQL attempts to use the largest size supported, up to 4MB. Under Solaris, a super large pages feature enables uses of pages up to 256MB. This feature is available for recent SPARC platforms. It can be enabled or disabled by using the --super-large-pages or --skip-super-large-pages option.

  • --symbolic-links, --skip-symbolic-links

    Command-Line Format--symbolic-links

    Enable or disable symbolic link support. This option has different effects on Windows and Unix:

  • --skip-safemalloc

    Removed5.5.6
    Command-Line Format--skip-safemalloc

    Previously, if MySQL was configured with full debugging support, all MySQL programs check for memory overruns during each memory allocation and memory freeing operation. This checking is very slow, so for the server you can avoid it when you do not need it by using the --skip-safemalloc option.

    safemalloc, along with this option, was removed in MySQL 5.5.6.

  • --skip-show-database

    Command-Line Format--skip-show-database
    System VariableNameskip_show_database
    Variable ScopeGlobal
    Dynamic VariableNo

    This option sets the skip_show_database system variable that controls who is permitted to use the SHOW DATABASES statement. See Section 5.1.4, “Server System Variables”.

  • --skip-stack-trace

    Command-Line Format--skip-stack-trace

    Do not write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See Section 24.5, “Debugging and Porting MySQL”.

  • --skip-thread-priority

    Deprecated5.1.29
    Command-Line Format--skip-thread-priority

    Disable using thread priorities for faster response time. This option is deprecated and is removed in MySQL 5.6.

  • --slow-query-log[={0|1}]

    Command-Line Format--slow-query-log
    System VariableNameslow_query_log
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Specify the initial slow query log state. With no argument or an argument of 1, the --slow-query-log option enables the log. If omitted or given with an argument of 0, the option disables the log.

  • --slow-start-timeout=timeout

    Introduced5.5.20
    Command-Line Format--slow-start-timeout=#
    Permitted Values (Windows)Typeinteger
    Default15000

    This option controls the Windows service control manager's service start timeout. The value is the maximum number of milliseconds that the service control manager waits before trying to kill the windows service during startup. The default value is 15000 (15 seconds). If the MySQL service takes too long to start, you may need to increase this value. A value of 0 means there is no timeout.

  • --socket=path

    Command-Line Format--socket={file_name|pipe_name}
    System VariableNamesocket
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring
    Default/tmp/mysql.sock

    On Unix, this option specifies the Unix socket file to use when listening for local connections. The default value is /tmp/mysql.sock. If this option is given, the server creates the file in the data directory unless an absolute path name is given to specify a different directory. On Windows, the option specifies the pipe name to use when listening for local connections that use a named pipe. The default value is MySQL (not case sensitive).

  • --sql-mode=value[,value[,value...]]

    Command-Line Format--sql-mode=name
    System VariableNamesql_mode
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeset
    Default''
    Valid ValuesALLOW_INVALID_DATES
    ANSI_QUOTES
    ERROR_FOR_DIVISION_BY_ZERO
    HIGH_NOT_PRECEDENCE
    IGNORE_SPACE
    NO_AUTO_CREATE_USER
    NO_AUTO_VALUE_ON_ZERO
    NO_BACKSLASH_ESCAPES
    NO_DIR_IN_CREATE
    NO_ENGINE_SUBSTITUTION
    NO_FIELD_OPTIONS
    NO_KEY_OPTIONS
    NO_TABLE_OPTIONS
    NO_UNSIGNED_SUBTRACTION
    NO_ZERO_DATE
    NO_ZERO_IN_DATE
    ONLY_FULL_GROUP_BY
    PAD_CHAR_TO_FULL_LENGTH
    PIPES_AS_CONCAT
    REAL_AS_FLOAT
    STRICT_ALL_TABLES
    STRICT_TRANS_TABLES

    Set the SQL mode. See Section 5.1.7, “Server SQL Modes”.

    Note

    MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.

  • --sysdate-is-now

    Command-Line Format--sysdate-is-now
    Permitted ValuesTypeboolean
    DefaultFALSE

    SYSDATE() by default returns the time at which it executes, not the time at which the statement in which it occurs begins executing. This differs from the behavior of NOW(). This option causes SYSDATE() to be an alias for NOW(). For information about the implications for binary logging and replication, see the description for SYSDATE() in Section 12.7, “Date and Time Functions” and for SET TIMESTAMP in Section 5.1.4, “Server System Variables”.

  • --tc-heuristic-recover={COMMIT|ROLLBACK}

    Command-Line Format--tc-heuristic-recover=name
    Permitted ValuesTypeenumeration
    DefaultCOMMIT
    Valid ValuesCOMMIT
    ROLLBACK

    The type of decision to use in the heuristic recovery process. This option is unused.

  • --temp-pool

    Command-Line Format--temp-pool
    Permitted ValuesTypeboolean
    DefaultTRUE

    This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to leak memory, because it is being allocated to the directory entry cache rather than to the disk cache. This option is ignored except on Linux.

  • --transaction-isolation=level

    Command-Line Format--transaction-isolation=name
    Permitted ValuesTypeenumeration
    DefaultREPEATABLE-READ
    Valid ValuesREAD-UNCOMMITTED
    READ-COMMITTED
    REPEATABLE-READ
    SERIALIZABLE

    Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 13.3.6, “SET TRANSACTION Syntax”.

    The default transaction isolation level can also be set at runtime using the SET TRANSACTION statement or by setting the tx_isolation system variable.

  • --tmpdir=dir_name, -t dir_name

    Command-Line Format--tmpdir=dir_name
    System VariableNametmpdir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. This option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (:) on Unix and semicolon characters (;) on Windows. If the MySQL server is acting as a replication slave, you should not set --tmpdir to point to a directory on a memory-based file system or to a directory that is cleared when the server host restarts. For more information about the storage location of temporary files, see Section B.5.3.5, “Where MySQL Stores Temporary Files”. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails.

  • --user={user_name|user_id}, -u {user_name|user_id}

    Command-Line Format--user=name
    Permitted ValuesTypestring

    Run the mysqld server as the user having the name user_name or the numeric user ID user_id. (User in this context refers to a system login account, not a MySQL user listed in the grant tables.)

    This option is mandatory when starting mysqld as root. The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as root. See Section 6.1.1, “Security Guidelines”.

    To avoid a possible security hole where a user adds a --user=root option to a my.cnf file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in /etc/my.cnf and $MYSQL_HOME/my.cnf are processed before command-line options, so it is recommended that you put a --user option in /etc/my.cnf and specify a value other than root. The option in /etc/my.cnf is found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.

  • --verbose, -v

    Use this option with the --help option for detailed help.

  • --version, -V

    Display version information and exit.

5.1.4 Server System Variables

The MySQL server maintains many system variables that indicate how it is configured. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. Setting the global value of a system variable requires the SUPER privilege. For some system variables, setting the session value also requires the SUPER privilege; if so, it is indicated in the variable description. You can also use system variable values in expressions.

There are several ways to see the names and values of system variables:

  • To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command:

    mysqld --verbose --help
    
  • To see the values that a server will use based on its compiled-in defaults, ignoring the settings in any option files, use this command:

    mysqld --no-defaults --verbose --help
    
  • To see the current values used by a running server, use the SHOW VARIABLES statement.

This section provides a description of each system variable. Variables with no version indicated are present in all MySQL 5.5 releases. For more information about manipulation of system variables, see Section 5.1.5, “Using System Variables”.

The following table lists all available system variables.

Table 5.2 System Variable Summary

NameCmd-LineOption FileSystem VarVar ScopeDynamic
audit_log_buffer_sizeYesYesYesGlobalNo
audit_log_fileYesYesYesGlobalNo
audit_log_flush  YesGlobalYes
audit_log_formatYesYesYesGlobalNo
audit_log_policyYesYesYesGlobalYes
audit_log_rotate_on_sizeYesYesYesGlobalYes
audit_log_strategyYesYesYesGlobalNo
auto_increment_increment  YesBothYes
auto_increment_offset  YesBothYes
autocommitYesYesYesVariesYes
automatic_sp_privileges  YesGlobalYes
back_log  YesGlobalNo
basedirYesYesYesGlobalNo
big-tablesYesYes  Yes
- Variable: big_tables  YesVariesYes
binlog_cache_sizeYesYesYesGlobalYes
binlog_direct_non_transactional_updatesYesYesYesBothYes
binlog-formatYesYes  Yes
- Variable: binlog_format  YesBothYes
binlog_stmt_cache_sizeYesYesYesGlobalYes
bulk_insert_buffer_sizeYesYesYesBothYes
character_set_client  YesBothYes
character_set_connection  YesBothYes
character_set_database[a]  YesBothYes
character-set-filesystemYesYes  Yes
- Variable: character_set_filesystem  YesBothYes
character_set_results  YesBothYes
character-set-serverYesYes  Yes
- Variable: character_set_server  YesBothYes
character_set_system  YesGlobalNo
character-sets-dirYesYes  No
- Variable: character_sets_dir  YesGlobalNo
collation_connection  YesBothYes
collation_database[b]  YesBothYes
collation-serverYesYes  Yes
- Variable: collation_server  YesBothYes
completion_typeYesYesYesBothYes
concurrent_insertYesYesYesGlobalYes
connect_timeoutYesYesYesGlobalYes
datadirYesYesYesGlobalNo
date_format  YesVariesNo
datetime_format  YesVariesNo
debugYesYesYesBothYes
debug_sync  YesVariesYes
default-storage-engineYesYes  Yes
- Variable: default_storage_engine  YesBothYes
default_week_formatYesYesYesBothYes
delay-key-writeYesYes  Yes
- Variable: delay_key_write  YesGlobalYes
delayed_insert_limitYesYesYesGlobalYes
delayed_insert_timeoutYesYesYesGlobalYes
delayed_queue_sizeYesYesYesGlobalYes
div_precision_incrementYesYesYesBothYes
engine-condition-pushdownYesYes  Yes
- Variable: engine_condition_pushdown  YesBothYes
error_count  YesSessionNo
event-schedulerYesYes  Yes
- Variable: event_scheduler  YesGlobalYes
expire_logs_daysYesYesYesGlobalYes
external_user  YesSessionNo
flushYesYesYesGlobalYes
flush_timeYesYesYesGlobalYes
foreign_key_checks  YesVariesYes
ft_boolean_syntaxYesYesYesGlobalYes
ft_max_word_lenYesYesYesGlobalNo
ft_min_word_lenYesYesYesGlobalNo
ft_query_expansion_limitYesYesYesGlobalNo
ft_stopword_fileYesYesYesGlobalNo
general-logYesYes  Yes
- Variable: general_log  YesGlobalYes
general_log_fileYesYesYesGlobalYes
group_concat_max_lenYesYesYesBothYes
have_compress  YesGlobalNo
have_crypt  YesGlobalNo
have_csv  YesGlobalNo
have_dynamic_loading  YesGlobalNo
have_geometry  YesGlobalNo
have_innodb  YesGlobalNo
have_ndbcluster  YesGlobalNo
have_openssl  YesGlobalNo
have_partitioning  YesGlobalNo
have_profiling  YesGlobalNo
have_query_cache  YesGlobalNo
have_rtree_keys  YesGlobalNo
have_ssl  YesGlobalNo
have_symlink  YesGlobalNo
hostname  YesGlobalNo
identity  YesSessionYes
ignore-builtin-innodbYesYes  No
- Variable: ignore_builtin_innodb  YesGlobalNo
init_connectYesYesYesGlobalYes
init-fileYesYes  No
- Variable: init_file  YesGlobalNo
init_slaveYesYesYesGlobalYes
innodb_adaptive_flushingYesYesYesGlobalYes
innodb_adaptive_hash_indexYesYesYesGlobalYes
innodb_additional_mem_pool_sizeYesYesYesGlobalNo
innodb_autoextend_incrementYesYesYesGlobalYes
innodb_autoinc_lock_modeYesYesYesGlobalNo
innodb_buffer_pool_instancesYesYesYesGlobalNo
innodb_buffer_pool_sizeYesYesYesGlobalNo
innodb_change_bufferingYesYesYesGlobalYes
innodb_change_buffering_debugYesYesYesGlobalYes
innodb_checksumsYesYesYesGlobalNo
innodb_commit_concurrencyYesYesYesGlobalYes
innodb_concurrency_ticketsYesYesYesGlobalYes
innodb_data_file_pathYesYesYesGlobalNo
innodb_data_home_dirYesYesYesGlobalNo
innodb_doublewriteYesYesYesGlobalNo
innodb_fast_shutdownYesYesYesGlobalYes
innodb_file_formatYesYesYesGlobalYes
innodb_file_format_checkYesYesYesGlobalVaries
innodb_file_format_maxYesYesYesGlobalYes
innodb_file_per_tableYesYesYesGlobalYes
innodb_flush_log_at_trx_commitYesYesYesGlobalYes
innodb_flush_methodYesYesYesGlobalNo
innodb_force_load_corruptedYesYesYesGlobalNo
innodb_force_recoveryYesYesYesGlobalNo
innodb_io_capacityYesYesYesGlobalYes
innodb_large_prefixYesYesYesGlobalYes
innodb_limit_optimistic_insert_debugYesYesYesGlobalYes
innodb_lock_wait_timeoutYesYesYesBothYes
innodb_locks_unsafe_for_binlogYesYesYesGlobalNo
innodb_log_buffer_sizeYesYesYesGlobalNo
innodb_log_file_sizeYesYesYesGlobalNo
innodb_log_files_in_groupYesYesYesGlobalNo
innodb_log_group_home_dirYesYesYesGlobalNo
innodb_max_dirty_pages_pctYesYesYesGlobalYes
innodb_max_purge_lagYesYesYesGlobalYes
innodb_mirrored_log_groupsYesYesYesGlobalNo
innodb_old_blocks_pctYesYesYesGlobalYes
innodb_old_blocks_timeYesYesYesGlobalYes
innodb_open_filesYesYesYesGlobalNo
innodb_print_all_deadlocksYesYesYesGlobalYes
innodb_purge_batch_sizeYesYesYesGlobalYes
innodb_purge_threadsYesYesYesGlobalNo
innodb_random_read_aheadYesYesYesGlobalYes
innodb_read_ahead_thresholdYesYesYesGlobalYes
innodb_read_io_threadsYesYesYesGlobalNo
innodb_replication_delayYesYesYesGlobalYes
innodb_rollback_on_timeoutYesYesYesGlobalNo
innodb_rollback_segmentsYesYesYesGlobalYes
innodb_spin_wait_delayYesYesYesGlobalYes
innodb_stats_methodYesYesYesGlobalYes
innodb_stats_on_metadataYesYesYesGlobalYes
innodb_stats_sample_pagesYesYesYesGlobalYes
innodb_strict_modeYesYesYesBothYes
innodb_support_xaYesYesYesBothYes
innodb_sync_spin_loopsYesYesYesGlobalYes
innodb_table_locksYesYesYesBothYes
innodb_thread_concurrencyYesYesYesGlobalYes
innodb_thread_sleep_delayYesYesYesGlobalYes
innodb_trx_purge_view_update_only_debugYesYesYesGlobalYes
innodb_trx_rseg_n_slots_debugYesYesYesGlobalYes
innodb_use_native_aioYesYesYesGlobalNo
innodb_use_sys_mallocYesYesYesGlobalNo
innodb_version  YesGlobalNo
innodb_write_io_threadsYesYesYesGlobalNo
insert_id  YesSessionYes
interactive_timeoutYesYesYesBothYes
join_buffer_sizeYesYesYesBothYes
keep_files_on_createYesYesYesBothYes
key_buffer_sizeYesYesYesGlobalYes
key_cache_age_thresholdYesYesYesGlobalYes
key_cache_block_sizeYesYesYesGlobalYes
key_cache_division_limitYesYesYesGlobalYes
languageYesYesYesGlobalNo
large_files_support  YesGlobalNo
large_page_size  YesGlobalNo
large-pagesYesYes  No
- Variable: large_pages  YesGlobalNo
last_insert_id  YesSessionYes
lc-messagesYesYes  Yes
- Variable: lc_messages  YesBothYes
lc-messages-dirYesYes  No
- Variable: lc_messages_dir  YesGlobalNo
lc_time_names  YesBothYes
license  YesGlobalNo
local_infile  YesGlobalYes
lock_wait_timeoutYesYesYesBothYes
locked_in_memory  YesGlobalNo
logYesYesYesGlobalYes
log-binYesYesYesGlobalNo
log_bin  YesGlobalNo
log-bin-trust-function-creatorsYesYes  Yes
- Variable: log_bin_trust_function_creators  YesGlobalYes
log-bin-trust-routine-creatorsYesYes  Yes
- Variable: log_bin_trust_routine_creators  YesGlobalYes
log-bin-use-v1-row-eventsYesYes  No
- Variable: log_bin_use_v1_row_events  YesGlobalNo
log_bin_use_v1_row_eventsYesYesYesGlobalNo
log-errorYesYes  No
- Variable: log_error  YesGlobalNo
log-outputYesYes  Yes
- Variable: log_output  YesGlobalYes
log-queries-not-using-indexesYesYes  Yes
- Variable: log_queries_not_using_indexes  YesGlobalYes
log-slave-updatesYesYes  No
- Variable: log_slave_updates  YesGlobalNo
log_slave_updatesYesYesYesGlobalNo
log-slow-queriesYesYes  Yes
- Variable: log_slow_queries  YesGlobalYes
log-warningsYesYes  Yes
- Variable: log_warnings  YesBothYes
long_query_timeYesYesYesBothYes
low-priority-updatesYesYes  Yes
- Variable: low_priority_updates  YesBothYes
lower_case_file_system  YesGlobalNo
lower_case_table_namesYesYesYesGlobalNo
max_allowed_packetYesYesYesBothYes
max_binlog_cache_sizeYesYesYesGlobalYes
max_binlog_sizeYesYesYesGlobalYes
max_binlog_stmt_cache_sizeYesYesYesGlobalYes
max_connect_errorsYesYesYesGlobalYes
max_connectionsYesYesYesGlobalYes
max_delayed_threadsYesYesYesBothYes
max_error_countYesYesYesBothYes
max_heap_table_sizeYesYesYesBothYes
max_insert_delayed_threads  YesBothYes
max_join_sizeYesYesYesBothYes
max_length_for_sort_dataYesYesYesBothYes
max_long_data_sizeYesYesYesGlobalNo
max_prepared_stmt_countYesYesYesGlobalYes
max_relay_log_sizeYesYesYesGlobalYes
max_seeks_for_keyYesYesYesBothYes
max_sort_lengthYesYesYesBothYes
max_sp_recursion_depthYesYesYesBothYes
max_tmp_tables  YesBothYes
max_user_connectionsYesYesYesBothYes
max_write_lock_countYesYesYesGlobalYes
metadata_locks_cache_size  YesGlobalNo
min-examined-row-limitYesYesYesBothYes
multi_range_countYesYesYesBothYes
myisam_data_pointer_sizeYesYesYesGlobalYes
myisam_max_sort_file_sizeYesYesYesGlobalYes
myisam_mmap_sizeYesYesYesGlobalNo
myisam_recover_options  YesGlobalNo
myisam_repair_threadsYesYesYesBothYes
myisam_sort_buffer_sizeYesYesYesBothYes
myisam_stats_methodYesYesYesBothYes
myisam_use_mmapYesYesYesGlobalYes
named_pipe  YesGlobalNo
ndb_autoincrement_prefetch_szYesYesYesBothYes
ndb-batch-sizeYesYesYesGlobalNo
ndb-blob-read-batch-bytesYesYesYesBothYes
ndb-blob-write-batch-bytesYesYesYesBothYes
ndb_cache_check_timeYesYesYesGlobalYes
ndb-cluster-connection-poolYesYesYesGlobalNo
ndb-deferred-constraintsYesYes  Yes
- Variable: ndb_deferred_constraints  YesBothYes
ndb_deferred_constraintsYesYesYesBothYes
ndb-distributionYesYes  Yes
- Variable: ndb_distribution  YesGlobalYes
ndb_distributionYesYesYesGlobalYes
ndb_eventbuffer_max_allocYesYesYesGlobalYes
ndb_extra_loggingYesYesYesGlobalYes
ndb_force_sendYesYesYesBothYes
ndb_index_stat_cache_entriesYesYesYesBothYes
ndb_index_stat_enableYesYesYesBothYes
ndb_index_stat_optionYesYesYesBothYes
ndb_index_stat_update_freqYesYesYesBothYes
ndb_join_pushdown  YesBothYes
ndb-log-apply-statusYesYes  No
- Variable: ndb_log_apply_status  YesGlobalNo
ndb_log_apply_statusYesYesYesGlobalNo
ndb_log_binYes YesBothYes
ndb_log_binlog_indexYes YesGlobalYes
ndb-log-empty-epochsYesYesYesGlobalYes
ndb_log_empty_epochsYesYesYesGlobalYes
ndb-log-origYesYes  No
- Variable: ndb_log_orig  YesGlobalNo
ndb_log_origYesYesYesGlobalNo
ndb-log-transaction-idYesYes  No
- Variable: ndb_log_transaction_id  YesGlobalNo
ndb_log_transaction_id  YesGlobalNo
ndb-log-update-as-writeYesYesYesGlobalYes
ndb_log_updated_onlyYesYesYesGlobalYes
ndb_optimization_delay  YesGlobalYes
ndb_optimized_node_selectionYesYesYesGlobalNo
ndb_report_thresh_binlog_epoch_slipYesYesYesGlobalYes
ndb_report_thresh_binlog_mem_usageYesYesYesGlobalYes
ndb_table_no_logging  YesSessionYes
ndb_table_temporary  YesSessionYes
ndb_use_copying_alter_table  YesBothNo
ndb_use_exact_count  YesBothYes
ndb_use_transactionsYesYesYesBothYes
ndb_version  YesGlobalNo
ndb_version_string  YesGlobalNo
ndb-wait-connectedYesYesYesGlobalNo
ndb-wait-setupYesYesYesGlobalNo
ndbinfo_database  YesGlobalNo
ndbinfo_max_bytesYes YesBothYes
ndbinfo_max_rowsYes YesBothYes
ndbinfo_offline  YesGlobalYes
ndbinfo_show_hiddenYes YesBothYes
ndbinfo_table_prefixYes YesBothYes
ndbinfo_version  YesGlobalNo
net_buffer_lengthYesYesYesBothYes
net_read_timeoutYesYesYesBothYes
net_retry_countYesYesYesBothYes
net_write_timeoutYesYesYesBothYes
newYesYesYesBothYes
oldYesYesYesGlobalNo
old-alter-tableYesYes  Yes
- Variable: old_alter_table  YesBothYes
old_passwords  YesBothYes
open-files-limitYesYes  No
- Variable: open_files_limit  YesGlobalNo
optimizer_prune_levelYesYesYesBothYes
optimizer_search_depthYesYesYesBothYes
optimizer_switchYesYesYesBothYes
performance_schemaYesYesYesGlobalNo
performance_schema_events_waits_history_long_sizeYesYesYesGlobalNo
performance_schema_events_waits_history_sizeYesYesYesGlobalNo
performance_schema_max_cond_classesYesYesYesGlobalNo
performance_schema_max_cond_instancesYesYesYesGlobalNo
performance_schema_max_file_classesYesYesYesGlobalNo
performance_schema_max_file_handlesYesYesYesGlobalNo
performance_schema_max_file_instancesYesYesYesGlobalNo
performance_schema_max_mutex_classesYesYesYesGlobalNo
performance_schema_max_mutex_instancesYesYesYesGlobalNo
performance_schema_max_rwlock_classesYesYesYesGlobalNo
performance_schema_max_rwlock_instancesYesYesYesGlobalNo
performance_schema_max_table_handlesYesYesYesGlobalNo
performance_schema_max_table_instancesYesYesYesGlobalNo
performance_schema_max_thread_classesYesYesYesGlobalNo
performance_schema_max_thread_instancesYesYesYesGlobalNo
pid-fileYesYes  No
- Variable: pid_file  YesGlobalNo
plugin_dirYesYesYesGlobalNo
portYesYesYesGlobalNo
preload_buffer_sizeYesYesYesBothYes
profiling  YesVariesYes
profiling_history_sizeYesYesYesBothYes
protocol_version  YesGlobalNo
proxy_user  YesSessionNo
pseudo_slave_mode  YesSessionYes
pseudo_thread_id  YesSessionYes
query_alloc_block_sizeYesYesYesBothYes
query_cache_limitYesYesYesGlobalYes
query_cache_min_res_unitYesYesYesGlobalYes
query_cache_sizeYesYesYesGlobalYes
query_cache_typeYesYesYesBothYes
query_cache_wlock_invalidateYesYesYesBothYes
query_prealloc_sizeYesYesYesBothYes
rand_seed1  YesSessionYes
rand_seed2  YesSessionYes
range_alloc_block_sizeYesYesYesBothYes
read_buffer_sizeYesYesYesBothYes
read_onlyYesYesYesGlobalYes
read_rnd_buffer_sizeYesYesYesBothYes
relay-logYesYes  No
- Variable: relay_log  YesGlobalNo
relay-log-indexYesYes  No
- Variable: relay_log_index  YesGlobalNo
relay_log_indexYesYesYesGlobalNo
relay_log_info_fileYesYesYesGlobalNo
relay_log_purgeYesYesYesGlobalYes
relay_log_recoveryYesYesYesGlobalYes
relay_log_space_limitYesYesYesGlobalNo
report-hostYesYes  No
- Variable: report_host  YesGlobalNo
report-passwordYesYes  No
- Variable: report_password  YesGlobalNo
report-portYesYes  No
- Variable: report_port  YesGlobalNo
report-userYesYes  No
- Variable: report_user  YesGlobalNo
rpl_recovery_rank  YesGlobalYes
rpl_semi_sync_master_enabled  YesGlobalYes
rpl_semi_sync_master_timeout  YesGlobalYes
rpl_semi_sync_master_trace_level  YesGlobalYes
rpl_semi_sync_master_wait_no_slave  YesGlobalYes
rpl_semi_sync_slave_enabled  YesGlobalYes
rpl_semi_sync_slave_trace_level  YesGlobalYes
secure-authYesYes  Yes
- Variable: secure_auth  YesGlobalYes
secure-file-privYesYes  No
- Variable: secure_file_priv  YesGlobalNo
server-idYesYes  Yes
- Variable: server_id  YesGlobalYes
server-id-bitsYesYes  No
- Variable: server_id_bits  YesGlobalNo
server_id_bitsYesYesYesGlobalNo
shared_memoryYesYesYesGlobalNo
shared_memory_base_nameYesYesYesGlobalNo
skip_external_lockingYesYesYesGlobalNo
skip-name-resolveYesYes  No
- Variable: skip_name_resolve  YesGlobalNo
skip-networkingYesYes  No
- Variable: skip_networking  YesGlobalNo
skip-show-databaseYesYes  No
- Variable: skip_show_database  YesGlobalNo
slave_allow_batchingYesYesYesGlobalYes
slave_compressed_protocolYesYesYesGlobalYes
slave_exec_modeYesYesYesGlobalYes
slave-load-tmpdirYesYes  No
- Variable: slave_load_tmpdir  YesGlobalNo
slave_max_allowed_packet  YesGlobalYes
slave-net-timeoutYesYes  Yes
- Variable: slave_net_timeout  YesGlobalYes
slave-skip-errorsYesYes  No
- Variable: slave_skip_errors  YesGlobalNo
slave_transaction_retriesYesYesYesGlobalYes
slave_type_conversionsYesYesYesGlobalNo
slow_launch_timeYesYesYesGlobalYes
slow-query-logYesYes  Yes
- Variable: slow_query_log  YesGlobalYes
slow_query_log_fileYesYesYesGlobalYes
socketYesYesYesGlobalNo
sort_buffer_sizeYesYesYesBothYes
sql_auto_is_null  YesVariesYes
sql_big_selects  YesVariesYes
sql_big_tables  YesVariesYes
sql_buffer_result  YesVariesYes
sql_log_bin  YesVariesYes
sql_log_off  YesVariesYes
sql_log_update  YesSessionYes
sql_low_priority_updates  YesBothYes
sql_max_join_size  YesBothYes
sql-modeYesYes  Yes
- Variable: sql_mode  YesBothYes
sql_notes  YesVariesYes
sql_quote_show_create  YesVariesYes
sql_safe_updates  YesVariesYes
sql_select_limit  YesBothYes
sql_slave_skip_counter  YesGlobalYes
sql_warnings  YesVariesYes
ssl-caYesYes  No
- Variable: ssl_ca  YesGlobalNo
ssl-capathYesYes  No
- Variable: ssl_capath  YesGlobalNo
ssl-certYesYes  No
- Variable: ssl_cert  YesGlobalNo
ssl-cipherYesYes  No
- Variable: ssl_cipher  YesGlobalNo
ssl-keyYesYes  No
- Variable: ssl_key  YesGlobalNo
storage_engine  YesBothYes
stored_program_cacheYesYesYesGlobalYes
sync_binlogYesYesYesGlobalYes
sync_frmYesYesYesGlobalYes
sync_master_infoYesYesYesGlobalYes
sync_relay_logYesYesYesGlobalYes
sync_relay_log_infoYesYesYesGlobalYes
system_time_zone  YesGlobalNo
table_definition_cache  YesGlobalYes
table_lock_wait_timeoutYesYesYesGlobalYes
table_open_cache  YesGlobalYes
table_type  YesBothYes
thread_cache_sizeYesYesYesGlobalYes
thread_concurrencyYesYesYesGlobalNo
thread_handlingYesYesYesGlobalNo
thread_pool_algorithmYesYesYesGlobalNo
thread_pool_high_priority_connectionYesYesYesBothYes
thread_pool_max_unused_threadsYesYesYesGlobalYes
thread_pool_prio_kickup_timerYesYesYesBothYes
thread_pool_sizeYesYesYesGlobalNo
thread_pool_stall_limitYesYesYesGlobalYes
thread_stackYesYesYesGlobalNo
time_format  YesVariesNo
time_zone  YesBothYes
timed_mutexesYesYesYesGlobalYes
timestamp  YesSessionYes
tmp_table_sizeYesYesYesBothYes
tmpdirYesYesYesGlobalNo
transaction_alloc_block_sizeYesYesYesBothYes
transaction_allow_batching  YesSessionYes
transaction_prealloc_sizeYesYesYesBothYes
tx_isolation  YesBothYes
unique_checks  YesVariesYes
updatable_views_with_limitYesYesYesBothYes
version  YesGlobalNo
version_comment  YesGlobalNo
version_compile_machine  YesGlobalNo
version_compile_os  YesGlobalNo
wait_timeoutYesYesYesBothYes
warning_count  YesSessionNo

[a] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

[b] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.


For additional system variable information, see these sections:

Note

Some of the following variable descriptions refer to enabling or disabling a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, before MySQL 5.5.10, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not. As of MySQL 5.5.10, boolean variables can be set at startup to the values ON, TRUE, OFF, and FALSE (not case sensitive). See Section 4.2.5, “Program Option Modifiers”.

Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server will set the value to 1024.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Some system variables take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued variable is given as a relative path name, it will be located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.

  • authentication_windows_log_level

    Introduced5.5.16
    Command-Line Format--authentication_windows_log_level
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value4

    This variable is available only if the authentication_windows Windows authentication plugin is enabled and debugging code is enabled. See Section 6.5.1.4, “The Windows Native Authentication Plugin”.

    This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.

    ValueDescription
    0No logging
    1Log only error messages
    2Log level 1 messages and warning messages
    3Log level 2 messages and information notes
    4Log level 3 messages and debug messages

    This variable was added in MySQL 5.5.16.

  • authentication_windows_use_principal_name

    Introduced5.5.16
    Command-Line Format--authentication_windows_use_principal_name
    Permitted ValuesTypeboolean
    DefaultON

    This variable is available only if the authentication_windows Windows authentication plugin is enabled. See Section 6.5.1.4, “The Windows Native Authentication Plugin”.

    A client that authenticates using the InitSecurityContext() function should provide a string identifying the service to which it connects (targetName). MySQL uses the principal name (UPN) of the account under which the server is running. The UPN has the form user_id@computer_name and need not be registered anywhere to be used. This UPN is sent by the server at the beginning of authentication handshake.

    This variable controls whether the server sends the UPN in the initial challenge. By default, the variable is enabled. For security reasons, it can be disabled to avoid sending the server's account name to a client in clear text. If the variable is disabled, the server always sends a 0x00 byte in the first challenge, the client does not specify targetName, and as a result, NTLM authentication is used.

    If the server fails to obtain its UPN (which will happen primarily in environments that do not support Kerberos authentication), the UPN is not sent by the server and NTLM authentication is used.

    This variable was added in MySQL 5.5.16.

  • autocommit

    Command-Line Format--autocommit[=#] (>= 5.5.8)
    System Variable (<= 5.5.2)Nameautocommit
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Nameautocommit
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultON

    The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. If autocommit is 0 and you change it to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

    By default, client connections begin with autocommit set to 1. To cause clients to begin with a default of 0, set the global autocommit value by starting the server with the --autocommit=0 option. To set the variable using an option file, include these lines:

    [mysqld]
    autocommit=0
    

    Before MySQL 5.5.8, the global autocommit value cannot be set at startup. As a workaround, set the init_connect system variable:

    SET GLOBAL init_connect='SET autocommit=0';
    

    The init_connect variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:

    [mysqld]
    init_connect='SET autocommit=0'
    

    The content of init_connect is not executed for users that have the SUPER privilege (unlike the effect of setting the global autocommit value at startup).

  • automatic_sp_privileges

    System VariableNameautomatic_sp_privileges
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultTRUE

    When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (The ALTER ROUTINE privilege is required to drop the routine.) The server also automatically drops those privileges from the creator when the routine is dropped. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.

    The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.

    See also Section 20.2.2, “Stored Routines and MySQL Privileges”.

  • back_log

    System VariableNameback_log
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default50
    Min Value1
    Max Value65535

    The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.

    In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. back_log cannot be set higher than your operating system limit.

  • basedir

    Command-Line Format--basedir=dir_name
    System VariableNamebasedir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The MySQL installation base directory. This variable can be set with the --basedir option. Relative path names for other variables usually are resolved relative to the base directory.

  • big_tables

    Command-Line Format--big-tables
    System Variable (<= 5.5.2)Namebig_tables
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namebig_tables
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    If set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error The table tbl_name is full does not occur for SELECT operations that require a large temporary table. The default value for a new connection is 0 (use in-memory temporary tables). Normally, you should never need to set this variable, because in-memory tables are automatically converted to disk-based tables as required.

    Note

    This variable was formerly named sql_big_tables.

  • bulk_insert_buffer_size

    Command-Line Format--bulk_insert_buffer_size=#
    System VariableNamebulk_insert_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default8388608
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default8388608
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default8388608
    Min Value0
    Max Value18446744073709551615

    MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE when adding data to nonempty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.

  • character_set_client

    System VariableNamecharacter_set_client
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring

    The character set for statements that arrive from the client. The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 10.1.5, “Connection Character Sets and Collations”.) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests:

    • The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.

    • The client requests a character set not known to the server. For example, a Japanese-enabled client requests sjis when connecting to a server not configured with sjis support.

    • mysqld was started with the --skip-character-set-client-handshake option, which causes it to ignore client character set configuration. This reproduces MySQL 4.0 behavior and is useful should you wish to upgrade the server without upgrading all the clients.

    ucs2, utf16, and utf32 cannot be used as a client character set, which means that they also do not work for SET NAMES or SET CHARACTER SET.

  • character_set_connection

    System VariableNamecharacter_set_connection
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring

    The character set used for literals that do not have a character set introducer and for number-to-string conversion.

  • character_set_database

    System VariableNamecharacter_set_database
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    FootnoteThis option is dynamic, but only the server should set this information. You should not set the value of this variable manually.
    Permitted ValuesTypestring

    The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server.

  • character_set_filesystem

    Command-Line Format--character-set-filesystem=name
    System VariableNamecharacter_set_filesystem
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaultbinary

    The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, which means that no conversion occurs. For systems on which multibyte file names are permitted, a different value may be more appropriate. For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8'.

  • character_set_results

    System VariableNamecharacter_set_results
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring

    The character set used for returning query results such as result sets or error messages to the client.

  • character_set_server

    Command-Line Format--character-set-server
    System VariableNamecharacter_set_server
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaultlatin1

    The server's default character set.

  • character_set_system

    System VariableNamecharacter_set_system
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring
    Defaultutf8

    The character set used by the server for storing identifiers. The value is always utf8.

  • character_sets_dir

    Command-Line Format--character-sets-dir=dir_name
    System VariableNamecharacter_sets_dir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The directory where character sets are installed.

  • collation_connection

    System VariableNamecollation_connection
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring

    The collation of the connection character set.

  • collation_database

    System VariableNamecollation_database
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    FootnoteThis option is dynamic, but only the server should set this information. You should not set the value of this variable manually.
    Permitted ValuesTypestring

    The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server.

  • collation_server

    Command-Line Format--collation-server
    System VariableNamecollation_server
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaultlatin1_swedish_ci

    The server's default collation.

  • completion_type

    Command-Line Format--completion_type=#
    System VariableNamecompletion_type
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.2)Typeinteger
    Default0
    Valid Values0
    1
    2
    Permitted Values (>= 5.5.3)Typeenumeration
    DefaultNO_CHAIN
    Valid ValuesNO_CHAIN
    CHAIN
    RELEASE
    0
    1
    2

    The transaction completion type. This variable can take the values shown in the following table. As of MySQL 5.5.3, the variable can be assigned using either the name values or corresponding integer values. Before 5.5.3, only the integer values can be used.

    ValueDescription
    NO_CHAIN (or 0)COMMIT and ROLLBACK are unaffected. This is the default value.
    CHAIN (or 1)COMMIT and ROLLBACK are equivalent to COMMIT AND CHAIN and ROLLBACK AND CHAIN, respectively. (A new transaction starts immediately with the same isolation level as the just-terminated transaction.)
    RELEASE (or 2)COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)

    completion_type affects transactions that begin with START TRANSACTION or BEGIN and end with COMMIT or ROLLBACK. It does not apply to implicit commits resulting from execution of the statements listed in Section 13.3.3, “Statements That Cause an Implicit Commit”. It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.

  • concurrent_insert

    Command-Line Format--concurrent_insert[=#]
    System VariableNameconcurrent_insert
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (<= 5.5.2)Typeinteger
    Default1
    Valid Values0
    1
    2
    Permitted Values (>= 5.5.3)Typeenumeration
    DefaultAUTO
    Valid ValuesNEVER
    AUTO
    ALWAYS
    0
    1
    2

    If AUTO (the default), MySQL permits INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file. If you start mysqld with --skip-new, this variable is set to NEVER.

    This variable can take the values shown in the following table. As of MySQL 5.5.3, the variable can be assigned using either the name values or corresponding integer values. Before 5.5.3, only the integer values can be used.

    ValueDescription
    NEVER (or 0)Disables concurrent inserts
    AUTO (or 1)(Default) Enables concurrent insert for MyISAM tables that do not have holes
    ALWAYS (or 2)Enables concurrent inserts for all MyISAM tables, even those that have holes. For a table with a hole, new rows are inserted at the end of the table if it is in use by another thread. Otherwise, MySQL acquires a normal write lock and inserts the row into the hole.

    See also Section 8.11.3, “Concurrent Inserts”.

  • connect_timeout

    Command-Line Format--connect_timeout=#
    System VariableNameconnect_timeout
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default10
    Min Value2
    Max Value31536000

    The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 10 seconds.

    Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.

  • datadir

    Command-Line Format--datadir=dir_name
    System VariableNamedatadir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The MySQL data directory. This variable can be set with the --datadir option.

  • date_format

    This variable is unused.

  • datetime_format

    This variable is unused.

  • debug

    Command-Line Format--debug[=debug_options]
    System VariableNamedebug
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (Unix)Typestring
    Defaultd:t:i:o,/tmp/mysqld.trace
    Permitted Values (Windows)Typestring
    Defaultd:t:i:O,\mysqld.trace

    This variable indicates the current debugging settings. It is available only for servers built with debugging support. The initial value comes from the value of instances of the --debug option given at server startup. The global and session values may be set at runtime; the SUPER privilege is required, even for the session value.

    Assigning a value that begins with + or - cause the value to added to or subtracted from the current value:

    mysql> SET debug = 'T';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | T       |
    +---------+
    
    mysql> SET debug = '+P';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | P:T     |
    +---------+
    
    mysql> SET debug = '-P';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | T       |
    +---------+
    

    For more information, see Section 24.5.3, “The DBUG Package”.

  • debug_sync

    System Variable (<= 5.5.2)Namedebug_sync
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namedebug_sync
    Variable ScopeSession
    Dynamic VariableYes
    Permitted ValuesTypestring

    This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 CMake option (see Section 2.9.4, “MySQL Source-Configuration Options”). If Debug Sync is not compiled in, this system variable is not available.

    The global variable value is read only and indicates whether the facility is enabled. By default, Debug Sync is disabled and the value of debug_sync is OFF. If the server is started with --debug-sync-timeout=N, where N is a timeout value greater than 0, Debug Sync is enabled and the value of debug_sync is ON - current signal followed by the signal name. Also, N becomes the default timeout for individual synchronization points.

    The session value can be read by any user and will have the same value as the global variable. The session value can be set by users that have the SUPER privilege to control synchronization points.

    For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.

  • default_storage_engine

    Command-Line Format--default-storage-engine=name
    System Variable (<= 5.5.2)Namestorage_engine
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namedefault_storage_engine
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.4)Typeenumeration
    DefaultMyISAM
    Permitted Values (>= 5.5.5)Typeenumeration
    DefaultInnoDB

    The default storage engine. To set the storage engine at server startup, use the --default-storage-engine option. See Section 5.1.3, “Server Command Options”.

    To see which storage engines are available and enabled, use the SHOW ENGINES statement or query the INFORMATION_SCHEMA ENGINES table.

    If you disable the default storage engine at server startup, you must set the default engine to a different engine or the server will not start.

    This variable was added in MySQL 5.5.3 to be used in preference to storage_engine, which is now deprecated.

  • default_week_format

    Command-Line Format--default_week_format=#
    System VariableNamedefault_week_format
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value7

    The default mode value to use for the WEEK() function. See Section 12.7, “Date and Time Functions”.

  • delay_key_write

    Command-Line Format--delay-key-write[=name]
    System VariableNamedelay_key_write
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    DefaultON
    Valid ValuesON
    OFF
    ALL

    This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.

    OptionDescription
    OFFDELAY_KEY_WRITE is ignored.
    ONMySQL honors any DELAY_KEY_WRITE option specified in CREATE TABLE statements. This is the default value.
    ALLAll new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled.

    If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover-options option (for example, --myisam-recover-options=BACKUP,FORCE). See Section 5.1.3, “Server Command Options”, and Section 15.3.1, “MyISAM Startup Options”.

    Warning

    If you enable external locking with --external-locking, there is no protection against index corruption for tables that use delayed key writes.

  • delayed_insert_limit

    Command-Line Format--delayed_insert_limit=#
    System VariableNamedelayed_insert_limit
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default100
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default100
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default100
    Min Value1
    Max Value18446744073709551615

    After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it permits them to execute before continuing to insert delayed rows.

  • delayed_insert_timeout

    Command-Line Format--delayed_insert_timeout=#
    System VariableNamedelayed_insert_timeout
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default300

    How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.

  • delayed_queue_size

    Command-Line Format--delayed_queue_size=#
    System VariableNamedelayed_queue_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default1000
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default1000
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default1000
    Min Value1
    Max Value18446744073709551615

    This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement waits until there is room in the queue again.

  • div_precision_increment

    Command-Line Format--div_precision_increment=#
    System VariableNamediv_precision_increment
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default4
    Min Value0
    Max Value30

    This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.

    mysql> SELECT 1/7;
    +--------+
    | 1/7    |
    +--------+
    | 0.1429 |
    +--------+
    mysql> SET div_precision_increment = 12;
    mysql> SELECT 1/7;
    +----------------+
    | 1/7            |
    +----------------+
    | 0.142857142857 |
    +----------------+
    
  • engine_condition_pushdown

    Deprecated5.5.3, by optimizer_switch
    Command-Line Format--engine-condition-pushdown
    System VariableNameengine_condition_pushdown
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultON

    The engine condition pushdown optimization enables processing for certain comparisons to be pushed down to the storage engine level for more efficient execution. For more information, see Section 8.2.1.5, “Engine Condition Pushdown Optimization”.

    Engine condition pushdown is used only by the NDBCLUSTER storage engine. Enabling this optimization on a MySQL Server acting as a MySQL Cluster SQL node causes WHERE conditions on unindexed columns to be evaluated on the cluster's data nodes and only the rows that match to be sent back to the SQL node that issued the query. This greatly reduces the amount of cluster data that must be sent over the network, increasing the efficiency with which results are returned.

    The engine_condition_pushdown variable controls whether engine condition pushdown is enabled. By default, this variable is ON (1). Setting it to OFF (0) disables pushdown.

    This variable is deprecated as of MySQL 5.5.3 and is removed in MySQL 5.6. Use the engine_condition_pushdown flag of the optimizer_switch variable instead. See Section 8.9.2, “Controlling Switchable Optimizations”.

  • error_count

    The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.18, “SHOW ERRORS Syntax”.

  • event_scheduler

    Command-Line Format--event-scheduler[=value]
    System VariableNameevent_scheduler
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    DefaultOFF
    Valid ValuesON
    OFF
    DISABLED

    This variable indicates the status of the Event Scheduler; possible values are ON, OFF, and DISABLED, with the default being OFF. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in the Overview section of the Events chapter.

  • expire_logs_days

    Command-Line Format--expire_logs_days=#
    System VariableNameexpire_logs_days
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value99

    The number of days for automatic binary log file removal. The default is 0, which means no automatic removal. Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.4, “MySQL Server Logs”.

    To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

  • external_user

    Introduced5.5.7
    System VariableNameexternal_user
    Variable ScopeSession
    Dynamic VariableNo
    Permitted ValuesTypestring

    The external user name used during the authentication process, as set by the plugin used to authenticate the client. With native (built-in) MySQL authentication, or if the plugin does not set the value, this variable is NULL. See Section 6.3.7, “Proxy Users”.

    This variable was added in MySQL 5.5.7.

  • flush

    Command-Line Format--flush
    System VariableNameflush
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.5.3.3, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.

  • flush_time

    Command-Line Format--flush_time=#
    System VariableNameflush_time
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Permitted Values (Windows)Typeinteger
    Default1800
    Min Value0

    If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on systems with minimal resources.

  • foreign_key_checks

    If set to 1 (the default), foreign key constraints for InnoDB tables are checked. If set to 0, foreign key constraints are ignored, with a couple of exceptions. When re-creating a table that was dropped, an error is returned if the table definition does not conform to the foreign key constraints referencing the table. Likewise, an ALTER TABLE operation returns an error if a foreign key definition is incorrectly formed. For more information, see Section 13.1.17.3, “Using FOREIGN KEY Constraints”.

    Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. See Section 14.11.7, “InnoDB and FOREIGN KEY Constraints”.

    Setting foreign_key_checks to 0 also affects data definition statements: DROP SCHEMA drops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.

    Note

    Setting foreign_key_checks to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while foreign_key_checks=0 will not be verified for consistency.

    Warning

    With foreign_key_checks=0, dropping an index required by a foreign key constraint places the table in an inconsistent state and causes the foreign key check that occurs at table load to fail. To avoid this problem, remove the foreign key constraint before dropping the index (Bug #70260).

  • ft_boolean_syntax

    Command-Line Format--ft_boolean_syntax=name
    System VariableNameft_boolean_syntax
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypestring
    Default+ -><()~*:""&|

    The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 12.9.2, “Boolean Full-Text Searches”.

    The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:

    • Operator function is determined by position within the string.

    • The replacement value must be 14 characters.

    • Each character must be an ASCII nonalphanumeric character.

    • Either the first or second character must be a space.

    • No duplicates are permitted except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.

    • Positions 10, 13, and 14 (which by default are set to :, &, and |) are reserved for future extensions.

  • ft_max_word_len

    Command-Line Format--ft_max_word_len=#
    System VariableNameft_max_word_len
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Min Value10

    The maximum length of the word to be included in a FULLTEXT index.

    Note

    FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_min_word_len

    Command-Line Format--ft_min_word_len=#
    System VariableNameft_min_word_len
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default4
    Min Value1

    The minimum length of the word to be included in a FULLTEXT index.

    Note

    FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_query_expansion_limit

    Command-Line Format--ft_query_expansion_limit=#
    System VariableNameft_query_expansion_limit
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default20
    Min Value0
    Max Value1000

    The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION.

  • ft_stopword_file

    Command-Line Format--ft_stopword_file=file_name
    System VariableNameft_stopword_file
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The file from which to read the list of stopwords for full-text searches. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the storage/myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. See also Section 12.9.4, “Full-Text Stopwords”.

    Note

    FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.

  • general_log

    Command-Line Format--general-log
    System VariableNamegeneral_log
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Whether the general query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The default value depends on whether the --general_log option is given. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.

  • general_log_file

    Command-Line Format--general-log-file=file_name
    System VariableNamegeneral_log_file
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypefile name
    Defaulthost_name.log

    The name of the general query log file. The default value is host_name.log, but the initial value can be changed with the --general_log_file option.

  • group_concat_max_len

    Command-Line Format--group_concat_max_len=#
    System VariableNamegroup_concat_max_len
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default1024
    Min Value4
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default1024
    Min Value4
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default1024
    Min Value4
    Max Value18446744073709551615

    The maximum permitted result length in bytes for the GROUP_CONCAT() function. The default is 1024.

  • have_compress

    YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used.

  • have_crypt

    YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used.

  • have_csv

    YES if mysqld supports CSV tables, NO if not.

    This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.

  • have_dynamic_loading

    YES if mysqld supports dynamic loading of plugins, NO if not. If the value is NO, you cannot use options such as --plugin-load to load plugins at server startup, or the INSTALL PLUGIN statement to load plugins at runtime.

  • have_geometry

    YES if the server supports spatial data types, NO if not.

  • have_innodb

    YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.

    This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.

  • have_openssl

    This variable is an alias for have_ssl.

  • have_partitioning

    YES if mysqld supports partitioning.

    This variable is deprecated and is removed in MySQL 5.6. Use SHOW PLUGINS instead. For more information, see Chapter 19, Partitioning.

  • have_profiling

    YES if statement profiling capability is present, NO if not. If present, the profiling system variable controls whether this capability is enabled or disabled. See Section 13.7.5.32, “SHOW PROFILES Syntax”.

  • have_query_cache

    YES if mysqld supports the query cache, NO if not.

  • have_rtree_keys

    YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)

  • have_ssl

    YES if mysqld supports SSL connections, NO if not. DISABLED indicates that the server was compiled with SSL support, but was not started with the appropriate --ssl-xxx options. For more information, see Section 6.4.2, “Building MySQL with Support for Secure Connections”.

  • have_symlink

    YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options, and on Windows for support of data directory symlinks. If the server is started with the --skip-symbolic-links option, the value is DISABLED.

  • hostname

    System VariableNamehostname
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The server sets this variable to the server host name at startup.

  • identity

    This variable is a synonym for the last_insert_id variable. It exists for compatibility with other database systems. You can read its value with SELECT @@identity, and set it using SET identity.

  • init_connect

    Command-Line Format--init-connect=name
    System VariableNameinit_connect
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypestring

    A string to be executed by the server for each client that connects. The string consists of one or more SQL statements, separated by semicolon characters. For example, each client session begins by default with autocommit mode enabled. For older servers (before MySQL 5.5.8), there is no global autocommit system variable to specify that autocommit should be disabled by default, but as a workaround init_connect can be used to achieve the same effect:

    SET GLOBAL init_connect='SET autocommit=0';
    

    The init_connect variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:

    [mysqld]
    init_connect='SET autocommit=0'
    

    The content of init_connect is not executed for users that have the SUPER privilege. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.

    The server discards any result sets produced by statements in the value of of init_connect.

  • init_file

    Command-Line Format--init-file=file_name
    System VariableNameinit_file
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The name of the file specified with the --init-file option when you start the server. This should be a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. For more information, see the description of --init-file.

  • innodb_xxx

    InnoDB system variables are listed in Section 14.17, “InnoDB Startup Options and System Variables”. These variables control many aspects of storage, memory use, and I/O patterns for InnoDB tables, and are especially important now that InnoDB is the default storage engine.

  • insert_id

    The value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.

  • interactive_timeout

    Command-Line Format--interactive_timeout=#
    System VariableNameinteractive_timeout
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default28800
    Min Value1

    The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.

  • join_buffer_size

    Command-Line Format--join_buffer_size=#
    System VariableNamejoin_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (Windows, >= 5.5.3)Typeinteger
    Default131072
    Min Value128
    Max Value4294967295
    Permitted Values (Windows, 32-bit platforms, <= 5.5.2)Typeinteger
    Default131072
    Min Value8200
    Max Value4294967295
    Permitted Values (Windows, 64-bit platforms, <= 5.5.2)Typeinteger
    Default131072
    Min Value8228
    Max Value4294967295
    Permitted Values (Other, 32-bit platforms, <= 5.5.2)Typeinteger
    Default131072
    Min Value8200
    Max Value4294967295
    Permitted Values (Other, 32-bit platforms, >= 5.5.3)Typeinteger
    Default131072
    Min Value128
    Max Value4294967295
    Permitted Values (Other, 64-bit platforms, <= 5.5.2)Typeinteger
    Default131072
    Min Value8228
    Max Value18446744073709547520
    Permitted Values (Other, 64-bit platforms, >= 5.5.3)Typeinteger
    Default131072
    Min Value128
    Max Value18446744073709547520

    The minimum size of the buffer that is used for plain index scans, range index scans, and joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.

    There is no gain from setting the buffer larger than required to hold each matching row, and all joins allocate at least the minimum size, so use caution in setting this variable to a large value globally. It is better to keep the global setting small and change to a larger setting only in sessions that are doing large joins. Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.

    The maximum permissible setting for join_buffer_size is 4GB−1. Larger values are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB−1 with a warning).

    Prior to MySQL 5.5.3, the minimum allowed value for this variable was 8200 for 32-bit platforms and 8228 for 64-bit platforms. In MySQL 5.5.3 and later, the minimum is 128 for all platforms.

    For additional information about join buffering, see Section 8.2.1.8, “Nested-Loop Join Algorithms”.

  • keep_files_on_create

    Command-Line Format--keep_files_on_create=#
    System VariableNamekeep_files_on_create
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    If a MyISAM table is created with no DATA DIRECTORY option, the .MYD file is created in the database directory. By default, if MyISAM finds an existing .MYD file in this case, it overwrites it. The same applies to .MYI files for tables created with no INDEX DIRECTORY option. To suppress this behavior, set the keep_files_on_create variable to ON (1), in which case MyISAM will not overwrite existing files and returns an error instead. The default value is OFF (0).

    If a MyISAM table is created with a DATA DIRECTORY or INDEX DIRECTORY option and an existing .MYD or .MYI file is found, MyISAM always returns an error. It will not overwrite a file in the specified directory.

  • key_buffer_size

    Command-Line Format--key_buffer_size=#
    System VariableNamekey_buffer_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default8388608
    Min Value8
    Max Value4294967295
    Permitted Values (64-bit platforms)Typeinteger
    Default8388608
    Min Value8
    Max ValueOS_PER_PROCESS_LIMIT

    Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

    The maximum permissible setting for key_buffer_size is 4GB−1 on 32-bit platforms. Larger values are permitted for 64-bit platforms. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. The value of this variable indicates the amount of memory requested. Internally, the server allocates as much memory as possible up to this amount, but the actual allocation might be less.

    You can increase the value to get better index handling for all reads and multiple writes; on a system whose primary function is to run MySQL using the MyISAM storage engine, 25% of the machine's total memory is an acceptable value for this variable. However, you should be aware that, if you make the value too large (for example, more than 50% of the machine's total memory), your system might start to page and become extremely slow. This is because MySQL relies on the operating system to perform file system caching for data reads, so you must leave some room for the file system cache. You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.

    For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 8.2.2.1, “Speed of INSERT Statements”.

    You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 13.7.5, “SHOW Syntax”.) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.

    The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size, which is available from the key_cache_block_size system variable:

    1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)
    

    This value is an approximation because some space in the key buffer is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).

    It is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 8.10.2, “The MyISAM Key Cache”.

  • key_cache_age_threshold

    Command-Line Format--key_cache_age_threshold=#
    System VariableNamekey_cache_age_threshold
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default300
    Min Value100
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default300
    Min Value100
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default300
    Min Value100
    Max Value18446744073709551615

    This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See Section 8.10.2, “The MyISAM Key Cache”.

  • key_cache_block_size

    Command-Line Format--key_cache_block_size=#
    System VariableNamekey_cache_block_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default1024
    Min Value512
    Max Value16384

    The size in bytes of blocks in the key cache. The default value is 1024. See Section 8.10.2, “The MyISAM Key Cache”.

  • key_cache_division_limit

    Command-Line Format--key_cache_division_limit=#
    System VariableNamekey_cache_division_limit
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default100
    Min Value1
    Max Value100

    The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. See Section 8.10.2, “The MyISAM Key Cache”.

  • language

    Deprecated5.5.0, by lc-messages-dir
    Command-Line Format--language=name
    System VariableNamelanguage
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name
    Default/usr/local/mysql/share/mysql/english/

    The directory where error messages are located. See Section 10.2, “Setting the Error Message Language”.

    The language system variable is removed as of MySQL 5.5.0 (although the --language command-line option can still be used). Similar information is available from the lc_messages_dir and lc_messages variables.

  • large_files_support

    System VariableNamelarge_files_support
    Variable ScopeGlobal
    Dynamic VariableNo

    Whether mysqld was compiled with options for large file support.

  • large_pages

    Command-Line Format--large-pages
    System VariableNamelarge_pages
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificLinux
    Permitted Values (Linux)Typeboolean
    DefaultFALSE

    Whether large page support is enabled (via the --large-pages option). See Section 8.12.5.2, “Enabling Large Page Support”.

  • large_page_size

    System VariableNamelarge_page_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (Linux)Typeinteger
    Default0

    If large page support is enabled, this shows the size of memory pages. Large memory pages are supported only on Linux; on other platforms, the value of this variable is always 0. See Section 8.12.5.2, “Enabling Large Page Support”.

  • last_insert_id

    The value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.

  • lc_messages

    Command-Line Format--lc-messages=name
    System VariableNamelc_messages
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring
    Defaulten_US

    The locale to use for error messages. The default is en_US. The server converts the argument to a language name and combines it with the value of lc_messages_dir to produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”.

  • lc_messages_dir

    Command-Line Format--lc-messages-dir=dir_name
    System VariableNamelc_messages_dir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The directory where error messages are located. The server uses the value together with the value of lc_messages to produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”.

  • lc_time_names

    System VariableNamelc_time_names
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring

    This variable specifies the locale that controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting. For further information, see Section 10.7, “MySQL Server Locale Support”.

  • license

    System VariableNamelicense
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring
    DefaultGPL

    The type of license the server has.

  • local_infile

    System VariableNamelocal_infile
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Defaulttrue

    Whether LOCAL is supported for LOAD DATA INFILE statements. If this variable is disabled, clients cannot use LOCAL in LOAD DATA statements. While the default for this variable is true, whether LOAD DATA INFILE LOCAL is actually permitted depends on how MySQL was compiled, as well as a number of settings on both the server and the client; see Section 6.1.6, “Security Issues with LOAD DATA LOCAL”, for details.

  • lock_wait_timeout

    Introduced5.5.3
    Command-Line Format--lock_wait_timeout=#
    System VariableNamelock_wait_timeout
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default31536000
    Min Value1
    Max Value31536000

    This variable specifies the timeout in seconds for attempts to acquire metadata locks. The permissible values range from 1 to 31536000 (1 year). The default is 31536000.

    This timeout applies to all statements that use metadata locks. These include DML and DDL operations on tables, views, stored procedures, and stored functions, as well as LOCK TABLES, FLUSH TABLES WITH READ LOCK, and HANDLER statements.

    This timeout does not apply to implicit accesses to system tables in the mysql database, such as grant tables modified by GRANT or REVOKE statements or table logging statements. The timeout does apply to system tables accessed directly, such as with SELECT or UPDATE.

    The timeout value applies separately for each metadata lock attempt. A given statement can require more than one lock, so it is possible for the statement to block for longer than the lock_wait_timeout value before reporting a timeout error. When lock timeout occurs, ER_LOCK_WAIT_TIMEOUT is reported.

    lock_wait_timeout does not apply to delayed inserts, which always execute with a timeout of 1 year. This is done to avoid unnecessary timeouts because a session that issues a delayed insert receives no notification of delayed insert timeouts.

    This variable was added in MySQL 5.5.3.

  • locked_in_memory

    System VariableNamelocked_in_memory
    Variable ScopeGlobal
    Dynamic VariableNo

    Whether mysqld was locked in memory with --memlock.

  • log

    Deprecated5.1.29, by general-log
    Command-Line Format--log[=file_name]
    System VariableNamelog
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypefile name

    Whether logging of all statements to the general query log is enabled. See Section 5.4.3, “The General Query Log”.

    This variable is deprecated and is removed in MySQL 5.6. Use general_log instead.

  • log_bin_trust_function_creators

    Command-Line Format--log-bin-trust-function-creators
    System VariableNamelog_bin_trust_function_creators
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultFALSE

    This variable applies when binary logging is enabled. It controls whether stored function creators can be trusted not to create stored functions that will cause unsafe events to be written to the binary log. If set to 0 (the default), users are not permitted to create or alter stored functions unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege. A setting of 0 also enforces the restriction that a function must be declared with the DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. This variable also applies to trigger creation. See Section 20.7, “Binary Logging of Stored Programs”.

  • log_bin_trust_routine_creators

    This is the old name for log_bin_trust_function_creators. This variable is deprecated and was removed in MySQL 5.5.3.

  • log_error

    Command-Line Format--log-error[=file_name]
    System VariableNamelog_error
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The location of the error log, or empty if the server is writing error message to the standard error output. See Section 5.4.2, “The Error Log”.

  • log_output

    Command-Line Format--log-output=name
    System VariableNamelog_output
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeset
    DefaultFILE
    Valid ValuesTABLE
    FILE
    NONE

    The destination for general query log and slow query log output. The value can be a comma-separated list of one or more of the words TABLE (log to tables), FILE (log to files), or NONE (do not log to tables or files). The default value is FILE. NONE, if present, takes precedence over any other specifiers. If the value is NONE log entries are not written even if the logs are enabled. If the logs are not enabled, no logging occurs even if the value of log_output is not NONE. For more information, see Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”.

  • log_queries_not_using_indexes

    Command-Line Format--log-queries-not-using-indexes
    System VariableNamelog_queries_not_using_indexes
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Whether queries that do not use indexes are logged to the slow query log. See Section 5.4.5, “The Slow Query Log”.

  • log_slow_queries

    Deprecated5.1.29, by slow-query-log
    Command-Line Format--log-slow-queries[=name]
    System VariableNamelog_slow_queries
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean

    Whether slow queries should be logged. Slow is determined by the value of the long_query_time variable. See Section 5.4.5, “The Slow Query Log”.

    This variable is deprecated and is removed in MySQL 5.6. Use slow_query_log instead.

  • log_warnings

    Command-Line Format--log-warnings[=#]
    System VariableNamelog_warnings
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default1
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default1
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default1
    Min Value0
    Max Value18446744073709551615

    Whether to produce additional warning messages to the error log. This variable is enabled (1) by default and can be disabled by setting it to 0. The server logs messages about statements that are unsafe for statement-based logging if the value is greater than 0. Aborted connections and access-denied errors for new connection attempts are logged if the value is greater than 1.

  • long_query_time

    Command-Line Format--long_query_time=#
    System VariableNamelong_query_time
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypenumeric
    Default10
    Min Value0

    If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If the slow query log is enabled, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds. For logging to a file, times are written including the microseconds part. For logging to tables, only integer times are written; the microseconds part is ignored. See Section 5.4.5, “The Slow Query Log”.

  • low_priority_updates

    Command-Line Format--low-priority-updates
    System VariableNamelow_priority_updates
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultFALSE

    If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This affects only storage engines that use only table-level locking (such as MyISAM, MEMORY, and MERGE). This variable previously was named sql_low_priority_updates.

  • lower_case_file_system

    System VariableNamelower_case_file_system
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeboolean

    This variable describes the case sensitivity of file names on the file system where the data directory is located. OFF means file names are case sensitive, ON means they are not case sensitive. This variable is read only because it reflects a file system attribute and setting it would have no effect on the file system.

  • lower_case_table_names

    Command-Line Format--lower_case_table_names[=#]
    System VariableNamelower_case_table_names
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value2

    If set to 0, table names are stored as specified and comparisons are case sensitive. If set to 1, table names are stored in lowercase on disk and comparisons are not case sensitive. If set to 2, table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. For additional information, see Section 9.2.2, “Identifier Case Sensitivity”.

    On Windows the default value is 1. On OS X, the default value is 2.

    You should not set lower_case_table_names to 0 if you are running MySQL on a system where the data directory resides on a case-insensitive file system (such as on Windows or OS X). It is an unsupported combination that could result in a hang condition when running an INSERT INTO ... SELECT ... FROM tbl_name operation with the wrong tbl_name letter case. With MyISAM, accessing table names using different letter cases could cause index corruption.

    As of MySQL 5.5.46, an error message is printed and the server exits if you attempt to start the server with --lower_case_table_names=0 on a case-insensitive file system.

    If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.

    The setting of this variable has no effect on replication filtering options. This is a known issue which is fixed in MySQL 5.6. See Section 17.2.3, “How Servers Evaluate Replication Filtering Rules”, for more information.

    You should not use different settings for lower_case_table_names on replication masters and slaves. In particular, you should not do this when the slave uses a case-sensitive file system, as this can cause replication to fail. This is a known issue which is fixed in MySQL 5.6. For more information, see Section 17.4.1.38, “Replication and Variables”.

  • max_allowed_packet

    Command-Line Format--max_allowed_packet=#
    System VariableNamemax_allowed_packet
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default1048576
    Min Value1024
    Max Value1073741824

    The maximum size of one packet or any generated/intermediate string.

    The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch large (possibly incorrect) packets.

    You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 1GB. The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple.

    When you change the message buffer size by changing the value of the max_allowed_packet variable, you should also change the buffer size on the client side if your client program permits it. The default max_allowed_packet value built in to the client library is 1GB, but individual client programs might override this. For example, mysql and mysqldump have defaults of 16MB and 24MB, respectively. They also enable you to change the client-side value by setting max_allowed_packet on the command line or in an option file.

    The session value of this variable is read only. The client can receive up to as many bytes as the session value. However, the server will not send to the client more bytes than the current global max_allowed_packet value. (The global value could be less than the session value if the global value is changed after the client connects.)

  • max_connect_errors

    Command-Line Format--max_connect_errors=#
    System VariableNamemax_connect_errors
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default10
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default10
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default10
    Min Value1
    Max Value18446744073709551615

    If more than this many successive connection requests from a host are interrupted without a successful connection, the server blocks that host from further connections. You can unblock blocked hosts by flushing the host cache. To do so, issue a FLUSH HOSTS statement or execute a mysqladmin flush-hosts command. If a connection is established successfully within fewer than max_connect_errors attempts after a previous connection was interrupted, the error count for the host is cleared to zero. However, once a host is blocked, flushing the host cache is the only way to unblock it.

  • max_connections

    Command-Line Format--max_connections=#
    System VariableNamemax_connections
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default151
    Min Value1
    Max Value100000

    The maximum permitted number of simultaneous client connections. By default, this is 151. See Section B.5.2.7, “Too many connections”, for more information.

    Increasing this value increases the number of file descriptors that mysqld requires. See Section 8.4.3.1, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.

  • max_delayed_threads

    Command-Line Format--max_delayed_threads=#
    System VariableNamemax_delayed_threads
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default20
    Min Value0
    Max Value16384

    Do not start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row is inserted as if the DELAYED attribute was not specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, this disables DELAYED entirely.

    For the SESSION value of this variable, the only valid values are 0 or the GLOBAL value.

  • max_error_count

    Command-Line Format--max_error_count=#
    System VariableNamemax_error_count
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default64
    Min Value0
    Max Value65535

    The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS and SHOW WARNINGS statements.

  • max_heap_table_size

    Command-Line Format--max_heap_table_size=#
    System VariableNamemax_heap_table_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default16777216
    Min Value16384
    Max Value4294967295
    Permitted Values (64-bit platforms)Typeinteger
    Default16777216
    Min Value16384
    Max Value1844674407370954752

    This variable sets the maximum size to which user-created MEMORY tables are permitted to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or altered with ALTER TABLE or TRUNCATE TABLE. A server restart also sets the maximum size of existing MEMORY tables to the global max_heap_table_size value.

    This variable is also used in conjunction with tmp_table_size to limit the size of internal in-memory tables. See Section 8.4.4, “Internal Temporary Table Use in MySQL”.

    max_heap_table_size is not replicated. See Section 17.4.1.23, “Replication and MEMORY Tables”, and Section 17.4.1.38, “Replication and Variables”, for more information.

  • max_insert_delayed_threads

    System VariableNamemax_insert_delayed_threads
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger

    This variable is a synonym for max_delayed_threads.

  • max_join_size

    Command-Line Format--max_join_size=#
    System VariableNamemax_join_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default18446744073709551615
    Min Value1
    Max Value18446744073709551615

    Do not permit statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than max_join_size disk seeks. By setting this value, you can catch statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows.

    Setting this variable to a value other than DEFAULT resets the value of sql_big_selects to 0. If you set the sql_big_selects value again, the max_join_size variable is ignored.

    If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.

    This variable previously was named sql_max_join_size.

  • max_length_for_sort_data

    Command-Line Format--max_length_for_sort_data=#
    System VariableNamemax_length_for_sort_data
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default1024
    Min Value4
    Max Value8388608

    The cutoff on the size of index values that determines which filesort algorithm to use. See Section 8.2.1.11, “ORDER BY Optimization”.

  • max_long_data_size

    Introduced5.5.11
    Deprecated5.5.11
    Command-Line Format--max_long_data_size=#
    System VariableNamemax_long_data_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default1048576
    Min Value1024
    Max Value4294967295

    The maximum size of parameter values that can be sent with the mysql_stmt_send_long_data() C API function. If not set at server startup, the default is the value of the max_allowed_packet system variable. This variable is deprecated. In MySQL 5.6, it is removed and the maximum parameter size is controlled by max_allowed_packet.

  • max_prepared_stmt_count

    Command-Line Format--max_prepared_stmt_count=#
    System VariableNamemax_prepared_stmt_count
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default16382
    Min Value0
    Max Value1048576

    This variable limits the total number of prepared statements in the server. (The sum of the number of prepared statements across all sessions.) It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. The default value is 16,382. The permissible range of values is from 0 to 1 million. Setting the value to 0 disables prepared statements.

  • max_relay_log_size

    Command-Line Format--max_relay_log_size=#
    System VariableNamemax_relay_log_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value1073741824

    If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 17.2.1, “Replication Implementation Details”.

  • max_seeks_for_key

    Command-Line Format--max_seeks_for_key=#
    System VariableNamemax_seeks_for_key
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default4294967295
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default18446744073709547520
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default18446744073709551615
    Min Value1
    Max Value18446744073709551615

    Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 13.7.5.23, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

  • max_sort_length

    Command-Line Format--max_sort_length=#
    System VariableNamemax_sort_length
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default1024
    Min Value4
    Max Value8388608

    The number of bytes to use when sorting data values. The server uses only the first max_sort_length bytes of each value and ignores the rest. Consequently, values that differ only after the first max_sort_length bytes compare as equal for GROUP BY, ORDER BY, and DISTINCT operations.

    Increasing the value of max_sort_length may require increasing the value of sort_buffer_size as well. For details, see Section 8.2.1.11, “ORDER BY Optimization”

  • max_sp_recursion_depth

    Command-Line Format--max_sp_recursion_depth[=#]
    System VariableNamemax_sp_recursion_depth
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Max Value255

    The number of times that any given stored procedure may be called recursively. The default value for this option is 0, which completely disables recursion in stored procedures. The maximum value is 255.

    Stored procedure recursion increases the demand on thread stack space. If you increase the value of max_sp_recursion_depth, it may be necessary to increase thread stack size by increasing the value of thread_stack at server startup.

  • max_tmp_tables

    This variable is unused.

  • max_user_connections

    Command-Line Format--max_user_connections=#
    System VariableNamemax_user_connections
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value4294967295

    The maximum number of simultaneous connections permitted to any given MySQL user account. A value of 0 (the default) means no limit.

    This variable has a global value that can be set at server startup or runtime. It also has a read-only session value that indicates the effective simultaneous-connection limit that applies to the account associated with the current session. The session value is initialized as follows:

    • If the user account has a nonzero MAX_USER_CONNECTIONS resource limit, the session max_user_connections value is set to that limit.

    • Otherwise, the session max_user_connections value is set to the global value.

    Account resource limits are specified using the GRANT statement. See Section 6.3.4, “Setting Account Resource Limits”, and Section 13.7.1.3, “GRANT Syntax”.

  • max_write_lock_count

    Command-Line Format--max_write_lock_count=#
    System VariableNamemax_write_lock_count
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default4294967295
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default18446744073709547520
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default18446744073709551615
    Min Value1
    Max Value18446744073709551615

    After this many write locks, permit some pending read lock requests to be processed in between.

  • metadata_locks_cache_size

    Introduced5.5.19
    System VariableNamemetadata_locks_cache_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default1024
    Min Value1
    Max Value1048576

    The size of the metadata locks cache. The server uses this cache to avoid creation and destruction of synchronization objects. This is particularly helpful on systems where such operations are expensive, such as Windows XP. This variable was added in MySQL 5.5.19.

  • min_examined_row_limit

    Command-Line Format--min-examined-row-limit=#
    System VariableNamemin_examined_row_limit
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default0
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709551615

    Queries that examine fewer than this number of rows are not logged to the slow query log.

  • multi_range_count

    Command-Line Format--multi_range_count=#
    System VariableNamemulti_range_count
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default256
    Min Value1
    Max Value4294967295

    The maximum number of ranges to send to a table handler at once during range selects. The default value is 256. Sending multiple ranges to a handler at once can improve the performance of certain selects dramatically. This is especially true for the NDBCLUSTER table handler, which needs to send the range requests to all nodes. Sending a batch of those requests at once reduces communication costs significantly.

    This variable is deprecated in MySQL 5.1, and is no longer supported in MySQL 5.5, in which arbitrarily long lists of ranges can be processed.

  • myisam_data_pointer_size

    Command-Line Format--myisam_data_pointer_size=#
    System VariableNamemyisam_data_pointer_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default6
    Min Value2
    Max Value7

    The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 6. See Section B.5.2.12, “The table is full”.

  • myisam_max_sort_file_size

    Command-Line Format--myisam_max_sort_file_size=#
    System VariableNamemyisam_max_sort_file_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default2147483648
    Permitted Values (64-bit platforms)Typeinteger
    Default9223372036854775807

    The maximum size of the temporary file that MySQL is permitted to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. The value is given in bytes.

    If MyISAM index files exceed this size and disk space is available, increasing the value may help performance. The space must be available in the file system containing the directory where the original index file is located.

  • myisam_mmap_size

    Introduced5.5.1
    Command-Line Format--myisam_mmap_size=#
    System VariableNamemyisam_mmap_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (32-bit platforms)Typeinteger
    Default4294967295
    Min Value7
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default18446744073709547520
    Min Value7
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default18446744073709551615
    Min Value7
    Max Value18446744073709551615

    The maximum amount of memory to use for memory mapping compressed MyISAM files. If many compressed MyISAM tables are used, the value can be decreased to reduce the likelihood of memory-swapping problems. This variable was added in MySQL 5.5.1.

  • myisam_recover_options

    System VariableNamemyisam_recover_options
    Variable ScopeGlobal
    Dynamic VariableNo

    The value of the --myisam-recover-options option. See Section 5.1.3, “Server Command Options”.

  • myisam_repair_threads

    Command-Line Format--myisam_repair_threads=#
    System VariableNamemyisam_repair_threads
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default1
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default1
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default1
    Min Value1
    Max Value18446744073709551615

    If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1.

    Note

    Multi-threaded repair is still beta-quality code.

  • myisam_sort_buffer_size

    Command-Line Format--myisam_sort_buffer_size=#
    System VariableNamemyisam_sort_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (Windows, 32-bit platforms)Typeinteger
    Default8388608
    Min Value4096
    Max Value4294967295
    Permitted Values (Windows, 64-bit platforms, <= 5.5.21)Typeinteger
    Default8388608
    Min Value4096
    Max Value4294967295
    Permitted Values (Windows, 64-bit platforms, >= 5.5.22)Typeinteger
    Default8388608
    Min Value4096
    Max Value18446744073709551615
    Permitted Values (Other, 32-bit platforms)Typeinteger
    Default8388608
    Min Value4096
    Max Value4294967295
    Permitted Values (Other, 64-bit platforms, <= 5.5.2)Typeinteger
    Default8388608
    Min Value4096
    Max Value18446744073709547520
    Permitted Values (Other, 64-bit platforms, >= 5.5.3)Typeinteger
    Default8388608
    Min Value4096
    Max Value18446744073709551615

    The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE.

    The maximum permissible setting for myisam_sort_buffer_size is 4GB−1. Larger values are permitted for 64-bit platforms (except 64-bit Windows prior to MySQL 5.5.222, for which large values are truncated to 4GB−1 with a warning).

  • myisam_stats_method

    Command-Line Format--myisam_stats_method=name
    System VariableNamemyisam_stats_method
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    Defaultnulls_unequal
    Valid Valuesnulls_equal
    nulls_unequal
    nulls_ignored

    How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has three possible values, nulls_equal, nulls_unequal, and nulls_ignored. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1. For nulls_ignored, NULL values are ignored.

    The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 8.3.7, “InnoDB and MyISAM Index Statistics Collection”.

  • myisam_use_mmap

    Command-Line Format--myisam_use_mmap
    System VariableNamemyisam_use_mmap
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Use memory mapping for reading and writing MyISAM tables.

  • named_pipe

    System VariableNamenamed_pipe
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificWindows
    Permitted Values (Windows)Typeboolean
    DefaultOFF

    (Windows only.) Indicates whether the server supports connections over named pipes.

  • net_buffer_length

    Command-Line Format--net_buffer_length=#
    System VariableNamenet_buffer_length
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default16384
    Min Value1024
    Max Value1048576

    Each client thread is associated with a connection buffer and result buffer. Both begin with a size given by net_buffer_length but are dynamically enlarged up to max_allowed_packet bytes as needed. The result buffer shrinks to net_buffer_length after each SQL statement.

    This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of statements sent by clients. If statements exceed this length, the connection buffer is automatically enlarged. The maximum value to which net_buffer_length can be set is 1MB.

    The session value of this variable is read only.

  • net_read_timeout

    Command-Line Format--net_read_timeout=#
    System VariableNamenet_read_timeout
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default30
    Min Value1

    The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout.

  • net_retry_count

    Command-Line Format--net_retry_count=#
    System VariableNamenet_retry_count
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default10
    Min Value1
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default10
    Min Value1
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default10
    Min Value1
    Max Value18446744073709551615

    If a read or write on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads.

  • net_write_timeout

    Command-Line Format--net_write_timeout=#
    System VariableNamenet_write_timeout
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default60
    Min Value1

    The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout.

  • new

    Command-Line Format--new
    System VariableNamenew
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Disabled byskip-new
    Permitted ValuesTypeboolean
    DefaultFALSE

    This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. Its value is always OFF.

    In MySQL Cluster, setting this variable to ON makes it possible to employ partitioning types other than KEY or LINEAR KEY with NDB tables. This feature is experimental only, and not supported in production. For additional information, see User-defined partitioning and the NDB storage engine (MySQL Cluster).

  • old

    Command-Line Format--old
    System VariableNameold
    Variable ScopeGlobal
    Dynamic VariableNo

    old is a compatibility variable. It is disabled by default, but can be enabled at startup to revert the server to behaviors present in older versions.

    When old is enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with no FOR clause apply only to how indexes are used for row retrieval and not to resolution of ORDER BY or GROUP BY clauses. (See Section 8.9.3, “Index Hints”.) Take care about enabling this in a replication setup. With statement-based binary logging, having different modes for the master and slaves might lead to replication errors.

  • old_alter_table

    Command-Line Format--old-alter-table
    System VariableNameold_alter_table
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    When this variable is enabled, the server does not use the optimized method of processing an ALTER TABLE operation. It reverts to using a temporary table, copying over the data, and then renaming the temporary table to the original, as used by MySQL 5.0 and earlier. For more information on the operation of ALTER TABLE, see Section 13.1.7, “ALTER TABLE Syntax”.

  • old_passwords

    System VariableNameold_passwords
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default0

    This variable controls the password hashing method used by the PASSWORD() function. It also influences password hashing performed by CREATE USER and GRANT statements that specify a password using an IDENTIFIED BY clause.

    The following table shows the permitted values of old_passwords, the password hashing method for each value, and which authentication plugins use passwords hashed with each method.

    ValuePassword Hashing MethodAssociated Authentication Plugin
    0 or OFFMySQL 4.1 native hashingmysql_native_password
    1 or ONPre-4.1 (old) hashingmysql_old_password

    If old_passwords=1, PASSWORD(str) returns the same value as OLD_PASSWORD(str). The latter function is not affected by the value of old_passwords.

    For additional information about authentication plugins and hashing formats, see Section 6.3.6, “Pluggable Authentication”, and Section 6.1.2.4, “Password Hashing in MySQL”.

    Note

    Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.

  • one_shot

    This is not a variable, but it can be used when setting some variables. It is described in Section 13.7.4.1, “SET Syntax for Variable Assignment”.

  • open_files_limit

    Command-Line Format--open-files-limit=#
    System VariableNameopen_files_limit
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Valueplatform dependent

    The number of files that the operating system permits mysqld to open. This is the real value permitted by the system and might be different from the value you gave using the --open-files-limit option to mysqld or mysqld_safe. The value is 0 on systems where MySQL cannot change the number of open files.

  • optimizer_prune_level

    Command-Line Format--optimizer_prune_level[=#]
    System VariableNameoptimizer_prune_level
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default1

    Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans.

  • optimizer_search_depth

    Command-Line Format--optimizer_search_depth[=#]
    System VariableNameoptimizer_search_depth
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default62
    Min Value0
    Max Value63

    The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to 63, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. The value of 63 is deprecated and will be treated as invalid in a future MySQL release.

  • optimizer_switch

    Command-Line Format--optimizer_switch=value
    System VariableNameoptimizer_switch
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.2)Typeset
    Valid Valuesindex_merge={on|off}
    index_merge_intersection={on|off}
    index_merge_sort_union={on|off}
    index_merge_union={on|off}
    Permitted Values (>= 5.5.3)Typeset
    Valid Valuesengine_condition_pushdown={on|off}
    index_merge={on|off}
    index_merge_intersection={on|off}
    index_merge_sort_union={on|off}
    index_merge_union={on|off}

    The optimizer_switch system variable enables control over optimizer behavior. The value of this variable is a set of flags, each of which has a value of on or off to indicate whether the corresponding optimizer behavior is enabled or disabled. This variable has global and session values and can be changed at runtime. The global default can be set at server startup.

    To see the current set of optimizer flags, select the variable value:

    mysql> SELECT @@optimizer_switch\G
    *************************** 1. row ***************************
    @@optimizer_switch: index_merge=on,index_merge_union=on,
                        index_merge_sort_union=on,
                        index_merge_intersection=on,
                        engine_condition_pushdown=on
    

    For more information about the syntax of this variable and the optimizer behaviors that it controls, see Section 8.9.2, “Controlling Switchable Optimizations”.

  • performance_schema_xxx

    Performance Schema system variables are listed in Section 22.9, “Performance Schema System Variables”. These variables may be used to configure Performance Schema operation.

  • pid_file

    Command-Line Format--pid-file=file_name
    System VariableNamepid_file
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The path name of the process ID (PID) file. This variable can be set with the --pid-file option.

  • plugin_dir

    Command-Line Format--plugin_dir=dir_name
    System VariableNameplugin_dir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (>= 5.5.5)Typedirectory name
    DefaultBASEDIR/lib/plugin
    Permitted Values (Windows, <= 5.5.4)Typedirectory name
    DefaultBASEDIR/lib/plugin
    Permitted Values (Other, <= 5.5.4)Typedirectory name
    DefaultBASEDIR/lib/mysql/plugin

    The path name of the plugin directory.

    If the plugin directory is writable by the server, it may be possible for a user to write executable code to a file in the directory using SELECT ... INTO DUMPFILE. This can be prevented by making plugin_dir read only to the server or by setting --secure-file-priv to a directory where SELECT writes can be made safely.

  • port

    Command-Line Format--port=#
    System VariableNameport
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default3306
    Min Value0
    Max Value65535

    The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.

  • preload_buffer_size

    Command-Line Format--preload_buffer_size=#
    System VariableNamepreload_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default32768
    Min Value1024
    Max Value1073741824

    The size of the buffer that is allocated when preloading indexes.

  • profiling

    If set to 0 or OFF (the default), statement profiling is disabled. If set to 1 or ON, statement profiling is enabled and the SHOW PROFILE and SHOW PROFILES statements provide access to profiling information. See Section 13.7.5.32, “SHOW PROFILES Syntax”.

  • profiling_history_size

    The number of statements for which to maintain profiling information if profiling is enabled. The default value is 15. The maximum value is 100. Setting the value to 0 effectively disables profiling. See Section 13.7.5.32, “SHOW PROFILES Syntax”.

  • protocol_version

    System VariableNameprotocol_version
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger

    The version of the client/server protocol used by the MySQL server.

  • proxy_user

    Introduced5.5.7
    System VariableNameproxy_user
    Variable ScopeSession
    Dynamic VariableNo
    Permitted ValuesTypestring

    If the current client is a proxy for another user, this variable is the proxy user account name. Otherwise, this variable is NULL. See Section 6.3.7, “Proxy Users”.

    This variable was added in MySQL 5.5.7.

  • pseudo_slave_mode

    Introduced5.5.30
    System VariableNamepseudo_slave_mode
    Variable ScopeSession
    Dynamic VariableYes
    Permitted ValuesTypeinteger

    This variable is for internal server use. It was added in MySQL 5.5.30.

  • pseudo_thread_id

    System VariableNamepseudo_thread_id
    Variable ScopeSession
    Dynamic VariableYes
    Permitted ValuesTypeinteger

    This variable is for internal server use.

  • query_alloc_block_size

    Command-Line Format--query_alloc_block_size=#
    System VariableNamequery_alloc_block_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default8192
    Min Value1024
    Max Value4294967295
    Block Size1024
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default8192
    Min Value1024
    Max Value18446744073709547520
    Block Size1024
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default8192
    Min Value1024
    Max Value18446744073709551615
    Block Size1024

    The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this parameter.

  • query_cache_limit

    Command-Line Format--query_cache_limit=#
    System VariableNamequery_cache_limit
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default1048576
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default1048576
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default1048576
    Min Value0
    Max Value18446744073709551615

    Do not cache results that are larger than this number of bytes. The default value is 1MB.

  • query_cache_min_res_unit

    Command-Line Format--query_cache_min_res_unit=#
    System VariableNamequery_cache_min_res_unit
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default4096
    Min Value512
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default4096
    Min Value512
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default4096
    Min Value512
    Max Value18446744073709551615

    The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 8.10.3.3, “Query Cache Configuration”.

  • query_cache_size

    Command-Line Format--query_cache_size=#
    System VariableNamequery_cache_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default0
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709551615

    The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. To reduce overhead significantly, you should also start the server with query_cache_type=0 if you will not be using the query cache. The permissible values are multiples of 1024; other values are rounded down to the nearest multiple. query_cache_size bytes of memory are allocated even if query_cache_type is set to 0. See Section 8.10.3.3, “Query Cache Configuration”, for more information.

    The query cache needs a minimum size of about 40KB to allocate its structures. (The exact size depends on system architecture.) If you set the value of query_cache_size too small, a warning will occur, as described in Section 8.10.3.3, “Query Cache Configuration”.

  • query_cache_type

    Command-Line Format--query_cache_type=#
    System VariableNamequery_cache_type
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    Default1
    Valid Values0
    1
    2

    Set the query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache. Possible values are shown in the following table.

    OptionDescription
    0 or OFFDo not cache results in or retrieve results from the query cache. Note that this does not deallocate the query cache buffer. To do that, you should set query_cache_size to 0.
    1 or ONCache all cacheable query results except for those that begin with SELECT SQL_NO_CACHE.
    2 or DEMANDCache results only for cacheable queries that begin with SELECT SQL_CACHE.

    This variable defaults to ON.

    If the server is started with query_cache_type set to 0, it does not acquire the query cache mutex at all, which means that the query cache cannot be enabled at runtime and there is reduced overhead in query execution.

  • query_cache_wlock_invalidate

    Command-Line Format--query_cache_wlock_invalidate
    System VariableNamequery_cache_wlock_invalidate
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultFALSE

    Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing statements that read from the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect.

  • query_prealloc_size

    Command-Line Format--query_prealloc_size=#
    System VariableNamequery_prealloc_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default8192
    Min Value8192
    Max Value4294967295
    Block Size1024
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default8192
    Min Value8192
    Max Value18446744073709547520
    Block Size1024
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default8192
    Min Value8192
    Max Value18446744073709551615
    Block Size1024

    The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.

  • rand_seed1

    The rand_seed1 and rand_seed2 variables exist as session variables only, and can be set but not read. The variables—but not their values—are shown in the output of SHOW VARIABLES.

    The purpose of these variables is to support replication of the RAND() function. For statements that invoke RAND(), the master passes two values to the slave, where they are used to seed the random number generator. The slave uses these values to set the session variables rand_seed1 and rand_seed2 so that RAND() on the slave generates the same value as on the master.

  • rand_seed2

    See the description for rand_seed1.

  • range_alloc_block_size

    Command-Line Format--range_alloc_block_size=#
    System VariableNamerange_alloc_block_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (32-bit platforms)Typeinteger
    Default4096
    Min Value4096
    Max Value4294967295
    Block Size1024
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default4096
    Min Value4096
    Max Value18446744073709551615
    Block Size1024

    The size of blocks that are allocated when doing range optimization.

  • read_buffer_size

    Command-Line Format--read_buffer_size=#
    System VariableNameread_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default131072
    Min Value8200
    Max Value2147479552

    Each thread that does a sequential scan for a MyISAM table allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value, which defaults to 131072. The value of this variable should be a multiple of 4KB. If it is set to a value that is not a multiple of 4KB, its value will be rounded down to the nearest multiple of 4KB.

    This option is also used in the following context for all storage engines:

    • For caching the indexes in a temporary file (not a temporary table), when sorting rows for ORDER BY.

    • For bulk insert into partitions.

    • For caching results of nested queries.

    and in one other storage engine-specific way: to determine the memory block size for MEMORY tables.

    The maximum permissible setting for read_buffer_size is 2GB.

    For more information about memory use during different operations, see Section 8.12.5.1, “How MySQL Uses Memory”.

  • read_only

    Command-Line Format--read_only
    System VariableNameread_only
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    When the read_only system variable is enabled, the server permits no client updates except from users who have the SUPER privilege. This variable is disabled by default.

    Even with read_only enabled, the server permits these operations:

    read_only exists only as a GLOBAL variable, so changes to its value require the SUPER privilege. Changes to read_only on a master server are not replicated to slave servers. The value can be set on a slave server independent of the setting on the master.

    The following conditions apply to attempts to enable read_only:

    • The attempt fails and an error occurs if you have any explicit locks (acquired with LOCK TABLES) or have a pending transaction.

    • The attempt blocks while other clients hold explicit table locks or have pending transactions, until the locks are released and the transactions end. While the attempt to enable read_only is pending, requests by other clients for table locks or to begin transactions also block until read_only has been set.

    • As of MySQL 5.5.3, the attempt blocks if there are active transactions that hold metadata locks, until those transactions end.

    • read_only can be enabled while you hold a global read lock (acquired with FLUSH TABLES WITH READ LOCK) because that does not involve table locks.

  • read_rnd_buffer_size

    Command-Line Format--read_rnd_buffer_size=#
    System VariableNameread_rnd_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.2)Typeinteger
    Default262144
    Min Value8200
    Max Value2147483647
    Permitted Values (>= 5.5.3)Typeinteger
    Default262144
    Min Value1
    Max Value2147483647

    When reading rows from a MyISAM table in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. See Section 8.2.1.11, “ORDER BY Optimization”. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries.

    The maximum permissible setting for read_rnd_buffer_size is 2GB.

    For more information about memory use during different operations, see Section 8.12.5.1, “How MySQL Uses Memory”.

  • relay_log_purge

    Command-Line Format--relay_log_purge
    System VariableNamerelay_log_purge
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultTRUE

    Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (ON).

  • relay_log_space_limit

    Command-Line Format--relay_log_space_limit=#
    System VariableNamerelay_log_space_limit
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (32-bit platforms)Typeinteger
    Default0
    Min Value0
    Max Value4294967295
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709547520
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default0
    Min Value0
    Max Value18446744073709551615

    The maximum amount of space to use for all relay logs.

  • report_host

    Command-Line Format--report-host=host_name
    System VariableNamereport_host
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The value of the --report-host option.

  • report_password

    Command-Line Format--report-password=name
    System VariableNamereport_password
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The value of the --report-password option. Not the same as the password used for the MySQL replication user account.

  • report_port

    Command-Line Format--report-port=#
    System VariableNamereport_port
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (<= 5.5.22)Typeinteger
    Default3306
    Min Value0
    Max Value65535
    Permitted Values (>= 5.5.23)Typeinteger
    Default0
    Min Value0
    Max Value65535

    The value of the --report-port option.

  • report_user

    Command-Line Format--report-user=name
    System VariableNamereport_user
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The value of the --report-user option. Not the same as the name for the MySQL replication user account.

  • rpl_semi_sync_master_enabled

    System VariableNamerpl_semi_sync_master_enabled
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Controls whether semisynchronous replication is enabled on the master. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_timeout

    System VariableNamerpl_semi_sync_master_timeout
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default10000

    A value in milliseconds that controls how long the master waits on a commit for acknowledgment from a slave before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_trace_level

    System VariableNamerpl_semi_sync_master_trace_level
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default32

    The semisynchronous replication debug trace level on the master. Four levels are defined:

    • 1 = general level (for example, time function failures)

    • 16 = detail level (more verbose information)

    • 32 = net wait level (more information about network waits)

    • 64 = function level (information about function entry and exit)

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_no_slave

    System VariableNamerpl_semi_sync_master_wait_no_slave
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultON

    With semisynchronous replication, for each transaction, the master waits until timeout for acknowledgment of receipt from some semisynchronous slave. If no response occurs during this period, the master reverts to normal replication. This variable controls whether the master waits for the timeout to expire before reverting to normal replication even if the slave count drops to zero during the timeout period.

    If the value is ON (the default), it is permissible for the slave count to drop to zero during the timeout period (for example, if slaves disconnect). The master still waits for the timeout, so as long as some slave reconnects and acknowledges the transaction within the timeout interval, semisynchronous replication continues.

    If the value is OFF, the master reverts to normal replication if the slave count drops to zero during the timeout period.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_slave_enabled

    System VariableNamerpl_semi_sync_slave_enabled
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Controls whether semisynchronous replication is enabled on the slave. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.

    This variable is available only if the slave-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_slave_trace_level

    System VariableNamerpl_semi_sync_slave_trace_level
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default32

    The semisynchronous replication debug trace level on the slave. See rpl_semi_sync_master_trace_level for the permissible values.

    This variable is available only if the slave-side semisynchronous replication plugin is installed.

  • secure_auth

    Command-Line Format--secure-auth
    System VariableNamesecure_auth
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    If this variable is enabled, the server blocks connections by clients that attempt to use accounts that have passwords stored in the old (pre-4.1) format.

    Enable this variable to prevent all use of passwords employing the old format (and hence insecure communication over the network).

    Server startup fails with an error if this variable is enabled and the privilege tables are in pre-4.1 format. See Section B.5.2.4, “Client does not support authentication protocol”.

    Note

    Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.

  • secure_file_priv

    Command-Line Format--secure-file-priv=dir_name
    System VariableNamesecure_file_priv
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring
    Defaultempty
    Valid Valuesempty
    dirname

    This variable is used to limit the effect of data import and export operations, such as those performed by the LOAD DATA and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. By default, this variable is empty. If set to the name of a directory, it limits import and export operations to work only with files in that directory.

  • server_id

    Command-Line Format--server-id=#
    System VariableNameserver_id
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value4294967295

    The server ID, used in replication to give each master and slave a unique identity. This variable is set by the --server-id option. For each server participating in replication, you should pick a positive integer in the range from 1 to 232 − 1 to act as that server's ID.

  • shared_memory

    Command-Line Format--shared_memory[={0,1}]
    System VariableNameshared_memory
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificWindows
    Permitted ValuesTypeboolean
    DefaultFALSE

    (Windows only.) Whether the server permits shared-memory connections.

  • shared_memory_base_name

    Command-Line Format--shared_memory_base_name=name
    System VariableNameshared_memory_base_name
    Variable ScopeGlobal
    Dynamic VariableNo
    Platform SpecificWindows
    Permitted ValuesTypestring
    DefaultMYSQL

    (Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. The default name is MYSQL. The name is case sensitive.

  • skip_external_locking

    Command-Line Format--skip-external-locking
    System VariableNameskip_external_locking
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeboolean
    DefaultON

    This is OFF if mysqld uses external locking (system locking), ON if external locking is disabled. This affects only MyISAM table access.

    This variable is set by the --external-locking or --skip-external-locking option. External locking is disabled by default.

    External locking affects only MyISAM table access. For more information, including conditions under which it can and cannot be used, see Section 8.11.5, “External Locking”.

  • skip_name_resolve

    Command-Line Format--skip-name-resolve
    System VariableNameskip_name_resolve
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeboolean
    DefaultOFF

    This variable is set from the value of the --skip-name-resolve option. If it is OFF, mysqld resolves host names when checking client connections. If it is ON, mysqld uses only IP numbers; in this case, all Host column values in the grant tables must be IP addresses or localhost. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.

    This variable was added in MySQL 5.5.5.

  • skip_networking

    Command-Line Format--skip-networking
    System VariableNameskip_networking
    Variable ScopeGlobal
    Dynamic VariableNo

    This is ON if the server permits only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe or shared memory. This variable can be set to ON with the --skip-networking option.

  • skip_show_database

    Command-Line Format--skip-show-database
    System VariableNameskip_show_database
    Variable ScopeGlobal
    Dynamic VariableNo

    This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES privilege. This can improve security if you have concerns about users being able to see databases belonging to other users. Its effect depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is permitted only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is permitted to all users, but displays the names of only those databases for which the user has the SHOW DATABASES or other privilege. (Note that any global privilege is considered a privilege for the database.)

  • slow_launch_time

    Command-Line Format--slow_launch_time=#
    System VariableNameslow_launch_time
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default2

    If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable.

  • slow_query_log

    Command-Line Format--slow-query-log
    System VariableNameslow_query_log
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    Whether the slow query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The default value depends on whether the --slow_query_log option is given. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.

    Slow is determined by the value of the long_query_time variable. See Section 5.4.5, “The Slow Query Log”.

  • slow_query_log_file

    Command-Line Format--slow-query-log-file=file_name
    System VariableNameslow_query_log_file
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypefile name
    Defaulthost_name-slow.log

    The name of the slow query log file. The default value is host_name-slow.log, but the initial value can be changed with the --slow_query_log_file option.

  • socket

    Command-Line Format--socket={file_name|pipe_name}
    System VariableNamesocket
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring
    Default/tmp/mysql.sock

    On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)

    On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is MySQL (not case sensitive).

  • sort_buffer_size

    Command-Line Format--sort_buffer_size=#
    System VariableNamesort_buffer_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (Windows)Typeinteger
    Default2097144
    Min Value32768
    Max Value4294967295
    Permitted Values (Other, 32-bit platforms)Typeinteger
    Default2097144
    Min Value32768
    Max Value4294967295
    Permitted Values (Other, 64-bit platforms)Typeinteger
    Default2097144
    Min Value32768
    Max Value18446744073709551615

    Each session that must perform a sort allocates a buffer of this size. sort_buffer_size is not specific to any storage engine and applies in a general manner for optimization. At minimum the sort_buffer_size value must be large enough to accommodate fifteen tuples in the sort buffer. Also, increasing the value of max_sort_length may require increasing the value of sort_buffer_size. For more information, see Section 8.2.1.11, “ORDER BY Optimization”

    If you see many Sort_merge_passes per second in SHOW GLOBAL STATUS output, you can consider increasing the sort_buffer_size value to speed up ORDER BY or GROUP BY operations that cannot be improved with query optimization or improved indexing. The entire buffer is allocated even if it is not all needed, so setting it larger than required globally will slow down most queries that sort. It is best to increase it as a session setting, and only for the sessions that need a larger size. On Linux, there are thresholds of 256KB and 2MB where larger values may significantly slow down memory allocation, so you should consider staying below one of those values. Experiment to find the best value for your workload. See Section B.5.3.5, “Where MySQL Stores Temporary Files”.

    The maximum permissible setting for sort_buffer_size is 4GB−1. Larger values are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB−1 with a warning).

  • sql_auto_is_null

    System Variable (<= 5.5.2)Namesql_auto_is_null
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namesql_auto_is_null
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.2)Typeboolean
    Default1
    Permitted Values (>= 5.5.3)Typeboolean
    Default0

    If this variable is set to 1, then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:

    SELECT * FROM tbl_name WHERE auto_col IS NULL
    

    If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 12.14, “Information Functions”. If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.

    The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison is used by some ODBC programs, such as Access. See Obtaining Auto-Increment Values. This behavior can be disabled by setting sql_auto_is_null to 0.

    The default value of sql_auto_is_null is 0 as of MySQL 5.5.3, and 1 for earlier versions.

  • sql_big_selects

    System Variable (<= 5.5.2)Namesql_big_selects
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namesql_big_selects
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default1

    If set to 0, MySQL aborts SELECT statements that are likely to take a very long time to execute (that is, statements for which the optimizer estimates that the number of examined rows exceeds the value of max_join_size). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is 1, which permits all SELECT statements.

    If you set the max_join_size system variable to a value other than DEFAULT, sql_big_selects is set to 0.

  • sql_buffer_result

    System Variable (<= 5.5.2)Namesql_buffer_result
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namesql_buffer_result
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default0

    If set to 1, sql_buffer_result forces results from SELECT statements to be put into temporary tables. This helps MySQL free the table locks early and can be beneficial in cases where it takes a long time to send results to the client. The default value is 0.

  • sql_log_bin

    System Variable (<= 5.5.2)Namesql_log_bin
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3, <= 5.5.40)Namesql_log_bin
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    System Variable (>= 5.5.41)Namesql_log_bin
    Variable ScopeSession
    Dynamic VariableYes
    Permitted ValuesTypeboolean

    This variable controls whether logging to the binary log is done. The default value is 1 (do logging). To change logging for the current session, change the session value of this variable. The session user must have the SUPER privilege to set this variable.

    Beginning with MySQL 5.5.5, it is no longer possible to set @@session.sql_log_bin within a transaction or subquery. (Bug #53437)

  • sql_log_off

    System Variable (<= 5.5.2)Namesql_log_off
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Namesql_log_off
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default0

    This variable controls whether logging to the general query log is done. The default value is 0 (do logging). To change logging for the current session, change the session value of this variable. The session user must have the SUPER privilege to set this option. The default value is 0.

  • sql_log_update

    Deprecated5.0.0, by sql_log_bin
    Removed5.5.3
    System VariableNamesql_log_update
    Variable ScopeSession
    Dynamic VariableYes
    Permitted ValuesTypeboolean

    This variable is deprecated, and is mapped to sql_log_bin. It was removed in MySQL 5.5.3.

  • sql_mode

    Command-Line Format--sql-mode=name
    System VariableNamesql_mode
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeset
    Default''
    Valid ValuesALLOW_INVALID_DATES
    ANSI_QUOTES
    ERROR_FOR_DIVISION_BY_ZERO
    HIGH_NOT_PRECEDENCE
    IGNORE_SPACE
    NO_AUTO_CREATE_USER
    NO_AUTO_VALUE_ON_ZERO
    NO_BACKSLASH_ESCAPES
    NO_DIR_IN_CREATE
    NO_ENGINE_SUBSTITUTION
    NO_FIELD_OPTIONS
    NO_KEY_OPTIONS
    NO_TABLE_OPTIONS
    NO_UNSIGNED_SUBTRACTION
    NO_ZERO_DATE
    NO_ZERO_IN_DATE
    ONLY_FULL_GROUP_BY
    PAD_CHAR_TO_FULL_LENGTH
    PIPES_AS_CONCAT
    REAL_AS_FLOAT
    STRICT_ALL_TABLES
    STRICT_TRANS_TABLES

    The current server SQL mode, which can be set dynamically. For details, see Section 5.1.7, “Server SQL Modes”.

    Note

    MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.

  • sql_notes

    If set to 1 (the default), warnings of Note level increment warning_count and the server records them. If set to 0, Note warnings do not increment warning_count and the server does not record them. mysqldump includes output to set this variable to 0 so that reloading the dump file does not produce warnings for events that do not affect the integrity of the reload operation.

  • sql_quote_show_create

    If set to 1 (the default), the server quotes identifiers for SHOW CREATE TABLE and SHOW CREATE DATABASE statements. If set to 0, quoting is disabled. This option is enabled by default so that replication works for identifiers that require quoting. See Section 13.7.5.12, “SHOW CREATE TABLE Syntax”, and Section 13.7.5.8, “SHOW CREATE DATABASE Syntax”.

  • sql_safe_updates

    If set to 1, MySQL aborts UPDATE or DELETE statements that do not use a key in the WHERE clause or a LIMIT clause. (Specifically, UPDATE statements must have a WHERE clause that uses a key or a LIMIT clause, or both. DELETE statements must have both.) This makes it possible to catch UPDATE or DELETE statements where keys are not used properly and that would probably change or delete a large number of rows. The default value is 0.

  • sql_select_limit

    System VariableNamesql_select_limit
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger

    The maximum number of rows to return from SELECT statements. The default value for a new connection is the maximum number of rows that the server permits per table. Typical default values are (232)−1 or (264)−1. If you have changed the limit, the default value can be restored by assigning a value of DEFAULT.

    If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of sql_select_limit.

  • sql_warnings

    This variable controls whether single-row INSERT statements produce an information string if warnings occur. The default is 0. Set the value to 1 to produce an information string.

  • ssl_ca

    Command-Line Format--ssl-ca=file_name
    System VariableNamessl_ca
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The path to a file with a list of trusted SSL CAs.

  • ssl_capath

    Command-Line Format--ssl-capath=dir_name
    System VariableNamessl_capath
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The path to a directory that contains trusted SSL CA certificates in PEM format.

  • ssl_cert

    Command-Line Format--ssl-cert=file_name
    System VariableNamessl_cert
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The name of the SSL certificate file to use for establishing a secure connection.

  • ssl_cipher

    Command-Line Format--ssl-cipher=name
    System VariableNamessl_cipher
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    A list of permissible ciphers to use for SSL encryption.

  • ssl_key

    Command-Line Format--ssl-key=file_name
    System VariableNamessl_key
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypefile name

    The name of the SSL key file to use for establishing a secure connection.

  • storage_engine

    System VariableNamestorage_engine
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (<= 5.5.4)Typeenumeration
    DefaultMyISAM
    Permitted Values (>= 5.5.5)Typeenumeration
    DefaultInnoDB

    The default storage engine (table type). To set the storage engine at server startup, use the --default-storage-engine option. See Section 5.1.3, “Server Command Options”.

    This variable is deprecated as of MySQL 5.5.3. Use default_storage_engine instead.

  • stored_program_cache

    Introduced5.5.21
    Command-Line Format--stored-program-cache=#
    System VariableNamestored_program_cache
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default256
    Min Value256
    Max Value524288

    Sets a soft upper limit for the number of cached stored routines per connection. The value of this variable is specified in terms of the number of stored routines held in each of the two caches maintained by the MySQL Server for, respectively, stored procedures and stored functions.

    Whenever a stored routine is executed this cache size is checked before the first or top-level statement in the routine is parsed; if the number of routines of the same type (stored procedures or stored functions according to which is being executed) exceeds the limit specified by this variable, the corresponding cache is flushed and memory previously allocated for cached objects is freed. This allows the cache to be flushed safely, even when there are dependencies between stored routines.

  • sync_frm

    Command-Line Format--sync-frm
    System VariableNamesync_frm
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultTRUE

    If this variable is set to 1, when any nontemporary table is created its .frm file is synchronized to disk (using fdatasync()). This is slower but safer in case of a crash. The default is 1.

  • system_time_zone

    System VariableNamesystem_time_zone
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script.

    The system_time_zone variable differs from time_zone. Although they might have the same value, the latter variable is used to initialize the time zone for each client that connects. See Section 10.6, “MySQL Server Time Zone Support”.

  • table_definition_cache

    System VariableNametable_definition_cache
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default400
    Min Value400
    Max Value524288

    The number of table definitions (from .frm files) that can be stored in the definition cache. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache. The minimum and default values are both 400.

  • table_lock_wait_timeout

    Removed5.5.3
    Command-Line Format--table_lock_wait_timeout=#
    System VariableNametable_lock_wait_timeout
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default50
    Min Value1
    Max Value1073741824

    This variable is unused. It was removed in 5.5.3.

  • table_open_cache

    System VariableNametable_open_cache
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default400
    Min Value1
    Max Value524288

    The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See Section 5.1.6, “Server Status Variables”. If the value of Opened_tables is large and you do not use FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_open_cache variable. For more information about the table cache, see Section 8.4.3.1, “How MySQL Opens and Closes Tables”.

  • table_type

    This variable was removed in MySQL 5.5.3. Use storage_engine instead.

  • thread_cache_size

    Command-Line Format--thread_cache_size=#
    System VariableNamethread_cache_size
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value16384

    How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. Normally, this does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.1.6, “Server Status Variables”.

  • thread_concurrency

    Command-Line Format--thread_concurrency=#
    System VariableNamethread_concurrency
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default10
    Min Value1
    Max Value512

    This variable is specific to Solaris 8 and earlier systems, for which mysqld invokes the thr_setconcurrency() function with the variable value. This function enables applications to give the threads system a hint about the desired number of threads that should be run at the same time. Current Solaris versions document this as having no effect.

  • thread_handling

    Command-Line Format--thread_handling=name
    System VariableNamethread_handling
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (<= 5.5.13)Typeenumeration
    Defaultone-thread-per-connection
    Valid Valuesno-threads
    one-thread-per-connection
    Permitted Values (>= 5.5.16)Typeenumeration
    Defaultone-thread-per-connection
    Valid Valuesno-threads
    one-thread-per-connection
    dynamically-loaded

    The thread-handling model used by the server for connection threads. The permissible user-settable values are no-threads (the server uses a single thread to handle one connection) and one-thread-per-connection (the server uses one thread to handle each client connection; this is the default). no-threads is useful for debugging under Linux; see Section 24.5, “Debugging and Porting MySQL”.

    If the thread pool plugin is enabled, the server sets the thread_handling value to dynamically-loaded. See Section 5.5.4.2, “Thread Pool Installation”.

  • thread_pool_algorithm

    Introduced5.5.16
    Command-Line Format--thread_pool_algorithm=#
    System VariableNamethread_pool_algorithm
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value1

    This variable controls which algorithm the thread pool plugin uses:

    • A value of 0 (the default) uses a conservative low-concurrency algorithm which is most well tested and is known to produce very good results.

    • A value of 1 increases the concurrency and uses a more aggressive algorithm which at times has been known to perform 5–10% better on optimal thread counts, but has degrading performance as the number of connections increases. Its use should be considered as experimental and not supported.

      This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”

  • thread_pool_high_priority_connection

    Introduced5.5.16
    Command-Line Format--thread_pool_high_priority_connection=#
    System VariableNamethread_pool_high_priority_connection
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value1

    This variable affects queuing of new statements prior to execution. If the value is 0 (false, the default), statement queuing uses both the low-priority and high-priority queues. If the value is 1 (true), queued statements always go to the high-priority queue.

    This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”

  • thread_pool_max_unused_threads

    Introduced5.5.16
    Command-Line Format--thread_pool_max_unused_threads=#
    System VariableNamethread_pool_max_unused_threads
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default0
    Min Value0
    Max Value4096

    The maximum permitted number of unused threads in the thread pool. This variable makes it possible to limit the amount of memory used by sleeping threads.

    A value of 0 (the default) means no limit on the number of sleeping threads. A value of N where N is greater than 0 means 1 consumer thread and N−1 reserve threads. In this case, if a thread is ready to sleep but the number of sleeping threads is already at the maximum, the thread exits rather than going to sleep.

    A sleeping thread is either sleeping as a consumer thread or a reserve thread. The thread pool permits one thread to be the consumer thread when sleeping. If a thread goes to sleep and there is no existing consumer thread, it will sleep as a consumer thread. When a thread must be woken up, a consumer thread is selected if there is one. A reserve thread is selected only when there is no consumer thread to wake up.

    This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”

  • thread_pool_prio_kickup_timer

    Introduced5.5.16
    Command-Line Format--thread_pool_prio_kickup_timer=#
    System VariableNamethread_pool_prio_kickup_timer
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default1000
    Min Value0
    Max Value4294967294

    This variable affects statements waiting for execution in the low-priority queue. The value is the number of milliseconds before a waiting statement is moved to the high-priority queue. The default is 1000 (1 second). The range of values is 0 to 232 − 2.

    This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”

  • thread_pool_size

    Introduced5.5.16
    Command-Line Format--thread_pool_size=#
    System VariableNamethread_pool_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypeinteger
    Default16
    Min Value1
    Max Value64

    The number of thread groups in the thread pool. This is the most important parameter controlling thread pool performance. It affects how many statements can execute simultaneously. The default value is 16, with a range from 1 to 64 of permissible values. If a value outside this range is specified, the thread pool plugin does not load and the server writes a message to the error log.

    This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”

  • thread_pool_stall_limit

    Introduced5.5.16
    Command-Line Format--thread_pool_stall_limit=#
    System VariableNamethread_pool_stall_limit
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default6
    Min Value4
    Max Value600

    This variable affects executing statements. The value is the amount of time a statement has to finish after starting to execute before it becomes defined as stalled, at which point the thread pool permits the thread group to begin executing another statement. The value is measured in 10 millisecond units, so a value of 6 (the default) means 60ms. The range of values is 4 to 600 (40ms to 6s). Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.

    This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 5.5.4, “MySQL Enterprise Thread Pool”

  • thread_stack

    Command-Line Format--thread_stack=#
    System VariableNamethread_stack
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted Values (32-bit platforms)Typeinteger
    Default196608
    Min Value131072
    Max Value4294967295
    Block Size1024
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default262144
    Min Value131072
    Max Value18446744073709547520
    Block Size1024
    Permitted Values (64-bit platforms, >= 5.5.3)Typeinteger
    Default262144
    Min Value131072
    Max Value18446744073709551615
    Block Size1024

    The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. See Section 8.13.2, “The MySQL Benchmark Suite”. The default of 192KB (256KB for 64-bit systems) is large enough for normal operation. If the thread stack size is too small, it limits the complexity of the SQL statements that the server can handle, the recursion depth of stored procedures, and other memory-consuming actions.

  • time_format

    This variable is unused.

  • time_zone

    System VariableNametime_zone
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypestring

    The current time zone. This variable is used to initialize the time zone for each client that connects. By default, the initial value of this is 'SYSTEM' (which means, use the value of system_time_zone). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 10.6, “MySQL Server Time Zone Support”.

  • timed_mutexes

    Deprecated5.5.39
    Command-Line Format--timed_mutexes
    System VariableNametimed_mutexes
    Variable ScopeGlobal
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    DefaultOFF

    This variable is deprecated; it has no use. It will be removed in a future MySQL release.

  • timestamp

    System VariableNametimestamp
    Variable ScopeSession
    Dynamic VariableYes
    Permitted ValuesTypenumeric

    Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp (a value like that returned by UNIX_TIMESTAMP(), not a value in 'YYYY-MM-DD hh:mm:ss' format) or DEFAULT.

    Setting timestamp to a constant value causes it to retain that value until it is changed again. Setting timestamp to DEFAULT causes its value to be the current date and time as of the time it is accessed.

    SET timestamp affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE(). The server can be started with the --sysdate-is-now option to cause SYSDATE() to be an alias for NOW(), in which case SET timestamp affects both functions.

  • tmp_table_size

    Command-Line Format--tmp_table_size=#
    System VariableNametmp_table_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeinteger
    Default16777216
    Min Value1024
    Max Value18446744073709551615

    The maximum size of internal in-memory temporary tables. This variable does not apply to user-created MEMORY tables.

    The actual limit is determined from whichever of the values of tmp_table_size and max_heap_table_size is smaller. If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size (and max_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of memory.

    You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.

    See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.

  • tmpdir

    Command-Line Format--tmpdir=dir_name
    System VariableNametmpdir
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypedirectory name

    The directory used for temporary files and temporary tables. This variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (:) on Unix and semicolon characters (;) on Windows.

    The multiple-directory feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based file system or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails. You can set the slave's temporary directory using the slave_load_tmpdir variable. In that case, the slave will not use the general tmpdir value and you can set tmpdir to a nonpermanent location.

  • transaction_alloc_block_size

    Command-Line Format--transaction_alloc_block_size=#
    System VariableNametransaction_alloc_block_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (>= 5.5.43)Typeinteger
    Default8192
    Min Value1024
    Max Value131072
    Block Size1024
    Permitted Values (32-bit platforms, <= 5.5.42)Typeinteger
    Default8192
    Min Value1024
    Max Value4294967295
    Block Size1024
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default8192
    Min Value1024
    Max Value18446744073709547520
    Block Size1024
    Permitted Values (64-bit platforms, >= 5.5.3, <= 5.5.42)Typeinteger
    Default8192
    Min Value1024
    Max Value18446744073709551615
    Block Size1024

    The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size.

  • transaction_prealloc_size

    Command-Line Format--transaction_prealloc_size=#
    System VariableNametransaction_prealloc_size
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (>= 5.5.43)Typeinteger
    Default4096
    Min Value1024
    Max Value131072
    Block Size1024
    Permitted Values (32-bit platforms, <= 5.5.42)Typeinteger
    Default4096
    Min Value1024
    Max Value4294967295
    Block Size1024
    Permitted Values (64-bit platforms, <= 5.5.2)Typeinteger
    Default4096
    Min Value1024
    Max Value18446744073709547520
    Block Size1024
    Permitted Values (64-bit platforms, >= 5.5.3, <= 5.5.42)Typeinteger
    Default4096
    Min Value1024
    Max Value18446744073709551615
    Block Size1024

    There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is transaction_prealloc_size. For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by transaction_alloc_block_size bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size bytes.

    By making transaction_prealloc_size sufficiently large to contain all statements within a single transaction, you can avoid many malloc() calls.

  • tx_isolation

    System VariableNametx_isolation
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeenumeration
    DefaultREPEATABLE-READ
    Valid ValuesREAD-UNCOMMITTED
    READ-COMMITTED
    REPEATABLE-READ
    SERIALIZABLE

    The default transaction isolation level. Defaults to REPEATABLE-READ.

    This variable can be set directly, or indirectly using the SET TRANSACTION statement. See Section 13.3.6, “SET TRANSACTION Syntax”. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotation marks, with the space replaced by a dash. For example:

    SET tx_isolation = 'READ-COMMITTED';
    

    Any unique prefix of a valid value may be used to set the value of this variable.

    The default transaction isolation level can also be set at startup using the --transaction-isolation server option.

  • unique_checks

    System Variable (<= 5.5.2)Nameunique_checks
    Variable ScopeSession
    Dynamic VariableYes
    System Variable (>= 5.5.3)Nameunique_checks
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default1

    If set to 1 (the default), uniqueness checks for secondary indexes in InnoDB tables are performed. If set to 0, storage engines are permitted to assume that duplicate keys are not present in input data. If you know for certain that your data does not contain uniqueness violations, you can set this to 0 to speed up large table imports to InnoDB.

    Setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still permitted to check for them and issue duplicate-key errors if it detects them.

  • updatable_views_with_limit

    Command-Line Format--updatable_views_with_limit=#
    System VariableNameupdatable_views_with_limit
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted ValuesTypeboolean
    Default1

    This variable controls whether updates to a view can be made when the view does not contain all columns of the primary key defined in the underlying table, if the update statement contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL.

    The variable can have two values:

    • 1 or YES: Issue a warning only (not an error message). This is the default value.

    • 0 or NO: Prohibit the update.

  • version

    The version number for the server. The value might also include a suffix indicating server build or configuration information. -log indicates that one or more of the general log, slow query log, or binary log are enabled. -debug indicates that the server was built with debugging support enabled.

  • version_comment

    System VariableNameversion_comment
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The CMake configuration program has a COMPILATION_COMMENT option that permits a comment to be specified when building MySQL. This variable contains the value of that comment. See Section 2.9.4, “MySQL Source-Configuration Options”.

  • version_compile_machine

    System VariableNameversion_compile_machine
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The type of machine or architecture on which MySQL was built.

  • version_compile_os

    System VariableNameversion_compile_os
    Variable ScopeGlobal
    Dynamic VariableNo
    Permitted ValuesTypestring

    The type of operating system on which MySQL was built.

  • wait_timeout

    Command-Line Format--wait_timeout=#
    System VariableNamewait_timeout
    Variable ScopeGlobal, Session
    Dynamic VariableYes
    Permitted Values (Windows)Typeinteger
    Default28800
    Min Value1
    Max Value2147483
    Permitted Values (Other)Typeinteger
    Default28800
    Min Value1
    Max Value31536000

    The number of seconds the server waits for activity on a noninteractive connection before closing it.

    On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

  • warning_count

    The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.41, “SHOW WARNINGS Syntax”.

5.1.5 Using System Variables

The MySQL server maintains many system variables that indicate how it is configured. Section 5.1.4, “Server System Variables”, describes the meaning of these variables. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can also use system variable values in expressions.

There are two scopes in which system variables exist. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:

  • When the server starts, it initializes each global variable to its default value. These defaults can be changed by options specified on the command line or in an option file. (See Section 4.2.3, “Specifying Program Options”.)

  • The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For example, a client's SQL mode is controlled by the session sql_mode value, which is initialized when the client connects to the value of the global sql_mode value.

    For some system variables, the session value is not initialized from the corresponding global value; if so, that is indicated in the variable description.

System variable values can be set globally at server startup by using options on the command line or in an option file. When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of K, M, or G (either uppercase or lowercase) to indicate a multiplier of 1024, 10242 or 10243; that is, units of kilobytes, megabytes, or gigabytes, respectively. Thus, the following command starts the server with a query cache size of 16 megabytes and a maximum packet size of one gigabyte:

mysqld --query_cache_size=16M --max_allowed_packet=1G

Within an option file, those variables are set like this:

[mysqld]
query_cache_size=16M
max_allowed_packet=1G

The lettercase of suffix letters does not matter; 16M and 16m are equivalent, as are 1G and 1g.

To restrict the maximum value to which a system variable can be set at runtime with the SET statement, specify this maximum by using an option of the form --maximum-var_name=value at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M.

Many system variables are dynamic and can be changed at runtime by using the SET statement. For a list, see Section 5.1.5.2, “Dynamic System Variables”. To change a system variable with SET, refer to it by name, optionally preceded by a modifier:

  • To indicate that a variable is a global variable, precede its name by the GLOBAL keyword or the @@global. qualifier:

    SET GLOBAL max_connections = 1000;
    SET @@global.max_connections = 1000;
    

    The SUPER privilege is required to set global variables.

  • To indicate that a variable is a session variable, precede its name by the SESSION keyword or either the @@session. or @@ qualifier:

    SET SESSION sql_mode = 'TRADITIONAL';
    SET @@session.sql_mode = 'TRADITIONAL';
    SET @@sql_mode = 'TRADITIONAL';
    

    Setting a session variable normally requires no special privilege, although there are exceptions that require the SUPER privilege (such as sql_log_bin). A client can change its own session variables, but not those of any other client.

  • LOCAL and @@local. are synonyms for SESSION and @@session..

  • If no modifier is present, SET changes the session variable. If the variable has no session value, an error occurs.

    mysql> SET max_connections = 1000;
    ERROR 1229 (HY000): Variable 'max_connections' is a
    GLOBAL variable and should be set with SET GLOBAL
    
  • An error occurs if you:

    • Use SET GLOBAL (or @@global.) when setting a variable that has only a session value

    • Omit GLOBAL (or @@global.) or PERSIST (or @@persist.) when setting a variable that has only a global value

    • Use SET SESSION (or @@SESSION.) when setting a variable that has only a global value

The preceding modifiers apply only to system variables. It is not permitted to apply them to user-defined variables, stored procedure or function parameters, or stored program local variables. An error occurs for such attempts.

A SET statement can contain multiple variable assignments, separated by commas. This statement assigns values to a user-defined variable and a system variable:

SET @x = 1, SESSION sql_mode = '';

If you set multiple system variables, the most recent GLOBAL or SESSION modifier in the statement is used for following assignments that have no modifier specified.

Examples of multiple-variable assignment:

SET GLOBAL sort_buffer_size = 1000000, SESSION sort_buffer_size = 1000000;
SET @@global.sort_buffer_size = 1000000, @@local.sort_buffer_size = 1000000;
SET GLOBAL max_connections = 1000, sort_buffer_size = 1000000;

If any variable assignment in a SET statement fails, the entire statement fails and no variables are changed.

If you change a session system variable, the value remains in effect within your session until you change the variable to a different value or the session ends. The change has no effect on other sessions.

If you change a global system variable, the value is remembered and used for new connections until you change the variable to a different value or the server restarts. The change is visible to any client that accesses the global variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any client that is currently connected (not even the session within which the SET GLOBAL statement occurred).

To make a global system variable setting permanent so that it applies across server restarts, you should also set it in an option file.

To set a GLOBAL value to the compiled-in MySQL default value or a SESSION variable to the current corresponding GLOBAL value, set the variable to the value DEFAULT. For example, the following two statements are identical in setting the session value of max_join_size to the current global value:

SET @@session.max_join_size=DEFAULT;
SET @@session.max_join_size=@@global.max_join_size;

Not all system variables can be set to DEFAULT. In such cases, assigning DEFAULT results in an error.

It is not permitted to assign DEFAULT to user-defined variables, and not supported for stored procedure or function parameters or stored program local variables. This results in an error for user-defined variables, and the results are undefined for parameters or local variables.

To refer to the value of a system variable in expressions, use one of the @@-modifiers. For example, you can retrieve values in a SELECT statement like this:

SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;

When you refer to a system variable in an expression as @@var_name (that is, when you do not specify @@global. or @@session.), MySQL returns the session value if it exists and the global value otherwise. This differs from SET @@var_name = value, which always refers to the session value.

Suffixes for specifying a value multiplier can be used when setting a variable at server startup, but not to set the value with SET at runtime. On the other hand, with SET you can assign a variable's value using an expression, which is not true when you set a variable at server startup. For example, the first of the following lines is legal at server startup, but the second is not:

shell> mysql --max_allowed_packet=16M
shell> mysql --max_allowed_packet=16*1024*1024

Conversely, the second of the following lines is legal at runtime, but the first is not:

mysql> SET GLOBAL max_allowed_packet=16M;
mysql> SET GLOBAL max_allowed_packet=16*1024*1024;
Note

Some system variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.

To display system variable names and values, use the SHOW VARIABLES statement:

mysql> SHOW VARIABLES;
+---------------------------------+-----------------------------------+
| Variable_name                   | Value                             |
+---------------------------------+-----------------------------------+
| auto_increment_increment        | 1                                 |
| auto_increment_offset           | 1                                 |
| automatic_sp_privileges         | ON                                |
| back_log                        | 50                                |
| basedir                         | /home/mysql/                      |
| binlog_cache_size               | 32768                             |
| bulk_insert_buffer_size         | 8388608                           |
| character_set_client            | latin1                            |
| character_set_connection        | latin1                            |
| character_set_database          | latin1                            |
| character_set_results           | latin1                            |
| character_set_server            | latin1                            |
| character_set_system            | utf8                              |
| character_sets_dir              | /home/mysql/share/mysql/charsets/ |
| collation_connection            | latin1_swedish_ci                 |
| collation_database              | latin1_swedish_ci                 |
| collation_server                | latin1_swedish_ci                 |
...
| innodb_additional_mem_pool_size | 1048576                           |
| innodb_autoextend_increment     | 8                                 |
| innodb_buffer_pool_size         | 8388608                           |
| innodb_checksums                | ON                                |
| innodb_commit_concurrency       | 0                                 |
| innodb_concurrency_tickets      | 500                               |
| innodb_data_file_path           | ibdata1:10M:autoextend            |
| innodb_data_home_dir            |                                   |
...
| version                         | 5.1.6-alpha-log                   |
| version_comment                 | Source distribution               |
| version_compile_machine         | i686                              |
| version_compile_os              | suse-linux                        |
| wait_timeout                    | 28800                             |
+---------------------------------+-----------------------------------+

With a LIKE clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a LIKE clause as shown:

SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';

To get a list of variables whose name match a pattern, use the % wildcard character in a LIKE clause:

SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';

Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because _ is a wildcard that matches any single character, you should escape it as \_ to match it literally. In practice, this is rarely necessary.

For SHOW VARIABLES, if you specify neither GLOBAL nor SESSION, MySQL returns SESSION values.

The reason for requiring the GLOBAL keyword when setting GLOBAL-only variables but not when retrieving them is to prevent problems in the future. If we were to remove a SESSION variable that has the same name as a GLOBAL variable, a client with the SUPER privilege might accidentally change the GLOBAL variable rather than just the SESSION variable for its own connection. If we add a SESSION variable with the same name as a GLOBAL variable, a client that intends to change the GLOBAL variable might find only its own SESSION variable changed.

5.1.5.1 Structured System Variables

A structured variable differs from a regular system variable in two respects:

  • Its value is a structure with components that specify server parameters considered to be closely related.

  • There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.

MySQL supports one structured variable type, which specifies parameters governing the operation of key caches. A key cache structured variable has these components:

This section describes the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 8.10.2, “The MyISAM Key Cache”.

To refer to a component of a structured variable instance, you can use a compound name in instance_name.component_name format. Examples:

hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size

For each structured system variable, an instance with the name of default is always predefined. If you refer to a component of a structured variable without any instance name, the default instance is used. Thus, default.key_buffer_size and key_buffer_size both refer to the same system variable.

Structured variable instances and components follow these naming rules:

  • For a given type of structured variable, each instance must have a name that is unique within variables of that type. However, instance names need not be unique across structured variable types. For example, each structured variable has an instance named default, so default is not unique across variable types.

  • The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.

  • If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, hot-cache is not legal, but `hot-cache` is.

  • global, session, and local are not legal instance names. This avoids a conflict with notation such as @@global.var_name for referring to nonstructured system variables.

Currently, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.

With one exception, you can refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:

shell> mysqld --hot_cache.key_buffer_size=64K

In an option file, use this syntax:

[mysqld]
hot_cache.key_buffer_size=64K

If you start the server with this option, it creates a key cache named hot_cache with a size of 64KB in addition to the default key cache that has a default size of 8MB.

Suppose that you start the server as follows:

shell> mysqld --key_buffer_size=256K \
         --extra_cache.key_buffer_size=128K \
         --extra_cache.key_cache_block_size=2048

In this case, the server sets the size of the default key cache to 256KB. (You could also have written --default.key_buffer_size=256K.) In addition, the server creates a second key cache named extra_cache that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.

The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:

shell> mysqld --key_buffer_size=6M \
         --hot_cache.key_buffer_size=2M \
         --cold_cache.key_buffer_size=2M

Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named hot_cache to a size of 10MB, use either of these statements:

mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@global.hot_cache.key_buffer_size = 10*1024*1024;

To retrieve the cache size, do this:

mysql> SELECT @@global.hot_cache.key_buffer_size;

However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a LIKE pattern-matching operation:

mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';

This is the exception to being able to use structured variable names anywhere a simple variable name may occur.

5.1.5.2 Dynamic System Variables

Many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also obtain their values using SELECT. See Section 5.1.5, “Using System Variables”.

The following table shows the full list of all dynamic system variables. The last column indicates for each variable whether GLOBAL or SESSION (or both) apply. The table also lists session options that can be set with the SET statement. Section 5.1.4, “Server System Variables”, discusses these options.

Variables that have a type of string take a string value. Variables that have a type of numeric take a numeric value. Variables that have a type of boolean can be set to 0, 1, ON or OFF. (If you set them on the command line or in an option file, use the numeric values.) Variables that are marked as enumeration normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumerated system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1.

Table 5.3 Dynamic Variable Summary

Variable NameVariable TypeVariable Scope
audit_log_flushbooleanGLOBAL
audit_log_policyenumerationGLOBAL
audit_log_rotate_on_sizeintegerGLOBAL
auto_increment_incrementintegerGLOBAL | SESSION
auto_increment_offsetintegerGLOBAL | SESSION
autocommitbooleanGLOBAL | SESSION
automatic_sp_privilegesbooleanGLOBAL
big_tablesbooleanGLOBAL | SESSION
binlog_cache_sizeintegerGLOBAL
binlog_direct_non_transactional_updatesbooleanGLOBAL | SESSION
binlog_formatenumerationGLOBAL | SESSION
binlog_stmt_cache_sizeintegerGLOBAL
bulk_insert_buffer_sizeintegerGLOBAL | SESSION
character_set_clientstringGLOBAL | SESSION
character_set_connectionstringGLOBAL | SESSION
character_set_databasestringGLOBAL | SESSION
character_set_filesystemstringGLOBAL | SESSION
character_set_resultsstringGLOBAL | SESSION
character_set_serverstringGLOBAL | SESSION
collation_connectionstringGLOBAL | SESSION
collation_databasestringGLOBAL | SESSION
collation_serverstringGLOBAL | SESSION
completion_typeenumerationGLOBAL | SESSION
concurrent_insertenumerationGLOBAL
connect_timeoutintegerGLOBAL
debugstringGLOBAL | SESSION
debug_syncstringSESSION
default_storage_engineenumerationGLOBAL | SESSION
default_week_formatintegerGLOBAL | SESSION
delay_key_writeenumerationGLOBAL
delayed_insert_limitintegerGLOBAL
delayed_insert_timeoutintegerGLOBAL
delayed_queue_sizeintegerGLOBAL
div_precision_incrementintegerGLOBAL | SESSION
engine_condition_pushdownbooleanGLOBAL | SESSION
event_schedulerenumerationGLOBAL
expire_logs_daysintegerGLOBAL
flushbooleanGLOBAL
flush_timeintegerGLOBAL
foreign_key_checksbooleanGLOBAL | SESSION
ft_boolean_syntaxstringGLOBAL
general_logbooleanGLOBAL
general_log_filefilenameGLOBAL
group_concat_max_lenintegerGLOBAL | SESSION
identityintegerSESSION
init_connectstringGLOBAL
init_slavestringGLOBAL
innodb_adaptive_flushingbooleanGLOBAL
innodb_adaptive_hash_indexbooleanGLOBAL
innodb_autoextend_incrementintegerGLOBAL
innodb_change_bufferingenumerationGLOBAL
innodb_change_buffering_debugintegerGLOBAL
innodb_commit_concurrencyintegerGLOBAL
innodb_concurrency_ticketsintegerGLOBAL
innodb_fast_shutdownintegerGLOBAL
innodb_file_formatstringGLOBAL
innodb_file_format_checkbooleanGLOBAL
innodb_file_format_maxstringGLOBAL
innodb_file_per_tablebooleanGLOBAL
innodb_flush_log_at_trx_commitenumerationGLOBAL
innodb_io_capacityintegerGLOBAL
innodb_large_prefixbooleanGLOBAL
innodb_limit_optimistic_insert_debugintegerGLOBAL
innodb_lock_wait_timeoutintegerGLOBAL | SESSION
innodb_max_dirty_pages_pctnumericGLOBAL
innodb_max_purge_lagintegerGLOBAL
innodb_old_blocks_pctintegerGLOBAL
innodb_old_blocks_timeintegerGLOBAL
innodb_print_all_deadlocksbooleanGLOBAL
innodb_purge_batch_sizeintegerGLOBAL
innodb_random_read_aheadbooleanGLOBAL
innodb_read_ahead_thresholdintegerGLOBAL
innodb_replication_delayintegerGLOBAL
innodb_rollback_segmentsintegerGLOBAL
innodb_spin_wait_delayintegerGLOBAL
innodb_stats_methodenumerationGLOBAL
innodb_stats_on_metadatabooleanGLOBAL
innodb_stats_sample_pagesintegerGLOBAL
innodb_strict_modebooleanGLOBAL | SESSION
innodb_support_xabooleanGLOBAL | SESSION
innodb_sync_spin_loopsintegerGLOBAL
innodb_table_locksbooleanGLOBAL | SESSION
innodb_thread_concurrencyintegerGLOBAL
innodb_thread_sleep_delayintegerGLOBAL
innodb_trx_purge_view_update_only_debugbooleanGLOBAL
innodb_trx_rseg_n_slots_debugintegerGLOBAL
insert_idintegerSESSION
interactive_timeoutintegerGLOBAL | SESSION
join_buffer_sizeintegerGLOBAL | SESSION
keep_files_on_createbooleanGLOBAL | SESSION
key_buffer_sizeintegerGLOBAL
key_cache_age_thresholdintegerGLOBAL
key_cache_block_sizeintegerGLOBAL
key_cache_division_limitintegerGLOBAL
last_insert_idintegerSESSION
lc_messagesstringGLOBAL | SESSION
lc_time_namesstringGLOBAL | SESSION
local_infilebooleanGLOBAL
lock_wait_timeoutintegerGLOBAL | SESSION
logfilenameGLOBAL
log_bin_trust_function_creatorsbooleanGLOBAL
log_bin_trust_routine_creatorsbooleanGLOBAL
log_outputsetGLOBAL
log_queries_not_using_indexesbooleanGLOBAL
log_slow_queriesbooleanGLOBAL
log_warningsintegerGLOBAL | SESSION
long_query_timenumericGLOBAL | SESSION
low_priority_updatesbooleanGLOBAL | SESSION
max_allowed_packetintegerGLOBAL | SESSION
max_binlog_cache_sizeintegerGLOBAL
max_binlog_sizeintegerGLOBAL
max_binlog_stmt_cache_sizeintegerGLOBAL
max_connect_errorsintegerGLOBAL
max_connectionsintegerGLOBAL
max_delayed_threadsintegerGLOBAL | SESSION
max_error_countintegerGLOBAL | SESSION
max_heap_table_sizeintegerGLOBAL | SESSION
max_insert_delayed_threadsintegerGLOBAL | SESSION
max_join_sizeintegerGLOBAL | SESSION
max_length_for_sort_dataintegerGLOBAL | SESSION
max_prepared_stmt_countintegerGLOBAL
max_relay_log_sizeintegerGLOBAL
max_seeks_for_keyintegerGLOBAL | SESSION
max_sort_lengthintegerGLOBAL | SESSION
max_sp_recursion_depthintegerGLOBAL | SESSION
max_tmp_tablesintegerGLOBAL | SESSION
max_user_connectionsintegerGLOBAL | SESSION
max_write_lock_countintegerGLOBAL
min_examined_row_limitintegerGLOBAL | SESSION
multi_range_countintegerGLOBAL | SESSION
myisam_data_pointer_sizeintegerGLOBAL
myisam_max_sort_file_sizeintegerGLOBAL
myisam_repair_threadsintegerGLOBAL | SESSION
myisam_sort_buffer_sizeintegerGLOBAL | SESSION
myisam_stats_methodenumerationGLOBAL | SESSION
myisam_use_mmapbooleanGLOBAL
ndb_autoincrement_prefetch_szintegerGLOBAL | SESSION
ndb_blob_read_batch_bytesintegerGLOBAL | SESSION
ndb_blob_write_batch_bytesintegerGLOBAL | SESSION
ndb_cache_check_timeintegerGLOBAL
ndb_deferred_constraintsintegerGLOBAL | SESSION
ndb_deferred_constraintsintegerGLOBAL | SESSION
ndb_distributionenumerationGLOBAL
ndb_distribution={KEYHASH|LINHASH}enumerationGLOBAL
ndb_eventbuffer_max_allocintegerGLOBAL
ndb_extra_loggingintegerGLOBAL
ndb_force_sendbooleanGLOBAL | SESSION
ndb_index_stat_cache_entriesintegerGLOBAL | SESSION
ndb_index_stat_enablebooleanGLOBAL | SESSION
ndb_index_stat_optionstringGLOBAL | SESSION
ndb_index_stat_update_freqintegerGLOBAL | SESSION
ndb_join_pushdownbooleanGLOBAL | SESSION
ndb_log_binbooleanGLOBAL | SESSION
ndb_log_binlog_indexbooleanGLOBAL
ndb_log_empty_epochsbooleanGLOBAL
ndb_log_empty_epochsbooleanGLOBAL
ndb_log_update_as_writebooleanGLOBAL
ndb_log_updated_onlybooleanGLOBAL
ndb_optimization_delayintegerGLOBAL
ndb_report_thresh_binlog_epoch_slipintegerGLOBAL
ndb_report_thresh_binlog_mem_usageintegerGLOBAL
ndb_table_no_loggingbooleanSESSION
ndb_table_temporarybooleanSESSION
ndb_use_exact_countbooleanGLOBAL | SESSION
ndb_use_transactionsbooleanGLOBAL | SESSION
ndbinfo_max_bytesintegerGLOBAL | SESSION
ndbinfo_max_rowsintegerGLOBAL | SESSION
ndbinfo_offlinebooleanGLOBAL
ndbinfo_show_hiddenbooleanGLOBAL | SESSION
ndbinfo_table_prefixstringGLOBAL | SESSION
net_buffer_lengthintegerGLOBAL | SESSION
net_read_timeoutintegerGLOBAL | SESSION
net_retry_countintegerGLOBAL | SESSION
net_write_timeoutintegerGLOBAL | SESSION
newbooleanGLOBAL | SESSION
old_alter_tablebooleanGLOBAL | SESSION
old_passwordsbooleanGLOBAL | SESSION
optimizer_prune_levelbooleanGLOBAL | SESSION
optimizer_search_depthintegerGLOBAL | SESSION
optimizer_switchsetGLOBAL | SESSION
preload_buffer_sizeintegerGLOBAL | SESSION
profilingbooleanGLOBAL | SESSION
profiling_history_sizeintegerGLOBAL | SESSION
pseudo_slave_modeintegerSESSION
pseudo_thread_idintegerSESSION
query_alloc_block_sizeintegerGLOBAL | SESSION
query_cache_limitintegerGLOBAL
query_cache_min_res_unitintegerGLOBAL
query_cache_sizeintegerGLOBAL
query_cache_typeenumerationGLOBAL | SESSION
query_cache_wlock_invalidatebooleanGLOBAL | SESSION
query_prealloc_sizeintegerGLOBAL | SESSION
rand_seed1integerSESSION
rand_seed2integerSESSION
range_alloc_block_sizeintegerGLOBAL | SESSION
read_buffer_sizeintegerGLOBAL | SESSION
read_onlybooleanGLOBAL
read_rnd_buffer_sizeintegerGLOBAL | SESSION
relay_log_purgebooleanGLOBAL
relay_log_recoverybooleanGLOBAL
rpl_recovery_rankintegerGLOBAL
rpl_semi_sync_master_enabledbooleanGLOBAL
rpl_semi_sync_master_timeoutintegerGLOBAL
rpl_semi_sync_master_trace_levelintegerGLOBAL
rpl_semi_sync_master_wait_no_slavebooleanGLOBAL
rpl_semi_sync_slave_enabledbooleanGLOBAL
rpl_semi_sync_slave_trace_levelintegerGLOBAL
secure_authbooleanGLOBAL
server_idintegerGLOBAL
slave_allow_batchingbooleanGLOBAL
slave_compressed_protocolbooleanGLOBAL
slave_exec_modeenumerationGLOBAL
slave_max_allowed_packetintegerGLOBAL
slave_net_timeoutintegerGLOBAL
slave_transaction_retriesintegerGLOBAL
slow_launch_timeintegerGLOBAL
slow_query_logbooleanGLOBAL
slow_query_log_filefilenameGLOBAL
sort_buffer_sizeintegerGLOBAL | SESSION
sql_auto_is_nullbooleanGLOBAL | SESSION
sql_big_selectsbooleanGLOBAL | SESSION
sql_big_tablesbooleanGLOBAL | SESSION
sql_buffer_resultbooleanGLOBAL | SESSION
sql_log_binbooleanSESSION
sql_log_offbooleanGLOBAL | SESSION
sql_log_updatebooleanSESSION
sql_low_priority_updatesbooleanGLOBAL | SESSION
sql_max_join_sizeintegerGLOBAL | SESSION
sql_modesetGLOBAL | SESSION
sql_notesbooleanGLOBAL | SESSION
sql_quote_show_createbooleanGLOBAL | SESSION
sql_safe_updatesbooleanGLOBAL | SESSION
sql_select_limitintegerGLOBAL | SESSION
sql_slave_skip_counterintegerGLOBAL
sql_warningsbooleanGLOBAL | SESSION
storage_engineenumerationGLOBAL | SESSION
stored_program_cacheintegerGLOBAL
sync_binlogintegerGLOBAL
sync_frmbooleanGLOBAL
sync_master_infointegerGLOBAL
sync_relay_logintegerGLOBAL
sync_relay_log_infointegerGLOBAL
table_definition_cacheintegerGLOBAL
table_lock_wait_timeoutintegerGLOBAL
table_open_cacheintegerGLOBAL
table_typeenumerationGLOBAL | SESSION
thread_cache_sizeintegerGLOBAL
thread_pool_high_priority_connectionintegerGLOBAL | SESSION
thread_pool_max_unused_threadsintegerGLOBAL
thread_pool_prio_kickup_timerintegerGLOBAL | SESSION
thread_pool_stall_limitintegerGLOBAL
time_zonestringGLOBAL | SESSION
timed_mutexesbooleanGLOBAL
timestampnumericSESSION
tmp_table_sizeintegerGLOBAL | SESSION
transaction_alloc_block_sizeintegerGLOBAL | SESSION
transaction_allow_batchingbooleanSESSION
transaction_prealloc_sizeintegerGLOBAL | SESSION
tx_isolationenumerationGLOBAL | SESSION
unique_checksbooleanGLOBAL | SESSION
updatable_views_with_limitbooleanGLOBAL | SESSION
wait_timeoutintegerGLOBAL | SESSION

5.1.6 Server Status Variables

The MySQL server maintains many status variables that provide information about its operation. You can view these variables and their values by using the SHOW [GLOBAL | SESSION] STATUS statement (see Section 13.7.5.36, “SHOW STATUS Syntax”). The optional GLOBAL keyword aggregates the values over all connections, and SESSION shows the values for the current connection.

mysql> SHOW GLOBAL STATUS;
+-----------------------------------+------------+
| Variable_name                     | Value      |
+-----------------------------------+------------+
| Aborted_clients                   | 0          |
| Aborted_connects                  | 0          |
| Bytes_received                    | 155372598  |
| Bytes_sent                        | 1176560426 |
...
| Connections                       | 30023      |
| Created_tmp_disk_tables           | 0          |
| Created_tmp_files                 | 3          |
| Created_tmp_tables                | 2          |
...
| Threads_created                   | 217        |
| Threads_running                   | 88         |
| Uptime                            | 1389872    |
+-----------------------------------+------------+

Several status variables provide statement counts. To determine the number of statements executed, use these relationships:

  SUM(Com_xxx) + Qcache_hits
= Questions + statements executed within stored programs
= Queries

Many status variables are reset to 0 by the FLUSH STATUS statement.

The following table lists all available server status variables:

Table 5.4 Status Variable Summary

Variable NameVariable TypeVariable Scope
Aborted_clientsintegerGLOBAL
Aborted_connectsintegerGLOBAL
Binlog_cache_disk_useintegerGLOBAL
Binlog_cache_useintegerGLOBAL
Binlog_stmt_cache_disk_useintegerGLOBAL
Binlog_stmt_cache_useintegerGLOBAL
Bytes_receivedintegerGLOBAL | SESSION
Bytes_sentintegerGLOBAL | SESSION
Com_admin_commandsintegerGLOBAL | SESSION
Com_alter_dbintegerGLOBAL | SESSION
Com_alter_db_upgradeintegerGLOBAL | SESSION
Com_alter_eventintegerGLOBAL | SESSION
Com_alter_functionintegerGLOBAL | SESSION
Com_alter_procedureintegerGLOBAL | SESSION
Com_alter_serverintegerGLOBAL | SESSION
Com_alter_tableintegerGLOBAL | SESSION
Com_alter_tablespaceintegerGLOBAL | SESSION
Com_analyzeintegerGLOBAL | SESSION
Com_assign_to_keycacheintegerGLOBAL | SESSION
Com_beginintegerGLOBAL | SESSION
Com_binlogintegerGLOBAL | SESSION
Com_call_procedureintegerGLOBAL | SESSION
Com_change_dbintegerGLOBAL | SESSION
Com_change_masterintegerGLOBAL | SESSION
Com_checkintegerGLOBAL | SESSION
Com_checksumintegerGLOBAL | SESSION
Com_commitintegerGLOBAL | SESSION
Com_create_dbintegerGLOBAL | SESSION
Com_create_eventintegerGLOBAL | SESSION
Com_create_functionintegerGLOBAL | SESSION
Com_create_indexintegerGLOBAL | SESSION
Com_create_procedureintegerGLOBAL | SESSION
Com_create_serverintegerGLOBAL | SESSION
Com_create_tableintegerGLOBAL | SESSION
Com_create_triggerintegerGLOBAL | SESSION
Com_create_udfintegerGLOBAL | SESSION
Com_create_userintegerGLOBAL | SESSION
Com_create_viewintegerGLOBAL | SESSION
Com_dealloc_sqlintegerGLOBAL | SESSION
Com_deleteintegerGLOBAL | SESSION
Com_delete_multiintegerGLOBAL | SESSION
Com_dointegerGLOBAL | SESSION
Com_drop_dbintegerGLOBAL | SESSION
Com_drop_eventintegerGLOBAL | SESSION
Com_drop_functionintegerGLOBAL | SESSION
Com_drop_indexintegerGLOBAL | SESSION
Com_drop_procedureintegerGLOBAL | SESSION
Com_drop_serverintegerGLOBAL | SESSION
Com_drop_tableintegerGLOBAL | SESSION
Com_drop_triggerintegerGLOBAL | SESSION
Com_drop_userintegerGLOBAL | SESSION
Com_drop_viewintegerGLOBAL | SESSION
Com_empty_queryintegerGLOBAL | SESSION
Com_execute_sqlintegerGLOBAL | SESSION
Com_flushintegerGLOBAL | SESSION
Com_grantintegerGLOBAL | SESSION
Com_ha_closeintegerGLOBAL | SESSION
Com_ha_openintegerGLOBAL | SESSION
Com_ha_readintegerGLOBAL | SESSION
Com_helpintegerGLOBAL | SESSION
Com_insertintegerGLOBAL | SESSION
Com_insert_selectintegerGLOBAL | SESSION
Com_install_pluginintegerGLOBAL | SESSION
Com_killintegerGLOBAL | SESSION
Com_loadintegerGLOBAL | SESSION
Com_lock_tablesintegerGLOBAL | SESSION
Com_optimizeintegerGLOBAL | SESSION
Com_preload_keysintegerGLOBAL | SESSION
Com_prepare_sqlintegerGLOBAL | SESSION
Com_purgeintegerGLOBAL | SESSION
Com_purge_before_dateintegerGLOBAL | SESSION
Com_release_savepointintegerGLOBAL | SESSION
Com_rename_tableintegerGLOBAL | SESSION
Com_rename_userintegerGLOBAL | SESSION
Com_repairintegerGLOBAL | SESSION
Com_replaceintegerGLOBAL | SESSION
Com_replace_selectintegerGLOBAL | SESSION
Com_resetintegerGLOBAL | SESSION
Com_resignalintegerGLOBAL | SESSION
Com_revokeintegerGLOBAL | SESSION
Com_revoke_allintegerGLOBAL | SESSION
Com_rollbackintegerGLOBAL | SESSION
Com_rollback_to_savepointintegerGLOBAL | SESSION
Com_savepointintegerGLOBAL | SESSION
Com_selectintegerGLOBAL | SESSION
Com_set_optionintegerGLOBAL | SESSION
Com_show_authorsintegerGLOBAL | SESSION
Com_show_binlog_eventsintegerGLOBAL | SESSION
Com_show_binlogsintegerGLOBAL | SESSION
Com_show_charsetsintegerGLOBAL | SESSION
Com_show_collationsintegerGLOBAL | SESSION
Com_show_contributorsintegerGLOBAL | SESSION
Com_show_create_dbintegerGLOBAL | SESSION
Com_show_create_eventintegerGLOBAL | SESSION
Com_show_create_funcintegerGLOBAL | SESSION
Com_show_create_procintegerGLOBAL | SESSION
Com_show_create_tableintegerGLOBAL | SESSION
Com_show_create_triggerintegerGLOBAL | SESSION
Com_show_databasesintegerGLOBAL | SESSION
Com_show_engine_logsintegerGLOBAL | SESSION
Com_show_engine_mutexintegerGLOBAL | SESSION
Com_show_engine_statusintegerGLOBAL | SESSION
Com_show_errorsintegerGLOBAL | SESSION
Com_show_eventsintegerGLOBAL | SESSION
Com_show_fieldsintegerGLOBAL | SESSION
Com_show_function_codeintegerGLOBAL | SESSION
Com_show_function_statusintegerGLOBAL | SESSION
Com_show_grantsintegerGLOBAL | SESSION
Com_show_keysintegerGLOBAL | SESSION
Com_show_master_statusintegerGLOBAL | SESSION
Com_show_ndb_statusintegerGLOBAL | SESSION
Com_show_new_masterintegerGLOBAL | SESSION
Com_show_open_tablesintegerGLOBAL | SESSION
Com_show_pluginsintegerGLOBAL | SESSION
Com_show_privilegesintegerGLOBAL | SESSION
Com_show_procedure_codeintegerGLOBAL | SESSION
Com_show_procedure_statusintegerGLOBAL | SESSION
Com_show_processlistintegerGLOBAL | SESSION
Com_show_profileintegerGLOBAL | SESSION
Com_show_profilesintegerGLOBAL | SESSION
Com_show_relaylog_eventsintegerGLOBAL | SESSION
Com_show_slave_hostsintegerGLOBAL | SESSION
Com_show_slave_statusintegerGLOBAL | SESSION
Com_show_statusintegerGLOBAL | SESSION
Com_show_storage_enginesintegerGLOBAL | SESSION
Com_show_table_statusintegerGLOBAL | SESSION
Com_show_tablesintegerGLOBAL | SESSION
Com_show_triggersintegerGLOBAL | SESSION
Com_show_variablesintegerGLOBAL | SESSION
Com_show_warningsintegerGLOBAL | SESSION
Com_signalintegerGLOBAL | SESSION
Com_slave_startintegerGLOBAL | SESSION
Com_slave_stopintegerGLOBAL | SESSION
Com_stmt_closeintegerGLOBAL | SESSION
Com_stmt_executeintegerGLOBAL | SESSION
Com_stmt_fetchintegerGLOBAL | SESSION
Com_stmt_prepareintegerGLOBAL | SESSION
Com_stmt_reprepareintegerGLOBAL | SESSION
Com_stmt_resetintegerGLOBAL | SESSION
Com_stmt_send_long_dataintegerGLOBAL | SESSION
Com_truncateintegerGLOBAL | SESSION
Com_uninstall_pluginintegerGLOBAL | SESSION
Com_unlock_tablesintegerGLOBAL | SESSION
Com_updateintegerGLOBAL | SESSION
Com_update_multiintegerGLOBAL | SESSION
Com_xa_commitintegerGLOBAL | SESSION
Com_xa_endintegerGLOBAL | SESSION
Com_xa_prepareintegerGLOBAL | SESSION
Com_xa_recoverintegerGLOBAL | SESSION
Com_xa_rollbackintegerGLOBAL | SESSION
Com_xa_startintegerGLOBAL | SESSION
CompressionintegerSESSION
ConnectionsintegerGLOBAL
Created_tmp_disk_tablesintegerGLOBAL | SESSION
Created_tmp_filesintegerGLOBAL
Created_tmp_tablesintegerGLOBAL | SESSION
Delayed_errorsintegerGLOBAL
Delayed_insert_threadsintegerGLOBAL
Delayed_writesintegerGLOBAL
Flush_commandsintegerGLOBAL
Handler_commitintegerGLOBAL | SESSION
Handler_deleteintegerGLOBAL | SESSION
Handler_discoverintegerGLOBAL | SESSION
Handler_prepareintegerGLOBAL | SESSION
Handler_read_firstintegerGLOBAL | SESSION
Handler_read_keyintegerGLOBAL | SESSION
Handler_read_lastintegerGLOBAL | SESSION
Handler_read_nextintegerGLOBAL | SESSION
Handler_read_previntegerGLOBAL | SESSION
Handler_read_rndintegerGLOBAL | SESSION
Handler_read_rnd_nextintegerGLOBAL | SESSION
Handler_rollbackintegerGLOBAL | SESSION
Handler_savepointintegerGLOBAL | SESSION
Handler_savepoint_rollbackintegerGLOBAL | SESSION
Handler_updateintegerGLOBAL | SESSION
Handler_writeintegerGLOBAL | SESSION
Innodb_buffer_pool_bytes_dataintegerGLOBAL
Innodb_buffer_pool_bytes_dirtyintegerGLOBAL
Innodb_buffer_pool_pages_dataintegerGLOBAL
Innodb_buffer_pool_pages_dirtyintegerGLOBAL
Innodb_buffer_pool_pages_flushedintegerGLOBAL
Innodb_buffer_pool_pages_freeintegerGLOBAL
Innodb_buffer_pool_pages_latchedintegerGLOBAL
Innodb_buffer_pool_pages_miscintegerGLOBAL
Innodb_buffer_pool_pages_totalintegerGLOBAL
Innodb_buffer_pool_read_aheadintegerGLOBAL
Innodb_buffer_pool_read_ahead_evictedintegerGLOBAL
Innodb_buffer_pool_read_ahead_rndintegerGLOBAL
Innodb_buffer_pool_read_requestsintegerGLOBAL
Innodb_buffer_pool_readsintegerGLOBAL
Innodb_buffer_pool_wait_freeintegerGLOBAL
Innodb_buffer_pool_write_requestsintegerGLOBAL
Innodb_data_fsyncsintegerGLOBAL
Innodb_data_pending_fsyncsintegerGLOBAL
Innodb_data_pending_readsintegerGLOBAL
Innodb_data_pending_writesintegerGLOBAL
Innodb_data_readintegerGLOBAL
Innodb_data_readsintegerGLOBAL
Innodb_data_writesintegerGLOBAL
Innodb_data_writtenintegerGLOBAL
Innodb_dblwr_pages_writtenintegerGLOBAL
Innodb_dblwr_writesintegerGLOBAL
Innodb_have_atomic_builtinsintegerGLOBAL
Innodb_log_waitsintegerGLOBAL
Innodb_log_write_requestsintegerGLOBAL
Innodb_log_writesintegerGLOBAL
Innodb_os_log_fsyncsintegerGLOBAL
Innodb_os_log_pending_fsyncsintegerGLOBAL
Innodb_os_log_pending_writesintegerGLOBAL
Innodb_os_log_writtenintegerGLOBAL
Innodb_page_sizeintegerGLOBAL
Innodb_pages_createdintegerGLOBAL
Innodb_pages_readintegerGLOBAL
Innodb_pages_writtenintegerGLOBAL
Innodb_row_lock_current_waitsintegerGLOBAL
Innodb_row_lock_timeintegerGLOBAL
Innodb_row_lock_time_avgintegerGLOBAL
Innodb_row_lock_time_maxintegerGLOBAL
Innodb_row_lock_waitsintegerGLOBAL
Innodb_rows_deletedintegerGLOBAL
Innodb_rows_insertedintegerGLOBAL
Innodb_rows_readintegerGLOBAL
Innodb_rows_updatedintegerGLOBAL
Innodb_truncated_status_writesintegerGLOBAL
Key_blocks_not_flushedintegerGLOBAL
Key_blocks_unusedintegerGLOBAL
Key_blocks_usedintegerGLOBAL
Key_read_requestsintegerGLOBAL
Key_readsintegerGLOBAL
Key_write_requestsintegerGLOBAL
Key_writesintegerGLOBAL
Last_query_costnumericSESSION
Max_used_connectionsintegerGLOBAL
Ndb_api_bytes_received_countintegerGLOBAL
Ndb_api_bytes_received_count_sessionintegerSESSION
Ndb_api_bytes_received_count_slaveintegerGLOBAL
Ndb_api_bytes_sent_countintegerGLOBAL
Ndb_api_bytes_sent_count_sessionintegerSESSION
Ndb_api_bytes_sent_count_slaveintegerGLOBAL
Ndb_api_event_bytes_countintegerGLOBAL
Ndb_api_event_bytes_count_injectorintegerGLOBAL
Ndb_api_event_data_countintegerGLOBAL
Ndb_api_event_data_count_injectorintegerGLOBAL
Ndb_api_event_nondata_countintegerGLOBAL
Ndb_api_event_nondata_count_injectorintegerGLOBAL
Ndb_api_pk_op_countintegerGLOBAL
Ndb_api_pk_op_count_sessionintegerSESSION
Ndb_api_pk_op_count_slaveintegerGLOBAL
Ndb_api_pruned_scan_countintegerGLOBAL
Ndb_api_pruned_scan_count_sessionintegerSESSION
Ndb_api_pruned_scan_count_slaveintegerGLOBAL
Ndb_api_range_scan_countintegerGLOBAL
Ndb_api_range_scan_count_sessionintegerSESSION
Ndb_api_range_scan_count_slaveintegerGLOBAL
Ndb_api_read_row_countintegerGLOBAL
Ndb_api_read_row_count_sessionintegerSESSION
Ndb_api_read_row_count_slaveintegerGLOBAL
Ndb_api_scan_batch_countintegerGLOBAL
Ndb_api_scan_batch_count_sessionintegerSESSION
Ndb_api_scan_batch_count_slaveintegerGLOBAL
Ndb_api_table_scan_countintegerGLOBAL
Ndb_api_table_scan_count_sessionintegerSESSION
Ndb_api_table_scan_count_slaveintegerGLOBAL
Ndb_api_trans_abort_countintegerGLOBAL
Ndb_api_trans_abort_count_sessionintegerSESSION
Ndb_api_trans_abort_count_slaveintegerGLOBAL
Ndb_api_trans_close_countintegerGLOBAL
Ndb_api_trans_close_count_sessionintegerSESSION
Ndb_api_trans_close_count_slaveintegerGLOBAL
Ndb_api_trans_commit_countintegerGLOBAL
Ndb_api_trans_commit_count_sessionintegerSESSION
Ndb_api_trans_commit_count_slaveintegerGLOBAL
Ndb_api_trans_local_read_row_countintegerGLOBAL
Ndb_api_trans_local_read_row_count_sessionintegerSESSION
Ndb_api_trans_local_read_row_count_slaveintegerGLOBAL
Ndb_api_trans_start_countintegerGLOBAL
Ndb_api_trans_start_count_sessionintegerSESSION
Ndb_api_trans_start_count_slaveintegerGLOBAL
Ndb_api_uk_op_countintegerGLOBAL
Ndb_api_uk_op_count_sessionintegerSESSION
Ndb_api_uk_op_count_slaveintegerGLOBAL
Ndb_api_wait_exec_complete_countintegerGLOBAL
Ndb_api_wait_exec_complete_count_sessionintegerSESSION
Ndb_api_wait_exec_complete_count_slaveintegerGLOBAL
Ndb_api_wait_meta_request_countintegerGLOBAL
Ndb_api_wait_meta_request_count_sessionintegerSESSION
Ndb_api_wait_meta_request_count_slaveintegerGLOBAL
Ndb_api_wait_nanos_countintegerGLOBAL
Ndb_api_wait_nanos_count_sessionintegerSESSION
Ndb_api_wait_nanos_count_slaveintegerGLOBAL
Ndb_api_wait_scan_result_countintegerGLOBAL
Ndb_api_wait_scan_result_count_sessionintegerSESSION
Ndb_api_wait_scan_result_count_slaveintegerGLOBAL
Ndb_cluster_node_idintegerGLOBAL | SESSION
Ndb_config_from_hostintegerGLOBAL | SESSION
Ndb_config_from_portintegerGLOBAL | SESSION
Ndb_conflict_fn_epochintegerGLOBAL
Ndb_conflict_fn_epoch_transintegerGLOBAL
Ndb_conflict_fn_maxintegerGLOBAL
Ndb_conflict_fn_oldintegerGLOBAL
Ndb_conflict_trans_conflict_commit_countintegerGLOBAL
Ndb_conflict_trans_detect_iter_countintegerGLOBAL
Ndb_conflict_trans_reject_countintegerGLOBAL
Ndb_conflict_trans_row_reject_countintegerGLOBAL
Ndb_execute_countintegerGLOBAL
Ndb_cluster_node_idintegerGLOBAL
Ndb_number_of_data_nodesintegerGLOBAL
Ndb_pruned_scan_countintegerGLOBAL
Ndb_pushed_queries_definedintegerGLOBAL
Ndb_pushed_queries_droppedintegerGLOBAL
Ndb_pushed_queries_executedintegerGLOBAL
Ndb_pushed_readsintegerGLOBAL
Ndb_scan_countintegerGLOBAL
Not_flushed_delayed_rowsintegerGLOBAL
Open_filesintegerGLOBAL
Open_streamsintegerGLOBAL
Open_table_definitionsintegerGLOBAL
Open_tablesintegerGLOBAL | SESSION
Opened_filesintegerGLOBAL
Opened_table_definitionsintegerGLOBAL | SESSION
Opened_tablesintegerGLOBAL | SESSION
Performance_schema_cond_classes_lostintegerGLOBAL
Performance_schema_cond_instances_lostintegerGLOBAL
Performance_schema_file_classes_lostintegerGLOBAL
Performance_schema_file_handles_lostintegerGLOBAL
Performance_schema_file_instances_lostintegerGLOBAL
Performance_schema_locker_lostintegerGLOBAL
Performance_schema_mutex_classes_lostintegerGLOBAL
Performance_schema_mutex_instances_lostintegerGLOBAL
Performance_schema_rwlock_classes_lostintegerGLOBAL
Performance_schema_rwlock_instances_lostintegerGLOBAL
Performance_schema_table_handles_lostintegerGLOBAL
Performance_schema_table_instances_lostintegerGLOBAL
Performance_schema_thread_classes_lostintegerGLOBAL
Performance_schema_thread_instances_lostintegerGLOBAL
Prepared_stmt_countintegerGLOBAL
Qcache_free_blocksintegerGLOBAL
Qcache_free_memoryintegerGLOBAL
Qcache_hitsintegerGLOBAL
Qcache_insertsintegerGLOBAL
Qcache_lowmem_prunesintegerGLOBAL
Qcache_not_cachedintegerGLOBAL
Qcache_queries_in_cacheintegerGLOBAL
Qcache_total_blocksintegerGLOBAL
QueriesintegerGLOBAL | SESSION
QuestionsintegerGLOBAL | SESSION
Rpl_semi_sync_master_clientsintegerGLOBAL
Rpl_semi_sync_master_net_avg_wait_timeintegerGLOBAL
Rpl_semi_sync_master_net_wait_timeintegerGLOBAL
Rpl_semi_sync_master_net_waitsintegerGLOBAL
Rpl_semi_sync_master_no_timesintegerGLOBAL
Rpl_semi_sync_master_no_txintegerGLOBAL
Rpl_semi_sync_master_statusbooleanGLOBAL
Rpl_semi_sync_master_timefunc_failuresintegerGLOBAL
Rpl_semi_sync_master_tx_avg_wait_timeintegerGLOBAL
Rpl_semi_sync_master_tx_wait_timeintegerGLOBAL
Rpl_semi_sync_master_tx_waitsintegerGLOBAL
Rpl_semi_sync_master_wait_pos_backtraverseintegerGLOBAL
Rpl_semi_sync_master_wait_sessionsintegerGLOBAL
Rpl_semi_sync_master_yes_txintegerGLOBAL
Rpl_semi_sync_slave_statusbooleanGLOBAL
Rpl_statusstringGLOBAL
Select_full_joinintegerGLOBAL | SESSION
Select_full_range_joinintegerGLOBAL | SESSION
Select_rangeintegerGLOBAL | SESSION
Select_range_checkintegerGLOBAL | SESSION
Select_scanintegerGLOBAL | SESSION
Slave_heartbeat_periodnumericGLOBAL
Slave_open_temp_tablesintegerGLOBAL
Slave_received_heartbeats GLOBAL
Slave_retried_transactionsintegerGLOBAL
Slave_runningbooleanGLOBAL
Slow_launch_threadsintegerGLOBAL | SESSION
Slow_queriesintegerGLOBAL | SESSION
Sort_merge_passesintegerGLOBAL | SESSION
Sort_rangeintegerGLOBAL | SESSION
Sort_rowsintegerGLOBAL | SESSION
Sort_scanintegerGLOBAL | SESSION
Ssl_accept_renegotiatesintegerGLOBAL
Ssl_acceptsintegerGLOBAL
Ssl_callback_cache_hitsintegerGLOBAL
Ssl_cipherstringGLOBAL | SESSION
Ssl_cipher_liststringGLOBAL | SESSION
Ssl_client_connectsintegerGLOBAL
Ssl_connect_renegotiatesintegerGLOBAL
Ssl_ctx_verify_depthintegerGLOBAL
Ssl_ctx_verify_modeintegerGLOBAL
Ssl_default_timeoutintegerGLOBAL | SESSION
Ssl_finished_acceptsintegerGLOBAL
Ssl_finished_connectsintegerGLOBAL
Ssl_session_cache_hitsintegerGLOBAL
Ssl_session_cache_missesintegerGLOBAL
Ssl_session_cache_modestringGLOBAL
Ssl_session_cache_overflowsintegerGLOBAL
Ssl_session_cache_sizeintegerGLOBAL
Ssl_session_cache_timeoutsintegerGLOBAL
Ssl_sessions_reusedintegerGLOBAL | SESSION
Ssl_used_session_cache_entriesintegerGLOBAL
Ssl_verify_depthintegerGLOBAL | SESSION
Ssl_verify_modeintegerGLOBAL | SESSION
Ssl_versionstringGLOBAL | SESSION
Table_locks_immediateintegerGLOBAL
Table_locks_waitedintegerGLOBAL
Tc_log_max_pages_usedintegerGLOBAL
Tc_log_page_sizeintegerGLOBAL
Tc_log_page_waitsintegerGLOBAL
Threads_cachedintegerGLOBAL
Threads_connectedintegerGLOBAL
Threads_createdintegerGLOBAL
Threads_runningintegerGLOBAL
UptimeintegerGLOBAL
Uptime_since_flush_statusintegerGLOBAL

The status variables have the following meanings. For meanings of status variables specific to MySQL Cluster, see Section 18.3.3.8.3, “MySQL Cluster Status Variables”.

  • Aborted_clients

    The number of connections that were aborted because the client died without closing the connection properly. See Section B.5.2.11, “Communication Errors and Aborted Connections”.

  • Aborted_connects

    The number of failed attempts to connect to the MySQL server. See Section B.5.2.11, “Communication Errors and Aborted Connections”.

  • Binlog_cache_disk_use

    The number of transactions that used the binary log cache but that exceeded the value of binlog_cache_size and used a temporary file to store changes from the transaction.

    In MySQL versions 5.5.3 through 5.5.8, this variable also included the number of nontransactional statements that caused the binary log transaction cache to be written to disk. Beginning with MySQL 5.5.9, the number of such nontransactional statements is tracked separately in the Binlog_stmt_cache_disk_use status variable.

  • Binlog_cache_use

    The number of transactions that used the binary log cache.

  • Binlog_stmt_cache_disk_use

    The number of nontransaction statements that used the binary log statement cache but that exceeded the value of binlog_stmt_cache_size and used a temporary file to store those statements.

  • Binlog_stmt_cache_use

    The number of nontransactional statements that used the binary log statement cache.

  • Bytes_received

    The number of bytes received from all clients.

  • Bytes_sent

    The number of bytes sent to all clients.

  • Com_xxx

    The Com_xxx statement counter variables indicate the number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_update count DELETE and UPDATE statements, respectively. Com_delete_multi and Com_update_multi are similar but apply to DELETE and UPDATE statements that use multiple-table syntax.

    If a query result is returned from query cache, the server increments the Qcache_hits status variable, not Com_select. See Section 8.10.3.4, “Query Cache Status and Maintenance”.

    The discussion at the beginning of this section indicates how to relate these statement-counting status variables to other such variables.

    All of the Com_stmt_xxx variables are increased even if a prepared statement argument is unknown or an error occurred during execution. In other words, their values correspond to the number of requests issued, not to the number of requests successfully completed.

    The Com_stmt_xxx status variables are as follows:

    • Com_stmt_prepare

    • Com_stmt_execute

    • Com_stmt_fetch

    • Com_stmt_send_long_data

    • Com_stmt_reset

    • Com_stmt_close

    Those variables stand for prepared statement commands. Their names refer to the COM_xxx command set used in the network layer. In other words, their values increase whenever prepared statement API calls such as mysql_stmt_prepare(), mysql_stmt_execute(), and so forth are executed. However, Com_stmt_prepare, Com_stmt_execute and Com_stmt_close also increase for PREPARE, EXECUTE, or DEALLOCATE PREPARE, respectively. Additionally, the values of the older statement counter variables Com_prepare_sql, Com_execute_sql, and Com_dealloc_sql increase for the PREPARE, EXECUTE, and DEALLOCATE PREPARE statements. Com_stmt_fetch stands for the total number of network round-trips issued when fetching from cursors.

    Com_stmt_reprepare indicates the number of times statements were automatically reprepared by the server after metadata changes to tables or views referred to by the statement. A reprepare operation increments Com_stmt_reprepare, and also Com_stmt_prepare.

  • Compression

    Whether the client connection uses compression in the client/server protocol.

  • Connections

    The number of connection attempts (successful or not) to the MySQL server.

  • Created_tmp_disk_tables

    The number of internal on-disk temporary tables created by the server while executing statements.

    If an internal temporary table is created initially as an in-memory table but becomes too large, MySQL automatically converts it to an on-disk table. The maximum size for in-memory temporary tables is the minimum of the tmp_table_size and max_heap_table_size values. If Created_tmp_disk_tables is large, you may want to increase the tmp_table_size or max_heap_table_size value to lessen the likelihood that internal temporary tables in memory will be converted to on-disk tables.

    You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.

    See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.

  • Created_tmp_files

    How many temporary files mysqld has created.

  • Created_tmp_tables

    The number of internal temporary tables created by the server while executing statements.

    You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.

    See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.

    Each invocation of the SHOW STATUS statement uses an internal temporary table and increments the global Created_tmp_tables value.

  • Delayed_errors

    The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).

  • Delayed_insert_threads

    The number of INSERT DELAYED handler threads in use.

  • Delayed_writes

    The number of INSERT DELAYED rows written.

  • Flush_commands

    The number of times the server flushes tables, whether because a user executed a FLUSH TABLES statement or due to internal server operation. It is also incremented by receipt of a COM_REFRESH packet. This is in contrast to Com_flush, which indicates how many FLUSH statements have been executed, whether FLUSH TABLES, FLUSH LOGS, and so forth.

  • Handler_commit

    The number of internal COMMIT statements.

  • Handler_delete

    The number of times that rows have been deleted from tables.

  • Handler_prepare

    A counter for the prepare phase of two-phase commit operations.

  • Handler_read_first

    The number of times the first entry in an index was read. If this value is high, it suggests that the server is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming that col1 is indexed.

  • Handler_read_key

    The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.

  • Handler_read_last

    The number of requests to read the last key in an index. With ORDER BY, the server will issue a first-key request followed by several next-key requests, whereas with ORDER BY DESC, the server will issue a last-key request followed by several previous-key requests. This variable was added in MySQL 5.5.7.

  • Handler_read_next

    The number of requests to read the next row in key order. This value is incremented if you are querying an index column with a range constraint or if you are doing an index scan.

  • Handler_read_prev

    The number of requests to read the previous row in key order. This read method is mainly used to optimize ORDER BY ... DESC.

  • Handler_read_rnd

    The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that do not use keys properly.

  • Handler_read_rnd_next

    The number of requests to read the next row in the data file. This value is high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.

  • Handler_rollback

    The number of requests for a storage engine to perform a rollback operation.

  • Handler_savepoint

    The number of requests for a storage engine to place a savepoint.

  • Handler_savepoint_rollback

    The number of requests for a storage engine to roll back to a savepoint.

  • Handler_update

    The number of requests to update a row in a table.

  • Handler_write

    The number of requests to insert a row in a table.

  • Innodb_buffer_pool_bytes_data

    The total number of bytes in the InnoDB buffer pool containing data. The number includes both dirty and clean pages. For more accurate memory usage calculations than with Innodb_buffer_pool_pages_data, when compressed tables cause the buffer pool to hold pages of different sizes.

  • Innodb_buffer_pool_pages_data

    The number of pages in the InnoDB buffer pool containing data. The number includes both dirty and clean pages. When using compressed tables, the reported Innodb_buffer_pool_pages_data value may be larger than Innodb_buffer_pool_pages_total (Bug #59550).

  • Innodb_buffer_pool_bytes_dirty

    The total current number of bytes held in dirty pages in the InnoDB buffer pool. For more accurate memory usage calculations than with Innodb_buffer_pool_pages_dirty, when compressed tables cause the buffer pool to hold pages of different sizes.

  • Innodb_buffer_pool_pages_dirty

    The number of pages currently dirty.

  • Innodb_buffer_pool_pages_flushed

    The number of buffer pool page-flush requests.

  • Innodb_buffer_pool_pages_free

    The number of free pages.

  • Innodb_buffer_pool_pages_latched

    The number of latched pages in InnoDB buffer pool. These are pages currently being read or written or that cannot be flushed or removed for some other reason. Calculation of this variable is expensive, so it is available only when the UNIV_DEBUG system is defined at server build time.

  • Innodb_buffer_pool_pages_misc

    The number of pages that are busy because they have been allocated for administrative overhead such as row locks or the adaptive hash index. This value can also be calculated as Innodb_buffer_pool_pages_totalInnodb_buffer_pool_pages_freeInnodb_buffer_pool_pages_data. When using compressed tables, Innodb_buffer_pool_pages_misc may report an out-of-bounds value (Bug #59550).

  • Innodb_buffer_pool_pages_total

    The total size of the InnoDB buffer pool, in pages. When using compressed tables, the reported Innodb_buffer_pool_pages_data value may be larger than Innodb_buffer_pool_pages_total (Bug #59550)

  • Innodb_buffer_pool_read_ahead

    The number of pages read into the InnoDB buffer pool by the read-ahead background thread.

  • Innodb_buffer_pool_read_ahead_evicted

    The number of pages read into the InnoDB buffer pool by the read-ahead background thread that were subsequently evicted without having been accessed by queries.

  • Innodb_buffer_pool_read_ahead_rnd

    The number of random read-aheads initiated by InnoDB. This happens when a query scans a large portion of a table but in random order.

  • Innodb_buffer_pool_read_requests

    The number of logical read requests.

  • Innodb_buffer_pool_reads

    The number of logical reads that InnoDB could not satisfy from the buffer pool, and had to read directly from the disk.

  • Innodb_buffer_pool_wait_free

    Normally, writes to the InnoDB buffer pool happen in the background. However, if it is necessary to read or create a page and no clean pages are available, it is also necessary to wait for pages to be flushed first. This counter counts instances of these waits. If the buffer pool size has been set properly, this value should be small.

  • Innodb_buffer_pool_write_requests

    The number writes done to the InnoDB buffer pool.

  • Innodb_data_fsyncs

    The number of fsync() operations so far.

  • Innodb_data_pending_fsyncs

    The current number of pending fsync() operations.

  • Innodb_data_pending_reads

    The current number of pending reads.

  • Innodb_data_pending_writes

    The current number of pending writes.

  • Innodb_data_read

    The amount of data read since the server was started (in bytes).

  • Innodb_data_reads

    The total number of data reads (OS file reads).

  • Innodb_data_writes

    The total number of data writes.

  • Innodb_data_written

    The amount of data written so far, in bytes.

  • Innodb_dblwr_pages_written

    The number of pages that have been written for doublewrite operations. See Section 14.15.1, “InnoDB Disk I/O”.

  • Innodb_dblwr_writes

    The number of doublewrite operations that have been performed. See Section 14.15.1, “InnoDB Disk I/O”.

  • Innodb_have_atomic_builtins

    Indicates whether the server was built with atomic instructions.

  • Innodb_log_waits

    The number of times that the log buffer was too small and a wait was required for it to be flushed before continuing.

  • Innodb_log_write_requests

    The number of log write requests.

  • Innodb_log_writes

    The number of physical writes to the log file.

  • Innodb_os_log_fsyncs

    The number of fsync() writes done to the log file.

  • Innodb_os_log_pending_fsyncs

    The number of pending log file fsync() operations.

  • Innodb_os_log_pending_writes

    The number of pending log file writes.

  • Innodb_os_log_written

    The number of bytes written to the log file.

  • Innodb_page_size

    The compiled-in InnoDB page size (default 16KB). Many values are counted in pages; the page size enables them to be easily converted to bytes.

  • Innodb_pages_created

    The number of pages created.

  • Innodb_pages_read

    The number of pages read from the InnoDB buffer pool by operations on InnoDB tables.

  • Innodb_pages_written

    The number of pages written.

  • Innodb_row_lock_current_waits

    The number of row locks currently being waited for.

  • Innodb_row_lock_time

    The total time spent in acquiring row locks, in milliseconds.

  • Innodb_row_lock_time_avg

    The average time to acquire a row lock, in milliseconds.

  • Innodb_row_lock_time_max

    The maximum time to acquire a row lock, in milliseconds.

  • Innodb_row_lock_waits

    The number of times a row lock had to be waited for.

  • Innodb_rows_deleted

    The number of rows deleted from InnoDB tables.

  • Innodb_rows_inserted

    The number of rows inserted into InnoDB tables.

  • Innodb_rows_read

    The number of rows read from InnoDB tables.

  • Innodb_rows_updated

    The number of rows updated in InnoDB tables.

  • Innodb_truncated_status_writes

    The number of times output from the SHOW ENGINE INNODB STATUS is truncated. Monitoring applications that parse the output from this command can test this value before and after issuing the SHOW ENGINE command, to confirm if the output is complete or not.

  • Key_blocks_not_flushed

    The number of key blocks in the key cache that have changed but have not yet been flushed to disk.

  • Key_blocks_unused

    The number of unused blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of key_buffer_size in Section 5.1.4, “Server System Variables”.

  • Key_blocks_used

    The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.

  • Key_read_requests

    The number of requests to read a key block from the cache.

  • Key_reads

    The number of physical reads of a key block from disk. If Key_reads is large, then your key_buffer_size value is probably too small. The cache miss rate can be calculated as Key_reads/Key_read_requests.

  • Key_write_requests

    The number of requests to write a key block to the cache.

  • Key_writes

    The number of physical writes of a key block to disk.

  • Last_query_cost

    The total cost of the last compiled query as computed by the query optimizer. This is useful for comparing the cost of different query plans for the same query. The default value of 0 means that no query has been compiled yet. The default value is 0. Last_query_cost has session scope.

    The Last_query_cost value can be computed accurately only for simple flat queries, not complex queries such as those with subqueries or UNION. For the latter, the value is set to 0.

  • Max_used_connections

    The maximum number of connections that have been in use simultaneously since the server started.

  • Not_flushed_delayed_rows

    The number of rows waiting to be written in INSERT DELAYED queues.

  • Open_files

    The number of files that are open. This count includes regular files opened by the server. It does not include other types of files such as sockets or pipes. Also, the count does not include files that storage engines open using their own internal functions rather than asking the server level to do so.

  • Open_streams

    The number of streams that are open (used mainly for logging).

  • Open_table_definitions

    The number of cached .frm files.

  • Open_tables

    The number of tables that are open.

  • Opened_files

    The number of files that have been opened with my_open() (a mysys library function). Parts of the server that open files without using this function do not increment the count.

  • Opened_table_definitions

    The number of .frm files that have been cached.

  • Opened_tables

    The number of tables that have been opened. If Opened_tables is big, your table_open_cache value is probably too small.

  • Performance_schema_xxx

    Performance Schema status variables are listed in Section 22.10, “Performance Schema Status Variables”. These variables provide information about instrumentation that could not be loaded or created due to memory constraints.

  • Prepared_stmt_count

    The current number of prepared statements. (The maximum number of statements is given by the max_prepared_stmt_count system variable.)

  • Qcache_free_blocks

    The number of free memory blocks in the query cache.

  • Qcache_free_memory

    The amount of free memory for the query cache.

  • Qcache_hits

    The number of query cache hits.

    The discussion at the beginning of this section indicates how to relate this statement-counting status variable to other such variables.

  • Qcache_inserts

    The number of queries added to the query cache.

  • Qcache_lowmem_prunes

    The number of queries that were deleted from the query cache because of low memory.

  • Qcache_not_cached

    The number of noncached queries (not cacheable, or not cached due to the query_cache_type setting).

  • Qcache_queries_in_cache

    The number of queries registered in the query cache.

  • Qcache_total_blocks

    The total number of blocks in the query cache.

  • Queries

    The number of statements executed by the server. This variable includes statements executed within stored programs, unlike the Questions variable. It does not count COM_PING or COM_STATISTICS commands.

    The discussion at the beginning of this section indicates how to relate this statement-counting status variable to other such variables.

  • Questions

    The number of statements executed by the server. This includes only statements sent to the server by clients and not statements executed within stored programs, unlike the Queries variable. This variable does not count COM_PING, COM_STATISTICS, COM_STMT_PREPARE, COM_STMT_CLOSE, or COM_STMT_RESET commands.

    The discussion at the beginning of this section indicates how to relate this statement-counting status variable to other such variables.

  • Rpl_semi_sync_master_clients

    The number of semisynchronous slaves.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_net_avg_wait_time

    The average time in microseconds the master waited for a slave reply.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_net_wait_time

    The total time in microseconds the master waited for slave replies.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_net_waits

    The total number of times the master waited for slave replies.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_no_times

    The number of times the master turned off semisynchronous replication.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_no_tx

    The number of commits that were not acknowledged successfully by a slave.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_status

    Whether semisynchronous replication currently is operational on the master. The value is ON if the plugin has been enabled and a commit acknowledgment has occurred. It is OFF if the plugin is not enabled or the master has fallen back to asynchronous replication due to commit acknowledgment timeout.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_timefunc_failures

    The number of times the master failed when calling time functions such as gettimeofday().

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_tx_avg_wait_time

    The average time in microseconds the master waited for each transaction.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_tx_wait_time

    The total time in microseconds the master waited for transactions.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_tx_waits

    The total number of times the master waited for transactions.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_wait_pos_backtraverse

    The total number of times the master waited for an event with binary coordinates lower than events waited for previously. This can occur when the order in which transactions start waiting for a reply is different from the order in which their binary log events are written.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_wait_sessions

    The number of sessions currently waiting for slave replies.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_master_yes_tx

    The number of commits that were acknowledged successfully by a slave.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • Rpl_semi_sync_slave_status

    Whether semisynchronous replication currently is operational on the slave. This is ON if the plugin has been enabled and the slave I/O thread is running, OFF otherwise.

    This variable is available only if the slave-side semisynchronous replication plugin is installed.

  • Rpl_status

    The status of fail-safe replication (not implemented). This variable is unused and is removed in MySQL 5.6.

  • Select_full_join

    The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables.

  • Select_full_range_join

    The number of joins that used a range search on a reference table.

  • Select_range

    The number of joins that used ranges on the first table. This is normally not a critical issue even if the value is quite large.

  • Select_range_check

    The number of joins without keys that check for key usage after each row. If this is not 0, you should carefully check the indexes of your tables.

  • Select_scan

    The number of joins that did a full scan of the first table.

  • Slave_heartbeat_period

    Shows the replication heartbeat interval (in seconds) on a replication slave.

  • Slave_open_temp_tables

    The number of temporary tables that the slave SQL thread currently has open. If the value is greater than zero, it is not safe to shut down the slave; see Section 17.4.1.24, “Replication and Temporary Tables”.

  • Slave_received_heartbeats

    This counter increments with each replication heartbeat received by a replication slave since the last time that the slave was restarted or reset, or a CHANGE MASTER TO statement was issued.

  • Slave_retried_transactions

    The total number of times since startup that the replication slave SQL thread has retried transactions.

  • Slave_running

    This is ON if this server is a replication slave that is connected to a replication master, and both the I/O and SQL threads are running; otherwise, it is OFF.

  • Slow_launch_threads

    The number of threads that have taken more than slow_launch_time seconds to create.

  • Slow_queries

    The number of queries that have taken more than long_query_time seconds. This counter increments regardless of whether the slow query log is enabled. For information about that log, see Section 5.4.5, “The Slow Query Log”.

  • Sort_merge_passes

    The number of merge passes that the sort algorithm has had to do. If this value is large, you should consider increasing the value of the sort_buffer_size system variable.

  • Sort_range

    The number of sorts that were done using ranges.

  • Sort_rows

    The number of sorted rows.

  • Sort_scan

    The number of sorts that were done by scanning the table.

  • Ssl_accept_renegotiates

    The number of negotiates needed to establish the connection.

  • Ssl_accepts

    The number of accepted SSL connections.

  • Ssl_callback_cache_hits

    The number of callback cache hits.

  • Ssl_cipher

    The current encryption cipher (empty for unencrypted connections).

  • Ssl_cipher_list

    The list of possible SSL ciphers (empty for non-SSL connections).

  • Ssl_client_connects

    The number of SSL connection attempts to an SSL-enabled master.

  • Ssl_connect_renegotiates

    The number of negotiates needed to establish the connection to an SSL-enabled master.

  • Ssl_ctx_verify_depth

    The SSL context verification depth (how many certificates in the chain are tested).

  • Ssl_ctx_verify_mode

    The SSL context verification mode.

  • Ssl_default_timeout

    The default SSL timeout.

  • Ssl_finished_accepts

    The number of successful SSL connections to the server.

  • Ssl_finished_connects

    The number of successful slave connections to an SSL-enabled master.

  • Ssl_session_cache_hits

    The number of SSL session cache hits.

  • Ssl_session_cache_misses

    The number of SSL session cache misses.

  • Ssl_session_cache_mode

    The SSL session cache mode.

  • Ssl_session_cache_overflows

    The number of SSL session cache overflows.

  • Ssl_session_cache_size

    The SSL session cache size.

  • Ssl_session_cache_timeouts

    The number of SSL session cache timeouts.

  • Ssl_sessions_reused

    How many SSL connections were reused from the cache.

  • Ssl_used_session_cache_entries

    How many SSL session cache entries were used.

  • Ssl_verify_depth

    The verification depth for replication SSL connections.

  • Ssl_verify_mode

    The verification mode for replication SSL connections.

  • Ssl_version

    The SSL protocol version of the connection; for example, TLSv1. If the connection is not encrypted, the value is empty.

  • Table_locks_immediate

    The number of times that a request for a table lock could be granted immediately.

  • Table_locks_waited

    The number of times that a request for a table lock could not be granted immediately and a wait was needed. If this is high and you have performance problems, you should first optimize your queries, and then either split your table or tables or use replication.

  • Tc_log_max_pages_used

    For the memory-mapped implementation of the log that is used by mysqld when it acts as the transaction coordinator for recovery of internal XA transactions, this variable indicates the largest number of pages used for the log since the server started. If the product of Tc_log_max_pages_used and Tc_log_page_size is always significantly less than the log size, the size is larger than necessary and can be reduced. (The size is set by the --log-tc-size option. This variable is unused: It is unneeded for binary log-based recovery, and the memory-mapped recovery log method is not used unless the number of storage engines that are capable of two-phase commit and that support XA transactions is greater than one. (InnoDB is the only applicable engine.)

  • Tc_log_page_size

    The page size used for the memory-mapped implementation of the XA recovery log. The default value is determined using getpagesize(). This variable is unused for the same reasons as described for Tc_log_max_pages_used.

  • Tc_log_page_waits

    For the memory-mapped implementation of the recovery log, this variable increments each time the server was not able to commit a transaction and had to wait for a free page in the log. If this value is large, you might want to increase the log size (with the --log-tc-size option). For binary log-based recovery, this variable increments each time the binary log cannot be closed because there are two-phase commits in progress. (The close operation waits until all such transactions are finished.)

  • Threads_cached

    The number of threads in the thread cache.

  • Threads_connected

    The number of currently open connections.

  • Threads_created

    The number of threads created to handle connections. If Threads_created is big, you may want to increase the thread_cache_size value. The cache miss rate can be calculated as Threads_created/Connections.

  • Threads_running

    The number of threads that are not sleeping.

  • Uptime

    The number of seconds that the server has been up.

  • Uptime_since_flush_status

    The number of seconds since the most recent FLUSH STATUS statement.

5.1.7 Server SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differently for different clients, depending on the value of the sql_mode system variable. DBAs can set the global SQL mode to match site server operating requirements, and each application can set its session SQL mode to its own requirements.

Modes affect the SQL syntax MySQL supports and the data validation checks it performs. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

For answers to questions often asked about server SQL modes in MySQL, see Section A.3, “MySQL 5.5 FAQ: Server SQL Mode”.

When working with InnoDB tables, consider also the innodb_strict_mode system variable. It enables additional error checks for InnoDB tables.

Setting the SQL Mode

The default SQL mode is empty (no modes set).

To set the SQL mode at server startup, use the --sql-mode="modes" option on the command line, or sql-mode="modes" in an option file such as my.cnf (Unix operating systems) or my.ini (Windows). modes is a list of different modes separated by commas. To clear the SQL mode explicitly, set it to an empty string using --sql-mode="" on the command line, or sql-mode="" in an option file.

Note

MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.

To change the SQL mode at runtime, set the global or session sql_mode system variable using a SET statement:

SET GLOBAL sql_mode = 'modes';
SET SESSION sql_mode = 'modes';

Setting the GLOBAL variable requires the SUPER privilege and affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. Each client can change its session sql_mode value at any time.

To determine the current global or session sql_mode value, use the following statements:

SELECT @@GLOBAL.sql_mode;
SELECT @@SESSION.sql_mode;
Important

SQL mode and user-defined partitioning.  Changing the server SQL mode after creating and inserting data into partitioned tables can cause major changes in the behavior of such tables, and could lead to loss or corruption of data. It is strongly recommended that you never change the SQL mode once you have created tables employing user-defined partitioning.

When replicating partitioned tables, differing SQL modes on master and slave can also lead to problems. For best results, you should always use the same server SQL mode on the master and on the slave.

See Section 19.5, “Restrictions and Limitations on Partitioning”, for more information.

The Most Important SQL Modes

The most important sql_mode values are probably these:

  • ANSI

    This mode changes syntax and behavior to conform more closely to standard SQL. It is one of the special combination modes listed at the end of this section.

  • STRICT_TRANS_TABLES

    If a value could not be inserted as given into a transactional table, abort the statement. For a nontransactional table, abort the statement if the value occurs in a single-row statement or the first row of a multiple-row statement. More details are given later in this section.

  • TRADITIONAL

    Make MySQL behave like a traditional SQL database system. A simple description of this mode is give an error instead of a warning when inserting an incorrect value into a column. It is one of the special combination modes listed at the end of this section.

    Note

    The INSERT or UPDATE aborts as soon as the error is noticed. This may not be what you want if you are using a nontransactional storage engine, because data changes made prior to the error may not be rolled back, resulting in a partially done update.

When this manual refers to strict mode, it means a mode with either or both STRICT_TRANS_TABLES or STRICT_ALL_TABLES enabled.

Full List of SQL Modes

The following list describes all supported SQL modes:

  • ALLOW_INVALID_DATES

    Do not perform full checking of dates. Check only that the month is in the range from 1 to 12 and the day is in the range from 1 to 31. This is very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation). This mode applies to DATE and DATETIME columns. It does not apply TIMESTAMP columns, which always require a valid date.

    The server requires that month and day values be legal, and not merely in the range 1 to 12 and 1 to 31, respectively. With strict mode disabled, invalid dates such as '2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled, invalid dates generate an error. To permit such dates, enable ALLOW_INVALID_DATES.

  • ANSI_QUOTES

    Treat " as an identifier quote character (like the ` quote character) and not as a string quote character. You can still use ` to quote identifiers with this mode enabled. With ANSI_QUOTES enabled, you cannot use double quotation marks to quote literal strings, because it is interpreted as an identifier.

  • ERROR_FOR_DIVISION_BY_ZERO

    The ERROR_FOR_DIVISION_BY_ZERO mode affects handling of division by zero, which includes MOD(N,0). For data-change operations (INSERT, UPDATE), its effect also depends on whether strict SQL mode is enabled.

    • If this mode is not enabled, division by zero inserts NULL and produces no warning.

    • If this mode is enabled, division by zero inserts NULL and produces a warning.

    • If this mode and strict mode are enabled, division by zero produces an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, division by zero inserts NULL and produces a warning.

    For SELECT, division by zero returns NULL. Enabling ERROR_FOR_DIVISION_BY_ZERO causes a warning to be produced as well, regardless of whether strict mode is enabled.

  • HIGH_NOT_PRECEDENCE

    The precedence of the NOT operator is such that expressions such as NOT a BETWEEN b AND c are parsed as NOT (a BETWEEN b AND c). In some older versions of MySQL, the expression was parsed as (NOT a) BETWEEN b AND c. The old higher-precedence behavior can be obtained by enabling the HIGH_NOT_PRECEDENCE SQL mode.

    mysql> SET sql_mode = '';
    mysql> SELECT NOT 1 BETWEEN -5 AND 5;
            -> 0
    mysql> SET sql_mode = 'HIGH_NOT_PRECEDENCE';
    mysql> SELECT NOT 1 BETWEEN -5 AND 5;
            -> 1
    
  • IGNORE_SPACE

    Permit spaces between a function name and the ( character. This causes built-in function names to be treated as reserved words. As a result, identifiers that are the same as function names must be quoted as described in Section 9.2, “Schema Object Names”. For example, because there is a COUNT() function, the use of count as a table name in the following statement causes an error:

    mysql> CREATE TABLE count (i INT);
    ERROR 1064 (42000): You have an error in your SQL syntax
    

    The table name should be quoted:

    mysql> CREATE TABLE `count` (i INT);
    Query OK, 0 rows affected (0.00 sec)
    

    The IGNORE_SPACE SQL mode applies to built-in functions, not to user-defined functions or stored functions. It is always permissible to have spaces after a UDF or stored function name, regardless of whether IGNORE_SPACE is enabled.

    For further discussion of IGNORE_SPACE, see Section 9.2.4, “Function Name Parsing and Resolution”.

  • NO_AUTO_CREATE_USER

    Prevent the GRANT statement from automatically creating new users if it would otherwise do so, unless authentication information is specified. The statement must specify a nonempty password using IDENTIFIED BY or an authentication plugin using IDENTIFIED WITH.

  • NO_AUTO_VALUE_ON_ZERO

    NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number.

    This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with contents different from the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. mysqldump now automatically includes in its output a statement that enables NO_AUTO_VALUE_ON_ZERO, to avoid this problem.

  • NO_BACKSLASH_ESCAPES

    Disable the use of the backslash character (\) as an escape character within strings. With this mode enabled, backslash becomes an ordinary character like any other.

  • NO_DIR_IN_CREATE

    When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers.

  • NO_ENGINE_SUBSTITUTION

    Control automatic substitution of the default storage engine when a statement such as CREATE TABLE or ALTER TABLE specifies a storage engine that is disabled or not compiled in.

    Because storage engines can be pluggable at runtime, unavailable engines are treated the same way:

    With NO_ENGINE_SUBSTITUTION disabled, for CREATE TABLE the default engine is used and a warning occurs if the desired engine is unavailable. For ALTER TABLE, a warning occurs and the table is not altered.

    With NO_ENGINE_SUBSTITUTION enabled, an error occurs and the table is not created or altered if the desired engine is unavailable.

  • NO_FIELD_OPTIONS

    Do not print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

  • NO_KEY_OPTIONS

    Do not print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

  • NO_TABLE_OPTIONS

    Do not print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

  • NO_UNSIGNED_SUBTRACTION

    Subtraction between integer values, where one is of type UNSIGNED, produces an unsigned result by default. Prior to MySQL 5.5.5, if the result would otherwise have been negative, it becomes the maximum integer value:

    mysql> SET sql_mode = '';
    mysql> SELECT CAST(0 AS UNSIGNED) - 1;
    +-------------------------+
    | CAST(0 AS UNSIGNED) - 1 |
    +-------------------------+
    |    18446744073709551615 |
    +-------------------------+
    

    As of MySQL 5.5.5, if the result would otherwise have been negative, an error results:

    mysql> SET sql_mode = '';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT CAST(0 AS UNSIGNED) - 1;
    ERROR 1690 (22003): BIGINT UNSIGNED value is out of range in '(cast(0 as unsigned) - 1)'
    

    If the NO_UNSIGNED_SUBTRACTION SQL mode is enabled, the result is negative:

    mysql> SET sql_mode = 'NO_UNSIGNED_SUBTRACTION';
    mysql> SELECT CAST(0 AS UNSIGNED) - 1;
    +-------------------------+
    | CAST(0 AS UNSIGNED) - 1 |
    +-------------------------+
    |                      -1 |
    +-------------------------+
    

    If the result of such an operation is used to update an UNSIGNED integer column, the result is clipped to the maximum value for the column type, or clipped to 0 if NO_UNSIGNED_SUBTRACTION is enabled. If strict SQL mode is enabled, an error occurs and the column remains unchanged.

    When NO_UNSIGNED_SUBTRACTION is enabled, the subtraction result is signed, even if any operand is unsigned. For example, compare the type of column c2 in table t1 with that of column c2 in table t2:

    
    mysql> SET sql_mode='';
    mysql> CREATE TABLE test (c1 BIGINT UNSIGNED NOT NULL);
    mysql> CREATE TABLE t1 SELECT c1 - 1 AS c2 FROM test;
    mysql> DESCRIBE t1;
    +-------+---------------------+------+-----+---------+-------+
    | Field | Type                | Null | Key | Default | Extra |
    +-------+---------------------+------+-----+---------+-------+
    | c2    | bigint(21) unsigned | NO   |     | 0       |       |
    +-------+---------------------+------+-----+---------+-------+
    
    mysql> SET sql_mode='NO_UNSIGNED_SUBTRACTION';
    mysql> CREATE TABLE t2 SELECT c1 - 1 AS c2 FROM test;
    mysql> DESCRIBE t2;
    +-------+------------+------+-----+---------+-------+
    | Field | Type       | Null | Key | Default | Extra |
    +-------+------------+------+-----+---------+-------+
    | c2    | bigint(21) | NO   |     | 0       |       |
    +-------+------------+------+-----+---------+-------+
    

    This means that BIGINT UNSIGNED is not 100% usable in all contexts. See Section 12.10, “Cast Functions and Operators”.

  • NO_ZERO_DATE

    The NO_ZERO_DATE mode affects whether the server permits '0000-00-00' as a valid date. Its effect also depends on whether strict SQL mode is enabled.

    • If this mode is not enabled, '0000-00-00' is permitted and inserts produce no warning.

    • If this mode is enabled, '0000-00-00' is permitted and inserts produce a warning.

    • If this mode and strict mode are enabled, '0000-00-00' is not permitted and inserts produce an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, '0000-00-00' is permitted and inserts produce a warning.

  • NO_ZERO_IN_DATE

    The NO_ZERO_IN_DATE mode affects whether the server permits dates in which the year part is nonzero but the month or day part is 0. (This mode affects dates such as '2010-00-01' or '2010-01-00', but not '0000-00-00'. To control whether the server permits '0000-00-00', use the NO_ZERO_DATE mode.) The effect of NO_ZERO_IN_DATE also depends on whether strict SQL mode is enabled.

    • If this mode is not enabled, dates with zero parts are permitted and inserts produce no warning.

    • If this mode is enabled, dates with zero parts are inserted as '0000-00-00' and produce a warning.

    • If this mode and strict mode are enabled, dates with zero parts are not permitted and inserts produce an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, dates with zero parts are inserted as '0000-00-00' and produce a warning.

  • ONLY_FULL_GROUP_BY

    Reject queries for which the select list or HAVING condition refer to nonaggregated columns that are not named in the GROUP BY clause.

    A MySQL extension to standard SQL permits references in the HAVING clause to aliased expressions in the select list. Enabling ONLY_FULL_GROUP_BY disables this extension, thus requiring the HAVING clause to be written using unaliased expressions.

    For additional discussion and examples, see Section 12.17.3, “MySQL Handling of GROUP BY”.

  • PAD_CHAR_TO_FULL_LENGTH

    By default, trailing spaces are trimmed from CHAR column values on retrieval. If PAD_CHAR_TO_FULL_LENGTH is enabled, trimming does not occur and retrieved CHAR values are padded to their full length. This mode does not apply to VARCHAR columns, for which trailing spaces are retained on retrieval.

    
    mysql> CREATE TABLE t1 (c1 CHAR(10));
    Query OK, 0 rows affected (0.37 sec)
    
    mysql> INSERT INTO t1 (c1) VALUES('xy');
    Query OK, 1 row affected (0.01 sec)
    
    mysql> SET sql_mode = '';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT c1, CHAR_LENGTH(c1) FROM t1;
    +------+-----------------+
    | c1   | CHAR_LENGTH(c1) |
    +------+-----------------+
    | xy   |               2 |
    +------+-----------------+
    1 row in set (0.00 sec)
    
    mysql> SET sql_mode = 'PAD_CHAR_TO_FULL_LENGTH';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT c1, CHAR_LENGTH(c1) FROM t1;
    +------------+-----------------+
    | c1         | CHAR_LENGTH(c1) |
    +------------+-----------------+
    | xy         |              10 |
    +------------+-----------------+
    1 row in set (0.00 sec)
    
  • PIPES_AS_CONCAT

    Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR.

  • REAL_AS_FLOAT

    Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE.

  • STRICT_ALL_TABLES

    Enable strict SQL mode for all storage engines. Invalid data values are rejected. For details, see Strict SQL Mode.

  • STRICT_TRANS_TABLES

    Enable strict SQL mode for transactional storage engines, and when possible for nontransactional storage engines. For details, see Strict SQL Mode.

Combination SQL Modes

The following special modes are provided as shorthand for combinations of mode values from the preceding list.

Strict SQL Mode

Strict mode controls how MySQL handles invalid or missing values in data-change statements such as INSERT or UPDATE. A value can be invalid for several reasons. For example, it might have the wrong data type for the column, or it might be out of range. A value is missing when a new row to be inserted does not contain a value for a non-NULL column that has no explicit DEFAULT clause in its definition. (For a NULL column, NULL is inserted if the value is missing.)

If strict mode is not in effect, MySQL inserts adjusted values for invalid or missing values and produces warnings (see Section 13.7.5.41, “SHOW WARNINGS Syntax”). In strict mode, you can produce this behavior by using INSERT IGNORE or UPDATE IGNORE.

For statements such as SELECT that do not change data, invalid values generate a warning in strict mode, not an error.

Strict mode does not affect whether foreign key constraints are checked. foreign_key_checks can be used for that. (See Section 5.1.4, “Server System Variables”.)

Strict SQL mode is in effect if either STRICT_ALL_TABLES or STRICT_TRANS_TABLES is enabled, although the effects of these modes differ somewhat:

  • For transactional tables, an error occurs for invalid or missing values in a data-change statement when either STRICT_ALL_TABLES or STRICT_TRANS_TABLES is enabled. The statement is aborted and rolled back.

  • For nontransactional tables, the behavior is the same for either mode if the bad value occurs in the first row to be inserted or updated: The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows and the bad value occurs in the second or later row, the result depends on which strict mode is enabled:

    • For STRICT_ALL_TABLES, MySQL returns an error and ignores the rest of the rows. However, because the earlier rows have been inserted or updated, the result is a partial update. To avoid this, use single-row statements, which can be aborted without changing the table.

    • For STRICT_TRANS_TABLES, MySQL converts an invalid value to the closest valid value for the column and inserts the adjusted value. If a value is missing, MySQL inserts the implicit default value for the column data type. In either case, MySQL generates a warning rather than an error and continues processing the statement. Implicit defaults are described in Section 11.6, “Data Type Default Values”.

Strict mode also affects handling of division by zero, zero dates, and zeros in dates, in conjunction with the ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE modes. For details, see the descriptions of those modes.

5.1.8 IPv6 Support

As of MySQL 5.5.3, support for IPv6 includes these capabilities:

  • MySQL Server can accept TCP/IP connections from clients connecting over IPv6. For example, this command connects over IPv6 to the MySQL server on the local host:

    shell> mysql -h ::1
    

    To use this capability, two things must be true:

  • MySQL account names permit IPv6 addresses to enable DBAs to specify privileges for clients that connect to the server over IPv6. See Section 6.2.3, “Specifying Account Names”. IPv6 addresses can be specified in account names in statements such as CREATE USER, GRANT, and REVOKE. For example:

    mysql> CREATE USER 'bill'@'::1' IDENTIFIED BY 'secret';
    mysql> GRANT SELECT ON mydb.* TO 'bill'@'::1';
    

The following sections describe how to set up MySQL so that clients can connect to the server over IPv6.

5.1.8.1 Verifying System Support for IPv6

Before MySQL Server can accept IPv6 connections, the operating system on your server host must support IPv6. As a simple test to determine whether that is true, try this command:

shell> ping6 ::1
16 bytes from ::1, icmp_seq=0 hlim=64 time=0.171 ms
16 bytes from ::1, icmp_seq=1 hlim=64 time=0.077 ms
...

To produce a description of your system's network interfaces, invoke ifconfig -a and look for IPv6 addresses in the output.

If your host does not support IPv6, consult your system documentation for instructions on enabling it. It might be that you need only reconfigure an existing network interface to add an IPv6 address. Or a more extensive change might be needed, such as rebuilding the kernel with IPv6 options enabled.

These links may be helpful in setting up IPv6 on various platforms:

5.1.8.2 Configuring the MySQL Server to Permit IPv6 Connections

The MySQL server listens on a single network socket for TCP/IP connections. This socket is bound to a single address, but it is possible for an address to map onto multiple network interfaces. The default address is 0.0.0.0. To specify an address explicitly, use the --bind-address=addr option at server startup, where addr is an IPv4 or IPv6 address or a host name. (IPv6 addresses are not supported before MySQL 5.5.3.) If addr is a host name, the server resolves the name to an IP address and binds to that address.

The server treats different types of addresses as follows:

  • If the address is 0.0.0.0, the server accepts TCP/IP connections on all server host IPv4 interfaces.

  • If the address is ::, the server accepts TCP/IP connections on all server host IPv4 and IPv6 interfaces. Use this address to permit both IPv4 and IPv6 connections on all server interfaces.

  • If the address is an IPv4-mapped address, the server accepts TCP/IP connections for that address, in either IPv4 or IPv6 format. For example, if the server is bound to ::ffff:127.0.0.1, clients can connect using --host=127.0.0.1 or --host=::ffff:127.0.0.1.

  • If the address is a regular IPv4 or IPv6 address (such as 127.0.0.1 or ::1), the server accepts TCP/IP connections only for that IPv4 or IPv6 address.

If you intend to bind the server to a specific address, be sure that the mysql.user grant table contains an account with administrative privileges that you can use to connect to that address. Otherwise, you will not be able to shut down the server. For example, if you bind the server to ::, you can connect to it using all existing accounts. But if you bind the server to ::1, it accepts connections only on that address. In that case, first make sure that the 'root'@'::1' account is present in the mysql.user table so you can still connect to the server to shut it down.

5.1.8.3 Connecting Using the IPv6 Local Host Address

The following procedure shows how to configure MySQL to permit IPv6 connections by clients that connect to the local server using the ::1 local host address. The instructions given here assume that your system supports IPv6.

  1. Start the MySQL server with an appropriate --bind-address option to permit it to accept IPv6 connections. For example, put the following lines in your server option file and restart the server:

    [mysqld]
    bind-address = ::
    

    Alternatively, you can bind the server to ::1, but that makes the server more restrictive for TCP/IP connections. It accepts only IPv6 connections for that single address and rejects IPv4 connections. For more information, see Section 5.1.8.2, “Configuring the MySQL Server to Permit IPv6 Connections”.

  2. As an administrator, connect to the server and create an account for a local user who will connect from the ::1 local IPv6 host address:

    mysql> CREATE USER 'ipv6user'@'::1' IDENTIFIED BY 'ipv6pass';
    

    For the permitted syntax of IPv6 addresses in account names, see Section 6.2.3, “Specifying Account Names”. In addition to the CREATE USER statement, you can issue GRANT statements that give specific privileges to the account, although that is not necessary for the remaining steps in this procedure.

  3. Invoke the mysql client to connect to the server using the new account:

    shell> mysql -h ::1 -u ipv6user -pipv6pass
    
  4. Try some simple statements that show connection information:

    mysql> STATUS
    ...
    Connection:   ::1 via TCP/IP
    ...
    
    mysql> SELECT CURRENT_USER();
    +----------------+
    | CURRENT_USER() |
    +----------------+
    | ipv6user@::1   |
    +----------------+
    

5.1.8.4 Connecting Using IPv6 Nonlocal Host Addresses

The following procedure shows how to configure MySQL to permit IPv6 connections by remote clients. It is similar to the preceding procedure for local clients, but the server and client hosts are distinct and each has its own nonlocal IPv6 address. The example uses these addresses:

Server host: 2001:db8:0:f101::1
Client host: 2001:db8:0:f101::2

These addresses are chosen from the nonroutable address range recommended by IANA for documentation purposes and suffice for testing on your local network. To accept IPv6 connections from clients outside the local network, the server host must have a public address. If your network provider assigns you an IPv6 address, you can use that. Otherwise, another way to obtain an address is to use an IPv6 broker; see Section 5.1.8.5, “Obtaining an IPv6 Address from a Broker”.

  1. Start the MySQL server with an appropriate --bind-address option to permit it to accept IPv6 connections. For example, put the following lines in your server option file and restart the server:

    [mysqld]
    bind-address = ::
    

    Alternatively, you can bind the server to 2001:db8:0:f101::1, but that makes the server more restrictive for TCP/IP connections. It accepts only IPv6 connections for that single address and rejects IPv4 connections. For more information, see Section 5.1.8.2, “Configuring the MySQL Server to Permit IPv6 Connections”.

  2. On the server host (2001:db8:0:f101::1), create an account for a user who will connect from the client host (2001:db8:0:f101::2):

    mysql> CREATE USER 'remoteipv6user'@'2001:db8:0:f101::2' IDENTIFIED BY 'remoteipv6pass';
    
  3. On the client host (2001:db8:0:f101::2), invoke the mysql client to connect to the server using the new account:

    shell> mysql -h 2001:db8:0:f101::1 -u remoteipv6user -premoteipv6pass
    
  4. Try some simple statements that show connection information:

    mysql> STATUS
    ...
    Connection:   2001:db8:0:f101::1 via TCP/IP
    ...
    
    mysql> SELECT CURRENT_USER();
    +-----------------------------------+
    | CURRENT_USER()                    |
    +-----------------------------------+
    | remoteipv6user@2001:db8:0:f101::2 |
    +-----------------------------------+
    

5.1.8.5 Obtaining an IPv6 Address from a Broker

If you do not have a public IPv6 address that enables your system to communicate over IPv6 outside your local network, you can obtain one from an IPv6 broker. The Wikipedia IPv6 Tunnel Broker page lists several brokers and their features, such as whether they provide static addresses and the supported routing protocols.

After configuring your server host to use a broker-supplied IPv6 address, start the MySQL server with an appropriate --bind-address option to permit the server to accept IPv6 connections. For example, put the following lines in the server option file and restart the server:

[mysqld]
bind-address = ::

Alternatively, you can bind the server to the specific IPv6 address provided by the broker, but that makes the server more restrictive for TCP/IP connections. It accepts only IPv6 connections for that single address and rejects IPv4 connections. For more information, see Section 5.1.8.2, “Configuring the MySQL Server to Permit IPv6 Connections”. In addition, if the broker allocates dynamic addresses, the address provided for your system might change the next time you connect to the broker. If so, any accounts you create that name the original address become invalid. To bind to a specific address but avoid this change-of-address problem, you may be able to arrange with the broker for a static IPv6 address.

The following example shows how to use Freenet6 as the broker and the gogoc IPv6 client package on Gentoo Linux.

  1. Create an account at Freenet6 by visiting this URL and signing up:

    http://gogonet.gogo6.com
    
  2. After creating the account, go to this URL, sign in, and create a user ID and password for the IPv6 broker:

    http://gogonet.gogo6.com/page/freenet6-registration
    

  3. As root, install gogoc:

    shell> emerge gogoc
    
  4. Edit /etc/gogoc/gogoc.conf to set the userid and password values. For example:

    userid=gogouser
    passwd=gogopass
    
  5. Start gogoc:

    shell> /etc/init.d/gogoc start
    

    To start gogoc each time your system boots, execute this command:

    shell> rc-update add gogoc default
    
  6. Use ping6 to try to ping a host:

    shell> ping6 ipv6.google.com
    
  7. To see your IPv6 address:

    shell> ifconfig tun
    

5.1.9 Server-Side Help

MySQL Server supports a HELP statement that returns information from the MySQL Reference manual (see Section 13.8.3, “HELP Syntax”). Several tables in the mysql system database contain the information needed to support this statement (see Section 5.3, “The mysql System Database”). The proper operation of this statement requires that these help tables be initialized, which is done by processing the contents of the fill_help_tables.sql script.

If you install MySQL using a binary or source distribution on Unix, help table content initialization occurs when you initialize the data directory (see Section 2.10.1, “Initializing the Data Directory”). For an RPM distribution on Linux or binary distribution on Windows, content initialization occurs as part of the MySQL installation process.

If you upgrade MySQL using a binary distribution, help table content is not upgraded automatically, but you can upgrade it manually. Locate the fill_help_tables.sql file in the share or share/mysql directory. Change location into that directory and process the file with the mysql client as follows:

shell> mysql -u root mysql < fill_help_tables.sql

You can also obtain the latest fill_help_tables.sql at any time to upgrade your help tables. Download the proper file for your version of MySQL from http://dev.mysql.com/doc/index-other.html. After downloading and uncompressing the file, process it with mysql as described previously.

If you are working with Git and a MySQL development source tree, you must use a downloaded copy of the fill_help_tables.sql file because the source tree contains only a stub version.

Note

For a server that participates in replication, the help table content upgrade process involves multiple servers. For details, see Section 17.4.1.30, “Replication of Server-Side Help Tables”.

5.1.10 Server Response to Signals

On Unix, signals can be sent to processes. mysqld responds to signals sent to it as follows:

  • SIGTERM causes the server to shut down.

  • SIGHUP causes the server to reload the grant tables and to flush tables, logs, the thread cache, and the host cache. These actions are like various forms of the FLUSH statement. The server also writes a status report to the error log that has this format:

    Status information:
    
    Current dir: /var/mysql/data/
    Running threads: 0  Stack size: 196608
    Current locks:
    
    Key caches:
    default
    Buffer_size:       8388600
    Block_size:           1024
    Division_limit:        100
    Age_limit:             300
    blocks used:             0
    not flushed:             0
    w_requests:              0
    writes:                  0
    r_requests:              0
    reads:                   0
    
    handler status:
    read_key:            0
    read_next:           0
    read_rnd             0
    read_first:          1
    write:               0
    delete               0
    update:              0
    
    Table status:
    Opened tables:          5
    Open tables:            0
    Open files:             7
    Open streams:           0
    
    Alarm status:
    Active alarms:   1
    Max used alarms: 2
    Next alarm time: 67
    

5.1.11 The Server Shutdown Process

The server shutdown process takes place as follows:

  1. The shutdown process is initiated.

    This can occur initiated several ways. For example, a user with the SHUTDOWN privilege can execute a mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating system-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a SIGTERM signal. A server running as a service on Windows shuts down when the services manager tells it to.

  2. The server creates a shutdown thread if necessary.

    Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a SIGTERM signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that appears in the error log:

    Error: Can't create thread to kill server
    
  3. The server stops accepting new connections.

    To prevent new activity from being initiated during shutdown, the server stops accepting new client connections by closing the handlers for the network interfaces to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.

  4. The server terminates current activity.

    For each thread associated with a client connection, the server breaks the connection to the client and marks the thread as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing statements check their state periodically and take longer to die. For additional information about thread termination, see Section 13.7.6.4, “KILL Syntax”, in particular for the instructions about killed REPAIR TABLE or OPTIMIZE TABLE operations on MyISAM tables.

    For threads that have an open transaction, the transaction is rolled back. If a thread is updating a nontransactional table, an operation such as a multiple-row UPDATE or INSERT may leave the table partially updated because the operation can terminate before completion.

    If the server is a master replication server, it treats threads associated with currently connected slaves like other client threads. That is, each one is marked as killed and exits when it next checks its state.

    If the server is a slave replication server, it stops the I/O and SQL threads, if they are active, before marking client threads as killed. The SQL thread is permitted to finish its current statement (to avoid causing replication problems), and then stops. If the SQL thread is in the middle of a transaction at this point, the server waits until the current replication event group (if any) has finished executing, or until the user issues a KILL QUERY or KILL CONNECTION statement. See also Section 13.4.2.6, “STOP SLAVE Syntax”.

    If the slave is updating a nontransactional table when it is forcibly killed, the slave's data may become inconsistent with the master.

  5. The server shuts down or closes storage engines.

    At this stage, the server flushes the table cache and closes all open tables.

    Each storage engine performs any actions necessary for tables that it manages. For example, MyISAM flushes any pending index writes for a table. InnoDB flushes its buffer pool to disk (unless innodb_fast_shutdown is 2), writes the current LSN to the tablespace, and terminates its own internal threads.

  6. The server exits.

5.2 The MySQL Data Directory

Information managed by the MySQL server is stored under a directory known as the data directory. The following list briefly describes the items typically found in the data directory, with cross references for additional information:

  • Data directory subdirectories. Each subdirectory of the data directory is a database directory and corresponds to a database managed by the server. All MySQL installations have certain standard databases:

    Other subdirectories correspond to databases created by users or applications.

    Note

    INFORMATION_SCHEMA is a standard database, but its implementation uses no corresponding database directory.

  • Log files written by the server. See Section 5.4, “MySQL Server Logs”.

  • InnoDB tablespace and log files. See Chapter 14, The InnoDB Storage Engine.

  • The server process ID file (while the server is running).

Some items in the preceding list can be relocated elsewhere by reconfiguring the server. In addition, the --datadir option enables the location of the data directory itself to be changed. For a given MySQL installation, check the server configuration to determine whether items have been moved.

5.3 The mysql System Database

The mysql database is the system database. It contains tables that store information required by the MySQL server as it runs.

Tables in the mysql database fall into these categories:

The remainder of this section enumerates the tables in each category, with cross references for additional information. System tables use the MyISAM storage engine.

Grant System Tables

These system tables contain grant information about user accounts and the privileges held by them:

  • user: User accounts, global privileges, and other non-privilege columns.

  • db: Database-level privileges.

  • host: Obsolete.

  • tables_priv: Table-level privileges.

  • columns_priv: Column-level privileges.

  • procs_priv: Stored procedure and function privileges.

  • proxies_priv: Proxy-user privileges.

For more information about the structure, contents, and purpose of the grant tables, see Section 6.2.2, “Grant Tables”.

Object Information System Tables

These system tables contain information about stored programs, user-defined functions, and server-side plugins:

Log System Tables

The server uses these system tables for logging:

  • general_log: The general query log table.

  • slow_log: The slow query log table.

Log tables use the CSV storage engine.

For more information, see Section 5.4, “MySQL Server Logs”.

Server-Side Help System Tables

These system tables contain server-side help information:

  • help_category: Information about help categories.

  • help_keyword: Keywords associated with help topics.

  • help_relation: Mappings between help keywords and topics.

  • help_topic: Help topic contents.

For more information, see Section 5.1.9, “Server-Side Help”.

Time Zone System Tables

These system tables contain time zone information:

  • time_zone: Time zone IDs and whether they use leap seconds.

  • time_zone_leap_second: When leap seconds occur.

  • time_zone_name: Mappings between time zone IDs and names.

  • time_zone_transition, time_zone_transition_type: Time zone descriptions.

For more information, see Section 10.6, “MySQL Server Time Zone Support”.

Replication System Tables

The server uses the ndb_binlog_index system table to store binary log information for MySQL Cluster replication. See Section 18.6.4, “MySQL Cluster Replication Schema and Tables”.

Miscellaneous System Tables

Other system tables do not fall into the preceding categories:

5.4 MySQL Server Logs

MySQL Server has several logs that can help you find out what activity is taking place.

Log TypeInformation Written to Log
Error logProblems encountered starting, running, or stopping mysqld
General query logEstablished client connections and statements received from clients
Binary logStatements that change data (also used for replication)
Relay logData changes received from a replication master server
Slow query logQueries that took more than long_query_time seconds to execute
DDL log (metadata log)Metadata operations performed by DDL statements

By default, no logs are enabled, except the error log on Windows. (The DDL log is always created when required, and has no user-configurable options; see Section 5.4.6, “The DDL Log”.) The following log-specific sections provide information about the server options that enable logging.

By default, the server writes files for all enabled logs in the data directory. You can force the server to close and reopen the log files (or in some cases switch to a new log file) by flushing the logs. Log flushing occurs when you issue a FLUSH LOGS statement; execute mysqladmin with a flush-logs or refresh argument; or execute mysqldump with a --flush-logs or --master-data option. See Section 13.7.6.3, “FLUSH Syntax”, Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In addition, the binary log is flushed when its size reaches the value of the max_binlog_size system variable.

You can control the general query and slow query logs during runtime. You can enable or disable logging, or change the log file name. You can tell the server to write general query and slow query entries to log tables, log files, or both. For details, see Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”, Section 5.4.3, “The General Query Log”, and Section 5.4.5, “The Slow Query Log”.

The relay log is used only on slave replication servers, to hold data changes from the master server that must also be made on the slave. For discussion of relay log contents and configuration, see Section 17.2.2.1, “The Slave Relay Log”.

For information about log maintenance operations such as expiration of old log files, see Section 5.4.7, “Server Log Maintenance”.

For information about keeping logs secure, see Section 6.1.2.3, “Passwords and Logging”.

5.4.1 Selecting General Query and Slow Query Log Output Destinations

MySQL Server provides flexible control over the destination of output to the general query log and the slow query log, if those logs are enabled. Possible destinations for log entries are log files or the general_log and slow_log tables in the mysql database. Either or both destinations can be selected.

Before MySQL 5.5.7, logging to tables incurs significantly more server overhead than logging to files. If you enable the general log or slow query log and require highest performance, you should use file logging, not table logging.

Log control at server startup. The --log-output option specifies the destination for log output. This option does not in itself enable the logs. Its syntax is --log-output[=value,...]:

  • If --log-output is given with a value, the value should be a comma-separated list of one or more of the words TABLE (log to tables), FILE (log to files), or NONE (do not log to tables or files). NONE, if present, takes precedence over any other specifiers.

  • If --log-output is omitted, the default logging destination is FILE.

The general_log system variable controls logging to the general query log for the selected log destinations. If specified at server startup, general_log takes an optional argument of 1 or 0 to enable or disable the log. To specify a file name other than the default for file logging, set the general_log_file variable. Similarly, the slow_query_log variable controls logging to the slow query log for the selected destinations and setting slow_query_log_file specifies a file name for file logging. If either log is enabled, the server opens the corresponding log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected.

Examples:

  • To write general query log entries to the log table and the log file, use --log-output=TABLE,FILE to select both log destinations and --general_log to enable the general query log.

  • To write general and slow query log entries only to the log tables, use --log-output=TABLE to select tables as the log destination and --general_log and --slow_query_log to enable both logs.

  • To write slow query log entries only to the log file, use --log-output=FILE to select files as the log destination and --slow_query_log to enable the slow query log. (In this case, because the default log destination is FILE, you could omit the --log-output option.)

Log control at runtime. The system variables associated with log tables and files enable runtime control over logging:

  • The global log_output system variable indicates the current logging destination. It can be modified at runtime to change the destination.

  • The global general_log and slow_query_log variables indicate whether the general query log and slow query log are enabled (ON) or disabled (OFF). You can set these variables at runtime to control whether the logs are enabled.

  • The global general_log_file and slow_query_log_file variables indicate the names of the general query log and slow query log files. You can set these variables at server startup or at runtime to change the names of the log files.

  • To disable or enable general query logging for the current connection, set the session sql_log_off variable to ON or OFF.

The use of tables for log output offers the following benefits:

  • Log entries have a standard format. To display the current structure of the log tables, use these statements:

    SHOW CREATE TABLE mysql.general_log;
    SHOW CREATE TABLE mysql.slow_log;
    
  • Log contents are accessible through SQL statements. This enables the use of queries that select only those log entries that satisfy specific criteria. For example, to select log contents associated with a particular client (which can be useful for identifying problematic queries from that client), it is easier to do this using a log table than a log file.

  • Logs are accessible remotely through any client that can connect to the server and issue queries (if the client has the appropriate log table privileges). It is not necessary to log in to the server host and directly access the file system.

The log table implementation has the following characteristics:

  • In general, the primary purpose of log tables is to provide an interface for users to observe the runtime execution of the server, not to interfere with its runtime execution.

  • CREATE TABLE, ALTER TABLE, and DROP TABLE are valid operations on a log table. For ALTER TABLE and DROP TABLE, the log table cannot be in use and must be disabled, as described later.

  • By default, the log tables use the CSV storage engine that writes data in comma-separated values format. For users who have access to the .CSV files that contain log table data, the files are easy to import into other programs such as spreadsheets that can process CSV input.

    The log tables can be altered to use the MyISAM storage engine. You cannot use ALTER TABLE to alter a log table that is in use. The log must be disabled first. No engines other than CSV or MyISAM are legal for the log tables.

  • To disable logging so that you can alter (or drop) a log table, you can use the following strategy. The example uses the general query log; the procedure for the slow query log is similar but uses the slow_log table and slow_query_log system variable.

    SET @old_log_state = @@global.general_log;
    SET GLOBAL general_log = 'OFF';
    ALTER TABLE mysql.general_log ENGINE = MyISAM;
    SET GLOBAL general_log = @old_log_state;
    
  • TRUNCATE TABLE is a valid operation on a log table. It can be used to expire log entries.

  • RENAME TABLE is a valid operation on a log table. You can atomically rename a log table (to perform log rotation, for example) using the following strategy:

    USE mysql;
    DROP TABLE IF EXISTS general_log2;
    CREATE TABLE general_log2 LIKE general_log;
    RENAME TABLE general_log TO general_log_backup, general_log2 TO general_log;
    
  • As of MySQL 5.5.7, CHECK TABLE is a valid operation on a log table.

  • LOCK TABLES cannot be used on a log table.

  • INSERT, DELETE, and UPDATE cannot be used on a log table. These operations are permitted only internally to the server itself.

  • FLUSH TABLES WITH READ LOCK and the state of the read_only system variable have no effect on log tables. The server can always write to the log tables.

  • Entries written to the log tables are not written to the binary log and thus are not replicated to slave servers.

  • To flush the log tables or log files, use FLUSH TABLES or FLUSH LOGS, respectively.

  • Partitioning of log tables is not permitted.

  • Before MySQL 5.5.25, mysqldump does not dump the general_log or slow_query_log tables for dumps of the mysql database. As of 5.5.25, the dump includes statements to recreate those tables so that they are not missing after reloading the dump file. Log table contents are not dumped.

5.4.2 The Error Log

The error log contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.

On some operating systems, the error log contains a stack trace if mysqld exits abnormally. The trace can be used to determine where mysqld exited. See Section 24.5, “Debugging and Porting MySQL”.

If mysqld_safe is used to start mysqld and mysqld exits abnormally, mysqld_safe notices this, restarts mysqld, and writes a mysqld restarted message to the error log.

In the following discussion, console means stderr, the standard error output; this is your terminal or console window unless the standard error output has been redirected. (For example, if invoked with the --syslog option, mysqld_safe arranges for the server's stderr to be sent to syslog, as described later.)

On Windows, the --log-error, --pid-file, and --console options affect error logging:

  • Without --log-error, mysqld writes error messages to the default log file.

  • With --log-error[=file_name], mysqld writes error messages to an error log file. If no file is named, mysqld writes to the default log file. If a file is named, mysqld writes to it, creating it in the data directory unless an absolute path name is given to specify a different directory.

  • The default log file is host_name.err in the data directory, unless the --pid-file option is specified. In that case, the default name is the PID file base name with a suffix of .err in the data directory. This default is used if --log-error is not given, or is given without naming a log file.

  • With --console, mysqld writes error messages to the console, unless --log-error is also given. If both options are present, --console is ignored and has no effect. Their order does not matter: --log-error takes precedence and error messages go to a log file. (Before MySQL 5.5.3, if both options are given, the last one takes precedence.)

In addition, on Windows, the server writes events and error messages to the Windows Event Log within the Application log. Entries marked as Warning and Note are written to the Event Log, but not informational messages such as information statements from individual storage engines. These log entries have a source of MySQL. You cannot disable writing information to the Windows Event Log.

On Unix and Unix-like systems, mysqld writes error log messages as follows:

  • Without --log-error, mysqld writes error messages to the console.

  • With --log-error[=file_name], mysqld writes error messages to an error log file. If no file is named, mysqld writes to the default log file. If a file is named, mysqld writes to it, creating it in the data directory unless an absolute path name is given to specify a different directory.

    Note

    It is common for Yum or APT package installations to configure the error log location to be under /var/log with an entry like log-error=/var/log/mysqld.log in a server configuration file; removing the file name from the entry causes the default log file to be used.

  • The default log file is host_name.err in the data directory. This default is used if --log-error is given without naming a log file.

The log_error system variable indicates the error log file name if error output is written to a file.

If you specify --log-error in an option file in a [mysqld], [server], or [mysqld_safe] section, mysqld_safe finds and uses the option.

If you flush the logs using FLUSH LOGS or mysqladmin flush-logs and mysqld is writing the error log to a file (for example, if it was started with the --log-error option), the effect is version dependent:

  • As of MySQL 5.5.7, the server closes and reopens the log file. To rename the file, do so manually before flushing. Flushing the logs then reopens a new file with the original file name. For example, to rename the file and create a new one, use the following commands (assuming a log file name of host_name.err):

    shell> mv host_name.err host_name.err-old
    shell> mysqladmin flush-logs
    shell> mv host_name.err-old backup-directory
    

    On Windows, use rename rather than mv.

  • Prior to MySQL 5.5.7, the server renames the current log file with the suffix -old, then creates a new empty log file. Be aware that a second log-flushing operation thus causes the original error log file to be lost unless you save it under a different name. On Windows, you cannot rename the error log while the server has it open before MySQL 5.5.7. To avoid a restart, flush the logs first to cause the server to rename the original file and create a new one, then save the renamed file. That also works on Unix, or you can use the commands shown earlier.

If the server is not writing to a named file, no error log renaming occurs when the logs are flushed.

If you use mysqld_safe to start mysqld, mysqld_safe arranges for mysqld to write error messages to a log file or to syslog. mysqld_safe has three error-logging options, --syslog, --skip-syslog, and --log-error. The default with no logging options or with --skip-syslog is to use the default log file. To explicitly specify use of an error log file, specify --log-error=file_name to mysqld_safe, and mysqld_safe will arrange for mysqld to write messages to a log file. To use syslog instead, specify the --syslog option.

The --log-warnings option or log_warnings system variable controls warning logging to the error log. The default value is enabled (1). To disable warning logging, set --log-warnings or log_warnings to 0. If the value is greater than 1, aborted connections are written to the error log, and access-denied errors for new connection attempts are written. See Section B.5.2.11, “Communication Errors and Aborted Connections”.

5.4.3 The General Query Log

The general query log is a general record of what mysqld is doing. The server writes information to this log when clients connect or disconnect, and it logs each SQL statement received from clients. The general query log can be very useful when you suspect an error in a client and want to know exactly what the client sent to mysqld.

mysqld writes statements to the query log in the order that it receives them, which might differ from the order in which they are executed. This logging order is in contrast with that of the binary log, for which statements are written after they are executed but before any locks are released. In addition, the query log may contain statements that only select data while such statements are never written to the binary log.

When using statement-based binary logging on a replication master server, statements received by its slaves are written to the query log of each slave. Statements are written to the query log of the master server if a client reads events with the mysqlbinlog utility and passes them to the server.

However, when using row-based binary logging, updates are sent as row changes rather than SQL statements, and thus these statements are never written to the query log when binlog_format is ROW. A given update also might not be written to the query log when this variable is set to MIXED, depending on the statement used. See Section 17.1.2.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”, for more information.

By default, the general query log is disabled. To specify the initial general query log state explicitly, use --general_log[={0|1}]. With no argument or an argument of 1, --general_log enables the log. With an argument of 0, this option disables the log. To specify a log file name, use --general_log_file=file_name. To specify the log destination, use --log-output (as described in Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”). The older options to enable the general query log, --log and -l, are deprecated.

If you specify no name for the general query log file, the default name is host_name.log. The server creates the file in the data directory unless an absolute path name is given to specify a different directory.

To disable or enable the general query log or change the log file name at runtime, use the global general_log and general_log_file system variables. Set general_log to 0 (or OFF) to disable the log or to 1 (or ON) to enable it. Set general_log_file to specify the name of the log file. If a log file already is open, it is closed and the new file is opened.

When the general query log is enabled, the server writes output to any destinations specified by the --log-output option or log_output system variable. If you enable the log, the server opens the log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected. If the destination is NONE, the server writes no queries even if the general log is enabled. Setting the log file name has no effect on logging if the log destination value does not contain FILE.

Server restarts and log flushing do not cause a new general query log file to be generated (although flushing closes and reopens it). To rename the file and create a new one, use the following commands:

shell> mv host_name.log host_name-old.log
shell> mysqladmin flush-logs
shell> mv host_name-old.log backup-directory

On Windows, use rename rather than mv.

You can also rename the general query log file at runtime by disabling the log:

SET GLOBAL general_log = 'OFF';

With the log disabled, rename the log file externally; for example, from the command line. Then enable the log again:

SET GLOBAL general_log = 'ON';

This method works on any platform and does not require a server restart.

The session sql_log_off variable can be set to ON or OFF to disable or enable general query logging for the current connection.

The general query log should be protected because logged statements might contain passwords. See Section 6.1.2.3, “Passwords and Logging”.

5.4.4 The Binary Log

The binary log contains events that describe database changes such as table creation operations or changes to table data. It also contains events for statements that potentially could have made changes (for example, a DELETE which matched no rows), unless row-based logging is used. The binary log also contains information about how long each statement took that updated data. The binary log has two important purposes:

  • For replication, the binary log on a master replication server provides a record of the data changes to be sent to slave servers. The master server sends the events contained in its binary log to its slaves, which execute those events to make the same data changes that were made on the master. See Section 17.2, “Replication Implementation”.

  • Certain data recovery operations require use of the binary log. After a backup has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup. See Section 7.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

The binary log is not used for statements such as SELECT or SHOW that do not modify data. To log all statements (for example, to identify a problem query), use the general query log. See Section 5.4.3, “The General Query Log”.

Running a server with binary logging enabled makes performance slightly slower. However, the benefits of the binary log in enabling you to set up replication and for restore operations generally outweigh this minor performance decrement.

The binary log should be protected because logged statements might contain passwords. See Section 6.1.2.3, “Passwords and Logging”.

The following discussion describes some of the server options and variables that affect the operation of binary logging. For a complete list, see Section 17.1.3.4, “Binary Log Options and Variables”.

To enable the binary log, start the server with the --log-bin[=base_name] option. If no base_name value is given, the default name is the value of the pid-file option (which by default is the name of host machine) followed by -bin. If the base name is given, the server writes the file in the data directory unless the base name is given with a leading absolute path name to specify a different directory. It is recommended that you specify a base name explicitly rather than using the default of the host name; see Section B.5.7, “Known Issues in MySQL”, for the reason.

If you supply an extension in the log name (for example, --log-bin=base_name.extension), the extension is silently removed and ignored.

mysqld appends a numeric extension to the binary log base name to generate binary log file names. The number increases each time the server creates a new log file, thus creating an ordered series of files. The server creates a new file in the series each time it starts or flushes the logs. The server also creates a new binary log file automatically after the current log's size reaches max_binlog_size. A binary log file may become larger than max_binlog_size if you are using large transactions because a transaction is written to the file in one piece, never split between files.

To keep track of which binary log files have been used, mysqld also creates a binary log index file that contains the names of all used binary log files. By default, this has the same base name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while mysqld is running; doing so would confuse mysqld.

The term binary log file generally denotes an individual numbered file containing database events. The term binary log collectively denotes the set of numbered binary log files plus the index file.

A client that has the SUPER privilege can disable binary logging of its own statements by using a SET sql_log_bin=0 statement. See Section 5.1.4, “Server System Variables”.

The format of the events recorded in the binary log is dependent on the binary logging format. Three format types are supported, row-based logging, statement-based logging and mixed-base logging. The binary logging format used depends on the MySQL version. For general descriptions of the logging formats, see Section 5.4.4.1, “Binary Logging Formats”. For detailed information about the format of the binary log, see MySQL Internals: The Binary Log.

The server evaluates the --binlog-do-db and --binlog-ignore-db options in the same way as it does the --replicate-do-db and --replicate-ignore-db options. For information about how this is done, see Section 17.2.3.1, “Evaluation of Database-Level Replication and Binary Logging Options”.

If you are replicating from a MySQL Cluster to a standalone MySQL Server, you should be aware that the NDB storage engine uses default values for some binary logging options (including options specific to NDB such as --ndb-log-update-as-write) that differ from those used by other storage engines. If not corrected for, these differences can lead to divergence of the master's and slave's binary logs. For more information, see Replication from NDB to other storage engines. In particular, if you are using a nontransactional storage engine such as MyISAM on the slave, see Replication from NDB to a nontransactional storage engine.

A replication slave server by default does not write to its own binary log any data modifications that are received from the replication master. To log these modifications, start the slave with the --log-slave-updates option in addition to the --log-bin option (see Section 17.1.3.3, “Replication Slave Options and Variables”). This is done when a slave is also to act as a master to other slaves in chained replication.

You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE BINARY LOGS. See Section 13.7.6.6, “RESET Syntax”, and Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

If you are using replication, you should not delete old binary log files on the master until you are sure that no slave still needs to use them. For example, if your slaves never run more than three days behind, once a day you can execute mysqladmin flush-logs on the master and then remove any logs that are more than three days old. You can remove the files manually, but it is preferable to use PURGE BINARY LOGS, which also safely updates the binary log index file for you (and which can take a date argument). See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

You can display the contents of binary log files with the mysqlbinlog utility. This can be useful when you want to reprocess statements in the log for a recovery operation. For example, you can update a MySQL server from the binary log as follows:

shell> mysqlbinlog log_file | mysql -h server_name

mysqlbinlog also can be used to display replication slave relay log file contents because they are written using the same format as binary log files. For more information on the mysqlbinlog utility and how to use it, see Section 4.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”. For more information about the binary log and recovery operations, see Section 7.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

Binary logging is done immediately after a statement or transaction completes but before any locks are released or any commit is done. This ensures that the log is logged in commit order.

Updates to nontransactional tables are stored in the binary log immediately after execution.

Within an uncommitted transaction, all updates (UPDATE, DELETE, or INSERT) that change transactional tables such as InnoDB tables are cached until a COMMIT statement is received by the server. At that point, mysqld writes the entire transaction to the binary log before the COMMIT is executed.

Modifications to nontransactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to nontransactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that the modifications to those tables are replicated.

When a thread that handles the transaction starts, it allocates a buffer of binlog_cache_size to buffer statements. If a statement is bigger than this, the thread opens a temporary file to store the transaction. The temporary file is deleted when the thread ends.

The Binlog_cache_use status variable shows the number of transactions that used this buffer (and possibly a temporary file) for storing statements. The Binlog_cache_disk_use status variable shows how many of those transactions actually had to use a temporary file. These two variables can be used for tuning binlog_cache_size to a large enough value that avoids the use of temporary files.

The max_binlog_cache_size system variable (default 4GB, which is also the maximum) can be used to restrict the total size used to cache a multiple-statement transaction. If a transaction is larger than this many bytes, it fails and rolls back. The minimum value is 4096.

If you are using the binary log and row based logging, concurrent inserts are converted to normal inserts for CREATE ... SELECT or INSERT ... SELECT statements. This is done to ensure that you can re-create an exact copy of your tables by applying the log during a backup operation. If you are using statement-based logging, the original statement is written to the log.

The binary log format has some known limitations that can affect recovery from backups. See Section 17.4.1, “Replication Features and Issues”.

Binary logging for stored programs is done as described in Section 20.7, “Binary Logging of Stored Programs”.

Note that the binary log format differs in MySQL 5.5 from previous versions of MySQL, due to enhancements in replication. See Section 17.4.2, “Replication Compatibility Between MySQL Versions”.

Writes to the binary log file and binary log index file are handled in the same way as writes to MyISAM tables. See Section B.5.3.4, “How MySQL Handles a Full Disk”.

By default, the binary log is not synchronized to disk at each write. So if the operating system or machine (not only the MySQL server) crashes, there is a chance that the last statements of the binary log are lost. To prevent this, you can make the binary log be synchronized to disk after every N writes to the binary log, with the sync_binlog system variable. See Section 5.1.4, “Server System Variables”. 1 is the safest value for sync_binlog, but also the slowest. Even with sync_binlog set to 1, there is still the chance of an inconsistency between the table content and binary log content in case of a crash. For example, if you are using InnoDB tables and the MySQL server processes a COMMIT statement, it writes the whole transaction to the binary log and then commits this transaction into InnoDB. If the server crashes between those two operations, the transaction is rolled back by InnoDB at restart but still exists in the binary log. To resolve this, you should set --innodb_support_xa to 1. Although this option is related to the support of XA transactions in InnoDB, it also ensures that the binary log and InnoDB data files are synchronized.

For this option to provide a greater degree of safety, the MySQL server should also be configured to synchronize the binary log and the InnoDB logs to disk at every transaction. The InnoDB logs are synchronized by default, and sync_binlog=1 can be used to synchronize the binary log. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server cuts rolled back InnoDB transactions from the binary log. This ensures that the binary log reflects the exact data of InnoDB tables, and so, that the slave remains in synchrony with the master (not receiving a statement which has been rolled back).

If the MySQL server discovers at crash recovery that the binary log is shorter than it should have been, it lacks at least one successfully committed InnoDB transaction. This should not happen if sync_binlog=1 and the disk/file system do an actual sync when they are requested to (some do not), so the server prints an error message The binary log file_name is shorter than its expected size. In this case, this binary log is not correct and replication should be restarted from a fresh snapshot of the master's data.

The session values of the following system variables are written to the binary log and honored by the replication slave when parsing the binary log:

5.4.4.1 Binary Logging Formats

The server uses several logging formats to record information in the binary log. The exact format employed depends on the version of MySQL being used. There are three logging formats:

  • Replication capabilities in MySQL originally were based on propagation of SQL statements from master to slave. This is called statement-based logging. You can cause this format to be used by starting the server with --binlog-format=STATEMENT.

  • In row-based logging, the master writes events to the binary log that indicate how individual table rows are affected. You can cause the server to use row-based logging by starting it with --binlog-format=ROW.

  • A third option is also available: mixed logging. With mixed logging, statement-based logging is used by default, but the logging mode switches automatically to row-based in certain cases as described below. You can cause MySQL to use mixed logging explicitly by starting mysqld with the option --binlog-format=MIXED.

In MySQL 5.5, the default binary logging format is STATEMENT.

The logging format can also be set or limited by the storage engine being used. This helps to eliminate issues when replicating certain statements between a master and slave which are using different storage engines.

With statement-based replication, there may be issues with replicating nondeterministic statements. In deciding whether or not a given statement is safe for statement-based replication, MySQL determines whether it can guarantee that the statement can be replicated using statement-based logging. If MySQL cannot make this guarantee, it marks the statement as potentially unreliable and issues the warning, Statement may not be safe to log in statement format.

You can avoid these issues by using MySQL's row-based replication instead.

5.4.4.2 Setting The Binary Log Format

You can select the binary logging format explicitly by starting the MySQL server with --binlog-format=type. The supported values for type are:

  • STATEMENT causes logging to be statement based.

  • ROW causes logging to be row based.

  • MIXED causes logging to use mixed format.

In MySQL 5.5, the default binary logging format is STATEMENT. This includes MySQL Cluster NDB 7.2.1 and later MySQL Cluster NDB 7.2 releases, which are based on MySQL 5.5.

The logging format also can be switched at runtime. To specify the format globally for all clients, set the global value of the binlog_format system variable:

mysql> SET GLOBAL binlog_format = 'STATEMENT';
mysql> SET GLOBAL binlog_format = 'ROW';
mysql> SET GLOBAL binlog_format = 'MIXED';

An individual client can control the logging format for its own statements by setting the session value of binlog_format:

mysql> SET SESSION binlog_format = 'STATEMENT';
mysql> SET SESSION binlog_format = 'ROW';
mysql> SET SESSION binlog_format = 'MIXED';
Note

Each MySQL Server can set its own and only its own binary logging format (true whether binlog_format is set with global or session scope). This means that changing the logging format on a replication master does not cause a slave to change its logging format to match. (When using STATEMENT mode, the binlog_format system variable is not replicated; when using MIXED or ROW logging mode, it is replicated but is ignored by the slave.) Changing the binary logging format on the master while replication is ongoing, or without also changing it on the slave can cause replication to fail with errors such as Error executing row event: 'Cannot execute statement: impossible to write to binary log since statement is in row format and BINLOG_FORMAT = STATEMENT.'

To change the global or session binlog_format value, you must have the SUPER privilege.

There are several reasons why a client might want to set binary logging on a per-session basis:

  • A session that makes many small changes to the database might want to use row-based logging.

  • A session that performs updates that match many rows in the WHERE clause might want to use statement-based logging because it will be more efficient to log a few statements than many rows.

  • Some statements require a lot of execution time on the master, but result in just a few rows being modified. It might therefore be beneficial to replicate them using row-based logging.

There are exceptions when you cannot switch the replication format at runtime:

  • From within a stored function or a trigger

  • If the NDBCLUSTER storage engine is enabled

  • If the session is currently in row-based replication mode and has open temporary tables

Trying to switch the format in any of these cases results in an error.

If you are using InnoDB tables and the transaction isolation level is READ COMMITTED or READ UNCOMMITTED, only row-based logging can be used. It is possible to change the logging format to STATEMENT, but doing so at runtime leads very rapidly to errors because InnoDB can no longer perform inserts.

Switching the replication format at runtime is not recommended when any temporary tables exist, because temporary tables are logged only when using statement-based replication, whereas with row-based replication they are not logged. With mixed replication, temporary tables are usually logged; exceptions happen with user-defined functions (UDFs) and with the UUID() function.

With the binary log format set to ROW, many changes are written to the binary log using the row-based format. Some changes, however, still use the statement-based format. Examples include all DDL (data definition language) statements such as CREATE TABLE, ALTER TABLE, or DROP TABLE.

The --binlog-row-event-max-size option is available for servers that are capable of row-based replication. Rows are stored into the binary log in chunks having a size in bytes not exceeding the value of this option. The value must be a multiple of 256. The default value is 1024.

Warning

When using statement-based logging for replication, it is possible for the data on the master and slave to become different if a statement is designed in such a way that the data modification is nondeterministic; that is, it is left to the will of the query optimizer. In general, this is not a good practice even outside of replication. For a detailed explanation of this issue, see Section B.5.7, “Known Issues in MySQL”.

5.4.4.3 Mixed Binary Logging Format

When running in MIXED logging format, the server automatically switches from statement-based to row-based logging under the following conditions:

Note

A warning is generated if you try to execute a statement using statement-based logging that should be written using row-based logging. The warning is shown both in the client (in the output of SHOW WARNINGS) and through the mysqld error log. A warning is added to the SHOW WARNINGS table each time such a statement is executed. However, only the first statement that generated the warning for each client session is written to the error log to prevent flooding the log.

In addition to the decisions above, individual engines can also determine the logging format used when information in a table is updated. The logging capabilities of an individual engine can be defined as follows:

  • If an engine supports row-based logging, the engine is said to be row-logging capable.

  • If an engine supports statement-based logging, the engine is said to be statement-logging capable.

A given storage engine can support either or both logging formats. The following table lists the formats supported by each engine.

Storage EngineRow Logging SupportedStatement Logging Supported
ARCHIVEYesYes
BLACKHOLEYesYes
CSVYesYes
EXAMPLEYesNo
FEDERATEDYesYes
HEAPYesYes
InnoDBYesYes when the transaction isolation level is REPEATABLE READ or SERIALIZABLE; No otherwise.
MyISAMYesYes
MERGEYesYes
NDBCLUSTERYesNo

In MySQL 5.5.3 and later, whether a statement is to be logged and the logging mode to be used is determined according to the type of statement (safe, unsafe, or binary injected), the binary logging format (STATEMENT, ROW, or MIXED), and the logging capabilities of the storage engine (statement capable, row capable, both, or neither). (Binary injection refers to logging a change that must be logged using ROW format.)

Statements may be logged with or without a warning; failed statements are not logged, but generate errors in the log. This is shown in the following decision table, where SLC stands for statement-logging capable and RLC stands for row-logging capable.

ConditionAction
Typebinlog_formatSLCRLCError / WarningLogged as
**NoNoError: Cannot execute statement: Binary logging is impossible since at least one engine is involved that is both row-incapable and statement-incapable.-
SafeSTATEMENTYesNo-STATEMENT
SafeMIXEDYesNo-STATEMENT
SafeROWYesNoError: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = ROW and at least one table uses a storage engine that is not capable of row-based logging.-
UnsafeSTATEMENTYesNoWarning: Unsafe statement binlogged in statement format, since BINLOG_FORMAT = STATEMENTSTATEMENT
UnsafeMIXEDYesNoError: Cannot execute statement: Binary logging of an unsafe statement is impossible when the storage engine is limited to statement-based logging, even if BINLOG_FORMAT = MIXED.-
UnsafeROWYesNoError: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = ROW and at least one table uses a storage engine that is not capable of row-based logging.-
Row InjectionSTATEMENTYesNoError: Cannot execute row injection: Binary logging is not possible since at least one table uses a storage engine that is not capable of row-based logging.-
Row InjectionMIXEDYesNoError: Cannot execute row injection: Binary logging is not possible since at least one table uses a storage engine that is not capable of row-based logging.-
Row InjectionROWYesNoError: Cannot execute row injection: Binary logging is not possible since at least one table uses a storage engine that is not capable of row-based logging.-
SafeSTATEMENTNoYesError: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine that is not capable of statement-based logging.-
SafeMIXEDNoYes-ROW
SafeROWNoYes-ROW
UnsafeSTATEMENTNoYesError: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine that is not capable of statement-based logging.-
UnsafeMIXEDNoYes-ROW
UnsafeROWNoYes-ROW
Row InjectionSTATEMENTNoYesError: Cannot execute row injection: Binary logging is not possible since BINLOG_FORMAT = STATEMENT.-
Row InjectionMIXEDNoYes-ROW
Row InjectionROWNoYes-ROW
SafeSTATEMENTYesYes-STATEMENT
SafeMIXEDYesYes-STATEMENT
SafeROWYesYes-ROW
UnsafeSTATEMENTYesYesWarning: Unsafe statement binlogged in statement format since BINLOG_FORMAT = STATEMENT.STATEMENT
UnsafeMIXEDYesYes-ROW
UnsafeROWYesYes-ROW
Row InjectionSTATEMENTYesYesError: Cannot execute row injection: Binary logging is not possible because BINLOG_FORMAT = STATEMENT.-
Row InjectionMIXEDYesYes-ROW
Row InjectionROWYesYes-ROW

Handling of mixed-format logging in MySQL 5.5.2 and earlier.  The decision-making process for binary logging changed in MySQL 5.5.3, due to the fix for Bug #39934. Prior to MySQL 5.5.3, when determining the logging mode to be used, the capabilities of all the tables affected by the event are combined, and the set of affected tables is then marked according to these rules:

  • A set of tables is defined as row-logging restricted if the tables are row-logging capable but not statement-logging capable.

  • A set of tables is defined as statement-logging restricted if the tables are statement-logging capable but not row-logging capable.

Once the determination of the possible logging formats required by the statement is complete it is compared to the current binlog_format setting. The following table is used in MySQL 5.5.2 and earlier to decide how the information is recorded in the binary log or, if appropriate, whether an error is raised. In the table, a safe operation is defined as one that is deterministic.

In MySQL 5.5.2 and earlier, several rules decide whether the statement is deterministic, as shown in the following table, where SLR stands for statement-logging restricted and RLR stands for row-logging restricted. A statement is statement-logging restricted if one or more of the tables it accesses is not row-logging capable. Similarly, a statement is row-logging restricted if any table accessed by the statement is not statement-logging capable.

ConditionAction
Safe/unsafebinlog_formatSLRRLRError/WarningLogged as
SafeSTATEMENTYesYesError: not loggable 
SafeSTATEMENTYesNo STATEMENT
SafeSTATEMENTNoYesError: not loggable 
SafeSTATEMENTNoNo STATEMENT
SafeMIXEDYesYesError: not loggable 
SafeMIXEDYesNo STATEMENT
SafeMIXEDNoYes ROW
SafeMIXEDNoNo STATEMENT
SafeROWYesYesError: not loggable 
SafeROWYesNoError: not loggable 
SafeROWNoYes ROW
SafeROWNoNo ROW
UnsafeSTATEMENTYesYesError: not loggable 
UnsafeSTATEMENTYesNoWarning: unsafeSTATEMENT
UnsafeSTATEMENTNoYesError: not loggable 
UnsafeSTATEMENTNoNoWarning: unsafeSTATEMENT
UnsafeMIXEDYesYesError: not loggable 
UnsafeMIXEDYesNoError: not loggable 
UnsafeMIXEDNoYes ROW
UnsafeMIXEDNoNo ROW
UnsafeROWYesYesError: not loggable 
UnsafeROWYesNoError: not loggable 
UnsafeROWNoYes ROW
UnsafeROWNoNo ROW

In all MySQL 5.5 releases, when a warning is produced by the determination, a standard MySQL warning is produced (and is available using SHOW WARNINGS). The information is also written to the mysqld error log. Only one error for each error instance per client connection is logged to prevent flooding the log. The log message includes the SQL statement that was attempted.

If a slave server was started with --log-warnings enabled, the slave prints messages to the error log to provide information about its status, such as the binary log and relay log coordinates where it starts its job, when it is switching to another relay log, when it reconnects after a disconnect, and so forth.

5.4.4.4 Logging Format for Changes to mysql Database Tables

The contents of the grant tables in the mysql database can be modified directly (for example, with INSERT or DELETE) or indirectly (for example, with GRANT or CREATE USER). Statements that affect mysql database tables are written to the binary log using the following rules:

CREATE TABLE ... SELECT is a combination of data definition and data manipulation. The CREATE TABLE part is logged using statement format and the SELECT part is logged according to the value of binlog_format.

5.4.5 The Slow Query Log

The slow query log consists of SQL statements that took more than long_query_time seconds to execute and required at least min_examined_row_limit rows to be examined. The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds. For logging to a file, times are written including the microseconds part. For logging to tables, only integer times are written; the microseconds part is ignored.

By default, administrative statements are not logged, nor are queries that do not use indexes for lookups. This behavior can be changed using --log-slow-admin-statements and log_queries_not_using_indexes, as described later.

The time to acquire the initial locks is not counted as execution time. mysqld writes a statement to the slow query log after it has been executed and after all locks have been released, so log order might differ from execution order.

By default, the slow query log is disabled. To specify the initial slow query log state explicitly, use --slow_query_log[={0|1}]. With no argument or an argument of 1, --slow_query_log enables the log. With an argument of 0, this option disables the log. To specify a log file name, use --slow_query_log_file=file_name. To specify the log destination, use --log-output (as described in Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”). The older option to enable the slow query log file, --log-slow-queries, is deprecated.

If you specify no name for the slow query log file, the default name is host_name-slow.log. The server creates the file in the data directory unless an absolute path name is given to specify a different directory.

To disable or enable the slow query log or change the log file name at runtime, use the global slow_query_log and slow_query_log_file system variables. Set slow_query_log to 0 (or OFF) to disable the log or to 1 (or ON) to enable it. Set slow_query_log_file to specify the name of the log file. If a log file already is open, it is closed and the new file is opened.

When the slow query log is enabled, the server writes output to any destinations specified by the --log-output option or log_output system variable. If you enable the log, the server opens the log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected. If the destination is NONE, the server writes no queries even if the slow query log is enabled. Setting the log file name has no effect on logging if the log destination value does not contain FILE.

The server writes less information to the slow query log if you use the --log-short-format option.

To include slow administrative statements in the statements written to the slow query log, use the --log-slow-admin-statements server option. Administrative statements include ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE.

To include queries that do not use indexes for row lookups in the statements written to the slow query log, enable the log_queries_not_using_indexes system variable. When such queries are logged, the slow query log may grow quickly.

The server uses the controlling parameters in the following order to determine whether to write a query to the slow query log:

  1. The query must either not be an administrative statement, or --log-slow-admin-statements must have been specified.

  2. The query must have taken at least long_query_time seconds, or log_queries_not_using_indexes must be enabled and the query used no indexes for row lookups.

  3. The query must have examined at least min_examined_row_limit rows.

The server does not write queries handled by the query cache to the slow query log, nor queries that would not benefit from the presence of an index because the table has zero rows or one row.

By default, a replication slave does not write replicated queries to the slow query log. To change this, use the --log-slow-slave-statements server option.

The slow query log should be protected because logged statements might contain passwords. See Section 6.1.2.3, “Passwords and Logging”.

The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can become a difficult task. To make this easier, you can process a slow query log file using the mysqldumpslow command to summarize the queries that appear in the log. See Section 4.6.8, “mysqldumpslow — Summarize Slow Query Log Files”.

5.4.6 The DDL Log

The DDL log, or metadata log, records metadata operations generated by data definition statements such as DROP TABLE and ALTER TABLE. MySQL uses this log to recover from crashes occurring in the middle of a metadata operation. When executing the statement DROP TABLE t1, t2, we need to ensure that both t1 and t2 are dropped, and that each table drop is complete. Another example of this type of SQL statement is ALTER TABLE t3 DROP PARTITION p2, where we must make certain that the partition is completely dropped and that its definition is removed from the list of partitions for table t3.

A record of metadata operations such as those just described are written to the file ddl_log.log, in the MySQL data directory. This is a binary file; it is not intended to be human-readable, and you should not attempt to modify it in any way.

ddl_log.log is not created until it is actually needed for recording metadata statements, so it is possible for this file not to be present on a MySQL server that is functioning in a completely normal manner.

There are no user-configurable server options or variables associated with this file.

5.4.7 Server Log Maintenance

As described in Section 5.4, “MySQL Server Logs”, MySQL Server can create several different log files to help you see what activity is taking place. However, you must clean up these files regularly to ensure that the logs do not take up too much disk space.

When using MySQL with logging enabled, you may want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See Section 7.2, “Database Backup Methods”.

On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, this script should have been installed automatically. Be careful with this script if you are using the binary log for replication. You should not remove binary logs until you are certain that their contents have been processed by all slaves.

On other systems, you must install a short script yourself that you start from cron (or its equivalent) for handling log files.

For the binary log, you can set the expire_logs_days system variable to expire binary log files automatically after a given number of days (see Section 5.1.4, “Server System Variables”). If you are using replication, you should set the variable no lower than the maximum number of days your slaves might lag behind the master. To remove binary logs on demand, use the PURGE BINARY LOGS statement (see Section 13.4.1.1, “PURGE BINARY LOGS Syntax”).

You can force MySQL to start using new log files by flushing the logs. Log flushing occurs when you issue a FLUSH LOGS statement or execute a mysqladmin flush-logs, mysqladmin refresh, mysqldump --flush-logs, or mysqldump --master-data command. See Section 13.7.6.3, “FLUSH Syntax”, Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In addition, the binary log is flushed when its size reaches the value of the max_binlog_size system variable.

As of MySQL 5.5.3, FLUSH LOGS supports optional modifiers to enable selective flushing of individual logs (for example, FLUSH BINARY LOGS).

A log-flushing operation does the following:

  • If general query logging or slow query logging to a log file is enabled, the server closes and reopens the general query log file or slow query log file.

  • If binary logging is enabled, the server closes the current binary log file and opens a new log file with the next sequence number.

  • If the server was started with the --log-error option to cause the error log to be written to a file, the result of a log-flushing operation is version dependent:

    • As of MySQL 5.5.7, the server closes and reopens the log file.

    • Prior to MySQL 5.5.7, the server renames the current log file with the suffix -old, then creates a new empty log file.

The server creates a new binary log file when you flush the logs. However, it just closes and reopens the general and slow query log files. To cause new files to be created on Unix, rename the current log files before flushing them. At flush time, the server opens new log files with the original names. For example, if the general and slow query log files are named mysql.log and mysql-slow.log, you can use a series of commands like this:

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mv mysql-slow.log mysql-slow.old
shell> mysqladmin flush-logs

On Windows, use rename rather than mv.

At this point, you can make a backup of mysql.old and mysql-slow.old and then remove them from disk.

A similar strategy can be used to back up the error log file, if there is one, except that, on Windows, you cannot rename the error log file while the server has it open before MySQL 5.5.7. To rename the error log file, a stop and restart can be avoided by flushing the logs to cause the server to rename the current log file with the suffix -old and create a new empty error log file. For further information, see Section 5.4.2, “The Error Log”.

You can rename the general query log or slow query log at runtime by disabling the log:

SET GLOBAL general_log = 'OFF';
SET GLOBAL slow_query_log = 'OFF';

With the logs disabled, rename the log files externally; for example, from the command line. Then enable the logs again:

SET GLOBAL general_log = 'ON';
SET GLOBAL slow_query_log = 'ON';

This method works on any platform and does not require a server restart.

5.5 MySQL Server Plugins

MySQL supports a plugin API that enables creation of server components. Plugins can be loaded at server startup, or loaded and unloaded at runtime without restarting the server. The components supported by this interface include, but are not limited to, storage engines, INFORMATION_SCHEMA tables, full-text parser plugins, partitioning support, and server extensions.

5.5.1 Server Plugins Available

MySQL distributions include several plugins that implement server extensions:

The following sections describe how to install and uninstall plugins, and how to determine at runtime which plugins are installed and obtain information about them. For information about writing plugins, see Section 24.2, “The MySQL Plugin API”.

5.5.2 Installing and Uninstalling Plugins

Server plugins must be loaded into the server before they can be used. MySQL supports plugin loading at server startup and runtime. It is also possible to control the activation state of loaded plugins at startup, and to unload them at runtime.

While a plugin is loaded, information about it is available at runtime from the INFORMATION_SCHEMA.PLUGINS table and the SHOW PLUGINS statement. See Section 5.5.3, “Obtaining Server Plugin Information”.

Installing Plugins

Before a server plugin can be used, it must be installed using one of the following methods. In the descriptions, plugin_name stands for a plugin name such as innodb or csv.

Built-in plugins:

A built-in plugin is known by the server automatically. Normally, the server enables the plugin at startup. Some built-in plugins permit this to be changed with the --plugin_name[=activation_state] option.

Plugins registered in the mysql.plugin system table:

The mysql.plugin table serves as a registry of plugins (other than built-in plugins, which need not be registered). At startup, the server loads each plugin listed in the table. Normally, for a plugin loaded from the mysql.plugin table, the server also enables the plugin. This can be changed with the --plugin_name[=activation_state] option.

If the server is started with the --skip-grant-tables option, it does not consult the mysql.plugin table and does not load the plugins listed there.

Plugins named with the --plugin-load option:

A plugin located in a plugin library file can be loaded at server startup with the --plugin-load or --plugin-load-add option. Normally, for a plugin loaded at startup, the server also enables the plugin. This can be changed with the --plugin_name[=activation_state] option.

The value of each plugin-loading option is a semicolon-separated list of name=plugin_library and plugin_library values. Each name is the name of a plugin to load, and plugin_library is the name of the library file that contains the plugin code. If a plugin library is named without any preceding plugin name, the server loads all plugins in the library. The server looks for plugin library files in the directory named by the plugin_dir system variable.

Plugin-loading options do not register any plugin in the mysql.plugin table. For subsequent restarts, the server loads the plugin again only if --plugin-load or --plugin-load-add is given again. That is, the option produces a one-time plugin-installation operation that persists for a single server invocation.

--plugin-load enables plugins to be loaded even when --skip-grant-tables is given (which causes the server to ignore the mysql.plugin table). --plugin-load also enables plugins to be loaded at startup under configurations when plugins cannot be loaded at runtime.

Plugins installed with the INSTALL PLUGIN statement:

A plugin located in a plugin library file can be loaded at runtime with the INSTALL PLUGIN statement. The statement also registers the plugin in the mysql.plugin table to cause the server to load it on subsequent restarts. For this reason, INSTALL PLUGIN requires the INSERT privilege for the mysql.plugin table.

The plugin library file base name depends on your platform. Common suffixes are .so for Unix and Unix-like systems, .dll for Windows.

Example: The --plugin-load option installs a plugin at server startup. To install a plugin named myplugin from a plugin library file named somepluglib.so, use these lines in a my.cnf file:

[mysqld]
plugin-load=myplugin=somepluglib.so

In this case, the plugin is not registered in mysql.plugin. Restarting the server without the --plugin-load option causes the plugin not to be loaded at startup.

Alternatively, the INSTALL PLUGIN statement causes the server to load the plugin code from the library file at runtime:

INSTALL PLUGIN myplugin SONAME 'somepluglib.so';

INSTALL PLUGIN also causes permanent plugin registration: The plugin is listed in the mysql.plugin table to ensure that the server loads it on subsequent restarts.

Many plugins can be loaded either at server startup or at runtime. However, if a plugin is designed such that it must be loaded and initialized during server startup, attempts to load it at runtime using INSTALL PLUGIN produce an error:

mysql> INSTALL PLUGIN myplugin SONAME 'somepluglib.so';
ERROR 1721 (HY000): Plugin 'myplugin' is marked as not dynamically
installable. You have to stop the server to install it.

In this case, you must use --plugin-load or --plugin-load-add.

Many plugins can be loaded either at server startup or at runtime. However, if a plugin is designed such that it must be loaded and initialized during server startup, use --plugin-load rather than INSTALL PLUGIN.

If a plugin is named both using a --plugin-load option and (as a result of an earlier INSTALL PLUGIN statement) in the mysql.plugin table, the server starts but writes these messages to the error log:

[ERROR] Function 'plugin_name' already exists
[Warning] Couldn't load plugin named 'plugin_name'
with soname 'plugin_object_file'.

Controlling Plugin Activation State

If the server knows about a plugin when it starts (for example, because the plugin is named using a --plugin-load option or is registered in the mysql.plugin table), the server loads and enables the plugin by default. It is possible to control activation state for such a plugin using a --plugin_name[=activation_state] startup option, where plugin_name is the name of the plugin to affect, such as innodb or csv. As with other options, dashes and underscores are interchangeable in option names. Also, activation state values are not case sensitive. For example, --my_plugin=ON and --my-plugin=on are equivalent.

  • --plugin_name=OFF

    Tells the server to disable the plugin. This may not be possible for certain built-in plugins, such as mysql_native_password.

  • --plugin_name[=ON]

    Tells the server to enable the plugin. (Specifying the option as --plugin_name without a value has the same effect.) If the plugin fails to initialize, the server runs with the plugin disabled.

  • --plugin_name=FORCE

    Tells the server to enable the plugin, but if plugin initialization fails, the server does not start. In other words, this option forces the server to run with the plugin enabled or not at all.

  • --plugin_name=FORCE_PLUS_PERMANENT

    Like FORCE, but in addition prevents the plugin from being unloaded at runtime. If a user attempts to do so with UNINSTALL PLUGIN, an error occurs. This value is available as of MySQL 5.5.7.

Plugin activation states are visible in the LOAD_OPTION column of the INFORMATION_SCHEMA.PLUGINS table.

Suppose that CSV, BLACKHOLE, and ARCHIVE are built-in pluggable storage engines and that you want the server to load them at startup, subject to these conditions: The server is permitted to run if CSV initialization fails, must require that BLACKHOLE initialization succeeds, and should disable ARCHIVE. To accomplish that, use these lines in an option file:

[mysqld]
csv=ON
blackhole=FORCE
archive=OFF

The --enable-plugin_name option format is a synonym for --plugin_name=ON. The --disable-plugin_name and --skip-plugin_name option formats are synonyms for --plugin_name=OFF.

Before MySQL 5.1.36, plugin options are boolean options (see Section 4.2.5, “Program Option Modifiers”). That is, any of these options enable the plugin:

--plugin_name
--plugin_name=1
--enable-plugin_name

And these options disable the plugin:

--plugin_name=0
--disable-plugin_name
--skip-plugin_name

If you upgrade to MySQL 5.5 from a version older than 5.1.36 and previously used options of the form --plugin_name=0 or --plugin_name=1, the equivalent options are now --plugin_name=OFF and --plugin_name=ON, respectively. You also have the choice of requiring plugins to start successfully by using --plugin_name=FORCE or --plugin_name=FORCE_PLUS_PERMANENT.

If a plugin is disabled, either explicitly with OFF or implicitly because it was enabled with ON but failed to initialize, aspects of server operation that require the plugin will change. For example, if the plugin implements a storage engine, existing tables for the storage engine become inaccessible, and attempts to create new tables for the storage engine result in tables that use the default storage engine unless the NO_ENGINE_SUBSTITUTION SQL mode is enabled to cause an error to occur instead.

Disabling a plugin may require adjustment to other options. For example, if you start the server using --skip-innodb to disable InnoDB, other innodb_xxx options likely will need to be omitted at startup. In addition, because InnoDB is the default storage engine, it will not start unless you specify another available storage engine with --default-storage-engine.

Uninstalling Plugins

At runtime, the UNINSTALL PLUGIN statement disables and uninstalls a plugin known to the server. The statement unloads the plugin and removes it from the mysql.plugin table, if it is registered there. For this reason, UNINSTALL PLUGIN statement requires the DELETE privilege for the mysql.plugin table. With the plugin no longer registered in the table, the server will not load the plugin automatically for subsequent restarts.

UNINSTALL PLUGIN can unload a plugin regardless of whether it was loaded at runtime with INSTALL PLUGIN or at startup with a plugin-loading option, subject to these conditions:

  • It cannot unload plugins that are built in to the server. These can be identified as those that have a library name of NULL in the output from INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS.

  • It cannot unload plugins for which the server was started with --plugin_name=FORCE_PLUS_PERMANENT, which prevents plugin unloading at runtime. These can be identified from the LOAD_OPTION column of the INFORMATION_SCHEMA.PLUGINS table.

To uninstall a plugin that currently is loaded at server startup with a plugin-loading option, use this procedure.

  1. Remove any options related to the plugin from the my.cnf file.

  2. Restart the server.

  3. Plugins normally are installed using either a plugin-loading option at startup or with INSTALL PLUGIN at runtime, but not both. However, removing options for a plugin from the my.cnf file may not be sufficient to uninstall it if at some point INSTALL PLUGIN has also been used. If the plugin still appears in the output from INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS, use UNINSTALL PLUGIN to remove it from the mysql.plugin table. Then restart the server again.

5.5.3 Obtaining Server Plugin Information

There are several ways to determine which plugins are installed in the server:

  • The INFORMATION_SCHEMA.PLUGINS table contains a row for each loaded plugin. Any that have a PLUGIN_LIBRARY value of NULL are built in and cannot be unloaded.

    mysql> SELECT * FROM information_schema.PLUGINS\G
    *************************** 1. row ***************************
               PLUGIN_NAME: binlog
            PLUGIN_VERSION: 1.0
             PLUGIN_STATUS: ACTIVE
               PLUGIN_TYPE: STORAGE ENGINE
       PLUGIN_TYPE_VERSION: 50158.0
            PLUGIN_LIBRARY: NULL
    PLUGIN_LIBRARY_VERSION: NULL
             PLUGIN_AUTHOR: MySQL AB
        PLUGIN_DESCRIPTION: This is a pseudo storage engine to represent the binlog in a transaction
            PLUGIN_LICENSE: GPL
               LOAD_OPTION: FORCE
    ...
    *************************** 10. row ***************************
               PLUGIN_NAME: InnoDB
            PLUGIN_VERSION: 1.0
             PLUGIN_STATUS: ACTIVE
               PLUGIN_TYPE: STORAGE ENGINE
       PLUGIN_TYPE_VERSION: 50158.0
            PLUGIN_LIBRARY: ha_innodb_plugin.so
    PLUGIN_LIBRARY_VERSION: 1.0
             PLUGIN_AUTHOR: Innobase Oy
        PLUGIN_DESCRIPTION: Supports transactions, row-level locking,
                            and foreign keys
            PLUGIN_LICENSE: GPL
               LOAD_OPTION: ON
    ...
    
  • The SHOW PLUGINS statement displays a row for each loaded plugin. Any that have a Library value of NULL are built in and cannot be unloaded.

    mysql> SHOW PLUGINS\G
    *************************** 1. row ***************************
       Name: binlog
     Status: ACTIVE
       Type: STORAGE ENGINE
    Library: NULL
    License: GPL
    ...
    *************************** 10. row ***************************
       Name: InnoDB
     Status: ACTIVE
       Type: STORAGE ENGINE
    Library: ha_innodb_plugin.so
    License: GPL
    ...
    
  • The mysql.plugin table shows which plugins have been registered with INSTALL PLUGIN. The table contains only plugin names and library file names, so it does not provide as much information as the PLUGINS table or the SHOW PLUGINS statement.

5.5.4 MySQL Enterprise Thread Pool

Note

MySQL Enterprise Thread Pool is an extension included in MySQL Enterprise Edition, a commercial product. To learn more about commercial products, http://www.mysql.com/products/.

As of MySQL 5.5.16, MySQL Enterprise Edition includes MySQL Enterprise Thread Pool, implemented using a server plugin. The default thread-handling model in MySQL Server executes statements using one thread per client connection. As more clients connect to the server and execute statements, overall performance degrades. The thread pool plugin provides an alternative thread-handling model designed to reduce overhead and improve performance. The plugin implements a thread pool that increases server performance by efficiently managing statement execution threads for large numbers of client connections.

The thread pool addresses several problems of the one thread per connection model:

  • Too many thread stacks make CPU caches almost useless in highly parallel execution workloads. The thread pool promotes thread stack reuse to minimize the CPU cache footprint.

  • With too many threads executing in parallel, context switching overhead is high. This also presents a challenging task to the operating system scheduler. The thread pool controls the number of active threads to keep the parallelism within the MySQL server at a level that it can handle and that is appropriate for the server host on which MySQL is executing.

  • Too many transactions executing in parallel increases resource contention. In InnoDB, this increases the time spent holding central mutexes. The thread pool controls when transactions start to ensure that not too many execute in parallel.

The thread pool plugin is included only in MySQL Enterprise Edition. It is not included in MySQL community distributions.

On Windows, the thread pool plugin requires Windows Vista or newer. On Linux, the plugin requires kernel 2.6.9 or higher.

Additional Resources

Section A.14, “MySQL 5.5 FAQ: MySQL Enterprise Thread Pool”

5.5.4.1 Thread Pool Components

The thread pool feature comprises these components:

5.5.4.2 Thread Pool Installation

This section describes how to install MySQL Enterprise Thread Pool. For general information about installing plugins, see Section 5.5.2, “Installing and Uninstalling Plugins”.

To be usable by the server, the plugin library file must be located in the MySQL plugin directory (the directory named by the plugin_dir system variable). If necessary, set the value of plugin_dir at server startup to tell the server the plugin directory location.

The plugin library file base name is thread_pool. The file name suffix differs per platform (for example, .so for Unix and Unix-like systems, .dll for Windows).

To enable thread pool capability, load the plugins to be used by starting the server with the --plugin-load option. For example, if you name just the plugin library file, the server loads all plugins that it contains (that is, the thread pool plugin and all the INFORMATION_SCHEMA tables). To do this, put these lines in your my.cnf file (adjust the .so suffix for your platform as necessary):

[mysqld]
plugin-load=thread_pool.so

That is equivalent to loading all thread pool plugins by naming them individually:

[mysqld]
plugin-load=thread_pool=thread_pool.so;tp_thread_state=thread_pool.so;tp_thread_group_state=thread_pool.so;tp_thread_group_stats=thread_pool.so

If desired, you can load individual plugins from the library file. To load the thread pool plugin but not the INFORMATION_SCHEMA tables, use an option like this:

[mysqld]
plugin-load=thread_pool=thread_pool.so

To load the thread pool plugin and only the TP_THREAD_STATE INFORMATION_SCHEMA table, use an option like this:

[mysqld]
plugin-load=thread_pool=thread_pool.so;tp_thread_state=thread_pool.so
Note

If you do not load all the INFORMATION_SCHEMA tables, some or all MySQL Enterprise Monitor thread pool graphs will be empty.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement (see Section 5.5.3, “Obtaining Server Plugin Information”). For example:

mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS FROM INFORMATION_SCHEMA.PLUGINS
    -> WHERE PLUGIN_NAME LIKE 'thread%' OR PLUGIN_NAME LIKE 'tp%';
+-----------------------+---------------+
| PLUGIN_NAME           | PLUGIN_STATUS |
+-----------------------+---------------+
| thread_pool           | ACTIVE        |
| TP_THREAD_STATE       | ACTIVE        |
| TP_THREAD_GROUP_STATE | ACTIVE        |
| TP_THREAD_GROUP_STATS | ACTIVE        |
+-----------------------+---------------+

If the server loads the thread pool plugin successfully, it sets the thread_handling system variable to dynamically-loaded. If the plugin fails to load, the server writes a message to the error log.

5.5.4.3 Thread Pool Operation

The thread pool consists of a number of thread groups, each of which manages a set of client connections. As connections are established, the thread pool assigns them to thread groups in round-robin fashion.

The number of thread groups is configurable using the thread_pool_size system variable. The default number of groups is 16. For guidelines on setting this variable, see Section 5.5.4.4, “Thread Pool Tuning”.

The maximum number of threads per group is 4096 (or 4095 on some systems where one thread is used internally).

The thread pool separates connections and threads, so there is no fixed relationship between connections and the threads that execute statements received from those connections. This differs from the default thread-handling model that associates one thread with one connection such that the thread executes all statements from the connection.

The thread pool tries to ensure a maximum of one thread executing in each group at any time, but sometimes permits more threads to execute temporarily for best performance. The algorithm works in the following manner:

  • Each thread group has a listener thread that listens for incoming statements from the connections assigned to the group. When a statement arrives, the thread group either begins executing it immediately or queues it for later execution:

    • Immediate execution occurs if the statement is the only one received and no statements are queued or currently executing.

    • Queuing occurs if the statement cannot begin executing immediately.

  • If immediate execution occurs, execution is performed by the listener thread. (This means that temporarily no thread in the group is listening.) If the statement finishes quickly, the executing thread returns to listening for statements. Otherwise, the thread pool considers the statement stalled and starts another thread as a listener thread (creating it if necessary). To ensure that no thread group becomes blocked by stalled statements, the thread pool has a background thread that regularly monitors thread group states.

    By using the listening thread to execute a statement that can begin immediately, there is no need to create an additional thread if the statement finishes quickly. This ensures the most efficient execution possible in the case of a low number of concurrent threads.

    When the thread pool plugin starts, it creates one thread per group (the listener thread), plus the background thread. Additional threads are created as necessary to execute statements.

  • The value of the thread_pool_stall_limit system variable determines the meaning of finishes quickly in the previous item. The default time before threads are considered stalled is 60ms but can be set to a maximum of 6s. This parameter is configurable to enable you to strike a balance appropriate for the server work load. Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.

  • The thread pool focuses on limiting the number of concurrent short-running statements. Before an executing statement reaches the stall time, it prevents other statements from beginning to execute. If the statement executes past the stall time, it is permitted to continue but no longer prevents other statements from starting. In this way, the thread pool tries to ensure that in each thread group there is never more than one short-running statement, although there might be multiple long-running statements. It is undesirable to let long-running statements prevent other statements from executing because there is no limit on the amount of waiting that might be necessary. For example, on a replication master, a thread that is sending binary log events to a slave effectively runs forever.

  • A statement becomes blocked if it encounters a disk I/O operation or a user level lock (row lock or table lock). The block would cause the thread group to become unused, so there are callbacks to the thread pool to ensure that the thread pool can immediately start a new thread in this group to execute another statement. When a blocked thread returns, the thread pool permits it to restart immediately.

  • There are two queues, a high-priority queue and a low-priority queue. The first statement in a transaction goes to the low-priority queue. Any following statements for the transaction go to the high-priority queue if the transaction is ongoing (statements for it have begun executing), or to the low-priority queue otherwise. Queue assignment can be affected by enabling the thread_pool_high_priority_connection system variable, which causes all queued statements for a session to go into the high-priority queue.

    Statements for a nontransactional storage engine, or a transactional engine if autocommit is enabled, are treated as low-priority statements because in this case each statement is a transaction. Thus, given a mix of statements for InnoDB and MyISAM tables, the thread pool prioritizes those for InnoDB over those for MyISAM unless autocommit is enabled. With autocommit enabled, all statements will be low priority.

  • When the thread group selects a queued statement for execution, it first looks in the high-priority queue, then in the low-priority queue. If a statement is found, it is removed from its queue and begins to execute.

  • If a statement stays in the low-priority queue too long, the thread pool moves to the high-priority queue. The value of the thread_pool_prio_kickup_timer system variable controls the time before movement. For each thread group, a maximum of one statement per 10ms or 100 per second will be moved from the low-priority queue to the high-priority queue.

  • The thread pool reuses the most active threads to obtain a much better use of CPU caches. This is a small adjustment that has a great impact on performance.

  • While a thread executes a statement from a user connection, Performance Schema instrumentation accounts thread activity to the user connection. Otherwise, Performance Schema accounts activity to the thread pool.

Here are examples of conditions under which a thread group might have multiple threads started to execute statements:

  • One thread begins executing a statement, but runs long enough to be considered stalled. The thread group permits another thread to begin executing another statement even through the first thread is still executing.

  • One thread begins executing a statement, then becomes blocked and reports this back to the thread pool. The thread group permits another thread to begin executing another statement.

  • One thread begins executing a statement, becomes blocked, but does not report back that it is blocked because the block does not occur in code that has been instrumented with thread pool callbacks. In this case, the thread appears to the thread group to be still running. If the block lasts long enough for the statement to be considered stalled, the group permits another thread to begin executing another statement.

The thread pool is designed to be scalable across an increasing number of connections. It is also designed to avoid deadlocks that can arise from limiting the number of actively executing statements. It is important that threads that do not report back to the thread pool do not prevent other statements from executing and thus cause the thread pool to become deadlocked. Examples of such statements follow:

  • Long-running statements. These would lead to all resources used by only a few statements and they could prevent all others from accessing the server.

  • Binary log dump threads that read the binary log and send it to slaves. This is a kind of long-running statement that runs for a very long time, and that should not prevent other statements from executing.

  • Statements blocked on a row lock, table lock, sleep, or any other blocking activity that has not been reported back to the thread pool by MySQL Server or a storage engine.

In each case, to prevent deadlock, the statement is moved to the stalled category when it does not complete quickly, so that the thread group can permit another statement to begin executing. With this design, when a thread executes or becomes blocked for an extended time, the thread pool moves the thread to the stalled category and for the rest of the statement's execution, it does not prevent other statements from executing.

The maximum number of threads that can occur is the sum of max_connections and thread_pool_size. This can happen in a situation where all connections are in execution mode and an extra thread is created per group to listen for more statements. This is not necessarily a state that happens often, but it is theoretically possible.

5.5.4.4 Thread Pool Tuning

This section provides guidelines on setting thread pool system variables for best performance, measured using a metric such as transactions per second.

thread_pool_size is the most important parameter controlling thread pool performance. It can be set only at server startup. Our experience in testing the thread pool indicates the following:

  • If the primary storage engine is InnoDB, the optimal thread_pool_size setting is likely to be between 16 and 36, with the most common optimal values tending to be from 24 to 36. We have not seen any situation where the setting has been optimal beyond 36. There may be special cases where a value smaller than 16 is optimal.

    For workloads such as DBT2 and Sysbench, the optimum for InnoDB seems to be usually around 36. For very write-intensive workloads, the optimal setting can sometimes be lower.

  • If the primary storage engine is MyISAM, the thread_pool_size setting should be fairly low. We tend to get optimal performance for values from 4 to 8. Higher values tend to have a slightly negative but not dramatic impact on performance.

Another system variable, thread_pool_stall_limit, is important for handling of blocked and long-running statements. If all calls that block the MySQL Server are reported to the thread pool, it would always know when execution threads are blocked. However, this may not always be true. For example, blocks could occur in code that has not been instrumented with thread pool callbacks. For such cases, the thread pool must be able to identify threads that appear to be blocked. This is done by means of a timeout, the length of which can be tuned using the thread_pool_stall_limit system variable. This parameter ensures that the server does not become completely blocked. The value of thread_pool_stall_limit has an upper limit of 6 seconds to prevent the risk of a deadlocked server.

thread_pool_stall_limit also enables the thread pool to handle long-running statements. If a long-running statement was permitted to block a thread group, all other connections assigned to the group would be blocked and unable to start execution until the long-running statement completed. In the worst case, this could take hours or even days.

The value of thread_pool_stall_limit should be chosen such that statements that execute longer than its value are considered stalled. Stalled statements generate a lot of extra overhead since they involve extra context switches and in some cases even extra thread creations. On the other hand, setting the thread_pool_stall_limit parameter too high means that long-running statements will block a number of short-running statements for longer than necessary. Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.

Suppose a server executes a workload where 99.9% of the statements complete within 100ms even when the server is loaded, and the remaining statements take between 100ms and 2 hours fairly evenly spread. In this case, it would make sense to set thread_pool_stall_limit to 10 (meaning 100ms). The default value of 60ms is okay for servers that primarily execute very simple statements.

The thread_pool_stall_limit parameter can be changed at runtime to enable you to strike a balance appropriate for the server work load. Assuming that the TP_THREAD_GROUP_STATS table is enabled, you can use the following query to determine the fraction of executed statements that stalled:

SELECT SUM(STALLED_QUERIES_EXECUTED) / SUM(QUERIES_EXECUTED)
FROM information_schema.TP_THREAD_GROUP_STATS;

This number should be as low as possible. To decrease the likelihood of statements stalling, increase the value of thread_pool_stall_limit.

When a statement arrives, what is the maximum time it can be delayed before it actually starts executing? Suppose that the following conditions apply:

In the worst case, the 10 high-priority statements represent 10 transactions that continue executing for a long time. Thus, in the worst case, no statements will be moved to the high-priority queue because it will always already contain statements awaiting execution. After 10 seconds, the new statement is eligible to be moved to the high-priority queue. However, before it can be moved, all the statements before it must be moved as well. This could take another 2 seconds because a maximum of 100 statements per second are moved to the high-priority queue. Now when the statement reaches the high-priority queue, there could potentially be many long-running statements ahead of it. In the worst case, every one of those will become stalled and it will take 1 second for each statement before the next statement is retrieved from the high-priority queue. Thus, in this scenario, it will take 222 seconds before the new statement starts executing.

This example shows a worst case for an application. How to handle it depends on the application. If the application has high requirements for the response time, it should most likely throttle users at a higher level itself. Otherwise, it can use the thread pool configuration parameters to set some kind of a maximum waiting time.

5.6 Running Multiple MySQL Instances on One Machine

In some cases, you might want to run multiple instances of MySQL on a single machine. You might want to test a new MySQL release while leaving an existing production setup undisturbed. Or you might want to give different users access to different mysqld servers that they manage themselves. (For example, you might be an Internet Service Provider that wants to provide independent MySQL installations for different customers.)

It is possible to use a different MySQL server binary per instance, or use the same binary for multiple instances, or any combination of the two approaches. For example, you might run a server from MySQL 5.1 and one from MySQL 5.5, to see how different versions handle a given workload. Or you might run multiple instances of the current production version, each managing a different set of databases.

Whether or not you use distinct server binaries, each instance that you run must be configured with unique values for several operating parameters. This eliminates the potential for conflict between instances. Parameters can be set on the command line, in option files, or by setting environment variables. See Section 4.2.3, “Specifying Program Options”. To see the values used by a given instance, connect to it and execute a SHOW VARIABLES statement.

The primary resource managed by a MySQL instance is the data directory. Each instance should use a different data directory, the location of which is specified using the --datadir=dir_name option. For methods of configuring each instance with its own data directory, and warnings about the dangers of failing to do so, see Section 5.6.1, “Setting Up Multiple Data Directories”.

In addition to using different data directories, several other options must have different values for each server instance:

  • --port=port_num

    --port controls the port number for TCP/IP connections. Alternatively, if the host has multiple network addresses, you can use --bind-address to cause each server to listen to a different address.

  • --socket={file_name|pipe_name}

    --socket controls the Unix socket file path on Unix or the named pipe name on Windows. On Windows, it is necessary to specify distinct pipe names only for those servers configured to permit named-pipe connections.

  • --shared-memory-base-name=name

    This option is used only on Windows. It designates the shared-memory name used by a Windows server to permit clients to connect using shared memory. It is necessary to specify distinct shared-memory names only for those servers configured to permit shared-memory connections.

  • --pid-file=file_name

    This option indicates the path name of the file in which the server writes its process ID.

If you use the following log file options, their values must differ for each server:

For further discussion of log file options, see Section 5.4, “MySQL Server Logs”.

To achieve better performance, you can specify the following option differently for each server, to spread the load between several physical disks:

Having different temporary directories also makes it easier to determine which MySQL server created any given temporary file.

If you have multiple MySQL installations in different locations, you can specify the base directory for each installation with the --basedir=dir_name option. This causes each instance to automatically use a different data directory, log files, and PID file because the default for each of those parameters is relative to the base directory. In that case, the only other options you need to specify are the --socket and --port options. Suppose that you install different versions of MySQL using tar file binary distributions. These install in different locations, so you can start the server for each installation using the command bin/mysqld_safe under its corresponding base directory. mysqld_safe determines the proper --basedir option to pass to mysqld, and you need specify only the --socket and --port options to mysqld_safe.

As discussed in the following sections, it is possible to start additional servers by specifying appropriate command options or by setting environment variables. However, if you need to run multiple servers on a more permanent basis, it is more convenient to use option files to specify for each server those option values that must be unique to it. The --defaults-file option is useful for this purpose.

5.6.1 Setting Up Multiple Data Directories

Each MySQL Instance on a machine should have its own data directory. The location is specified using the --datadir=dir_name option.

There are different methods of setting up a data directory for a new instance:

  • Create a new data directory.

  • Copy an existing data directory.

The following discussion provides more detail about each method.

Warning

Normally, you should never have two servers that update data in the same databases. This may lead to unpleasant surprises if your operating system does not support fault-free system locking. If (despite this warning) you run multiple servers using the same data directory and they have logging enabled, you must use the appropriate options to specify log file names that are unique to each server. Otherwise, the servers try to log to the same files.

Even when the preceding precautions are observed, this kind of setup works only with MyISAM and MERGE tables, and not with any of the other storage engines. Also, this warning against sharing a data directory among servers always applies in an NFS environment. Permitting multiple MySQL servers to access a common data directory over NFS is a very bad idea. The primary problem is that NFS is the speed bottleneck. It is not meant for such use. Another risk with NFS is that you must devise a way to ensure that two or more servers do not interfere with each other. Usually NFS file locking is handled by the lockd daemon, but at the moment there is no platform that performs locking 100% reliably in every situation.

Create a New Data Directory

With this method, the data directory will be in the same state as when you first install MySQL. It will have the default set of MySQL accounts and no user data.

On Unix, initialize the data directory. See Section 2.10, “Postinstallation Setup and Testing”.

On Windows, the data directory is included in the MySQL distribution:

  • MySQL Zip archive distributions for Windows contain an unmodified data directory. You can unpack such a distribution into a temporary location, then copy it data directory to where you are setting up the new instance.

  • As of MySQL 5.5.5, Windows MSI package installers create and set up the data directory that the installed server will use, but also create a pristine template data directory named data under the installation directory. After an installation has been performed using an MSI package, the template data directory can be copied to set up additional MySQL instances.

Copy an Existing Data Directory

With this method, any MySQL accounts or user data present in the data directory are carried over to the new data directory.

  1. Stop the existing MySQL instance using the data directory. This must be a clean shutdown so that the instance flushes any pending changes to disk.

  2. Copy the data directory to the location where the new data directory should be.

  3. Copy the my.cnf or my.ini option file used by the existing instance. This serves as a basis for the new instance.

  4. Modify the new option file so that any pathnames referring to the original data directory refer to the new data directory. Also, modify any other options that must be unique per instance, such as the TCP/IP port number and the log files. For a list of parameters that must be unique per instance, see Section 5.6, “Running Multiple MySQL Instances on One Machine”.

  5. Start the new instance, telling it to use the new option file.

5.6.2 Running Multiple MySQL Instances on Windows

You can run multiple servers on Windows by starting them manually from the command line, each with appropriate operating parameters, or by installing several servers as Windows services and running them that way. General instructions for running MySQL from the command line or as a service are given in Section 2.3, “Installing MySQL on Microsoft Windows”. The following sections describe how to start each server with different values for those options that must be unique per server, such as the data directory. These options are listed in Section 5.6, “Running Multiple MySQL Instances on One Machine”.

5.6.2.1 Starting Multiple MySQL Instances at the Windows Command Line

The procedure for starting a single MySQL server manually from the command line is described in Section 2.3.7.5, “Starting MySQL from the Windows Command Line”. To start multiple servers this way, you can specify the appropriate options on the command line or in an option file. It is more convenient to place the options in an option file, but it is necessary to make sure that each server gets its own set of options. To do this, create an option file for each server and tell the server the file name with a --defaults-file option when you run it.

Suppose that you want to run one instance of mysqld on port 3307 with a data directory of C:\mydata1, and another instance on port 3308 with a data directory of C:\mydata2. Use this procedure:

  1. Make sure that each data directory exists, including its own copy of the mysql database that contains the grant tables.

  2. Create two option files. For example, create one file named C:\my-opts1.cnf that looks like this:

    [mysqld]
    datadir = C:/mydata1
    port = 3307
    

    Create a second file named C:\my-opts2.cnf that looks like this:

    [mysqld]
    datadir = C:/mydata2
    port = 3308
    
  3. Use the --defaults-file option to start each server with its own option file:

    C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnf
    C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts2.cnf
    

    Each server starts in the foreground (no new prompt appears until the server exits later), so you will need to issue those two commands in separate console windows.

To shut down the servers, connect to each using the appropriate port number:

C:\> C:\mysql\bin\mysqladmin --port=3307 --host=127.0.0.1 --user=root --password shutdown
C:\> C:\mysql\bin\mysqladmin --port=3308 --host=127.0.0.1 --user=root --password shutdown

Servers configured as just described permit clients to connect over TCP/IP. If your version of Windows supports named pipes and you also want to permit named-pipe connections, specify options that enable the named pipe and specify its name. Each server that supports named-pipe connections must use a unique pipe name. For example, the C:\my-opts1.cnf file might be written like this:

[mysqld]
datadir = C:/mydata1
port = 3307
enable-named-pipe
socket = mypipe1

Modify C:\my-opts2.cnf similarly for use by the second server. Then start the servers as described previously.

A similar procedure applies for servers that you want to permit shared-memory connections. Enable such connections with the --shared-memory option and specify a unique shared-memory name for each server with the --shared-memory-base-name option.

5.6.2.2 Starting Multiple MySQL Instances as Windows Services

On Windows, a MySQL server can run as a Windows service. The procedures for installing, controlling, and removing a single MySQL service are described in Section 2.3.7.7, “Starting MySQL as a Windows Service”.

To set up multiple MySQL services, you must make sure that each instance uses a different service name in addition to the other parameters that must be unique per instance.

For the following instructions, suppose that you want to run the mysqld server from two different versions of MySQL that are installed at C:\mysql-5.1.55 and C:\mysql-5.5.53, respectively. (This might be the case if you are running 5.1.55 as your production server, but also want to conduct tests using 5.5.53.)

To install MySQL as a Windows service, use the --install or --install-manual option. For information about these options, see Section 2.3.7.7, “Starting MySQL as a Windows Service”.

Based on the preceding information, you have several ways to set up multiple services. The following instructions describe some examples. Before trying any of them, shut down and remove any existing MySQL services.

  • Approach 1: Specify the options for all services in one of the standard option files. To do this, use a different service name for each server. Suppose that you want to run the 5.1.55 mysqld using the service name of mysqld1 and the 5.5.53 mysqld using the service name mysqld2. In this case, you can use the [mysqld1] group for 5.1.55 and the [mysqld2] group for 5.5.53. For example, you can set up C:\my.cnf like this:

    # options for mysqld1 service
    [mysqld1]
    basedir = C:/mysql-5.1.55
    port = 3307
    enable-named-pipe
    socket = mypipe1
    
    # options for mysqld2 service
    [mysqld2]
    basedir = C:/mysql-5.5.53
    port = 3308
    enable-named-pipe
    socket = mypipe2
    

    Install the services as follows, using the full server path names to ensure that Windows registers the correct executable program for each service:

    C:\> C:\mysql-5.1.55\bin\mysqld --install mysqld1
    C:\> C:\mysql-5.5.53\bin\mysqld --install mysqld2
    

    To start the services, use the services manager, or use NET START with the appropriate service names:

    C:\> NET START mysqld1
    C:\> NET START mysqld2
    

    To stop the services, use the services manager, or use NET STOP with the appropriate service names:

    C:\> NET STOP mysqld1
    C:\> NET STOP mysqld2
    
  • Approach 2: Specify options for each server in separate files and use --defaults-file when you install the services to tell each server what file to use. In this case, each file should list options using a [mysqld] group.

    With this approach, to specify options for the 5.1.55 mysqld, create a file C:\my-opts1.cnf that looks like this:

    [mysqld]
    basedir = C:/mysql-5.1.55
    port = 3307
    enable-named-pipe
    socket = mypipe1
    

    For the 5.5.53 mysqld, create a file C:\my-opts2.cnf that looks like this:

    [mysqld]
    basedir = C:/mysql-5.5.53
    port = 3308
    enable-named-pipe
    socket = mypipe2
    

    Install the services as follows (enter each command on a single line):

    C:\> C:\mysql-5.1.55\bin\mysqld --install mysqld1
               --defaults-file=C:\my-opts1.cnf
    C:\> C:\mysql-5.5.53\bin\mysqld --install mysqld2
               --defaults-file=C:\my-opts2.cnf
    

    When you install a MySQL server as a service and use a --defaults-file option, the service name must precede the option.

    After installing the services, start and stop them the same way as in the preceding example.

To remove multiple services, use mysqld --remove for each one, specifying a service name following the --remove option. If the service name is the default (MySQL), you can omit it.

5.6.3 Running Multiple MySQL Instances on Unix

One way is to run multiple MySQL instances on Unix is to compile different servers with different default TCP/IP ports and Unix socket files so that each one listens on different network interfaces. Compiling in different base directories for each installation also results automatically in a separate, compiled-in data directory, log file, and PID file location for each server.

Assume that an existing 5.1 server is configured for the default TCP/IP port number (3306) and Unix socket file (/tmp/mysql.sock). To configure a new 5.5.53 server to have different operating parameters, use a CMake command something like this:

shell> cmake . -DMYSQL_TCP_PORT=port_number \
             -DMYSQL_UNIX_ADDR=file_name \
             -DCMAKE_INSTALL_PREFIX=/usr/local/mysql-5.5.53

Here, port_number and file_name must be different from the default TCP/IP port number and Unix socket file path name, and the CMAKE_INSTALL_PREFIX value should specify an installation directory different from the one under which the existing MySQL installation is located.

If you have a MySQL server listening on a given port number, you can use the following command to find out what operating parameters it is using for several important configurable variables, including the base directory and Unix socket file name:

shell> mysqladmin --host=host_name --port=port_number variables

With the information displayed by that command, you can tell what option values not to use when configuring an additional server.

If you specify localhost as the host name, mysqladmin defaults to using a Unix socket file connection rather than TCP/IP. To explicitly specify the connection protocol, use the --protocol={TCP|SOCKET|PIPE|MEMORY} option.

You need not compile a new MySQL server just to start with a different Unix socket file and TCP/IP port number. It is also possible to use the same server binary and start each invocation of it with different parameter values at runtime. One way to do so is by using command-line options:

shell> mysqld_safe --socket=file_name --port=port_number

To start a second server, provide different --socket and --port option values, and pass a --datadir=dir_name option to mysqld_safe so that the server uses a different data directory.

Alternatively, put the options for each server in a different option file, then start each server using a --defaults-file option that specifies the path to the appropriate option file. For example, if the option files for two server instances are named /usr/local/mysql/my.cnf and /usr/local/mysql/my.cnf2, start the servers like this: command:

shell> mysqld_safe --defaults-file=/usr/local/mysql/my.cnf
shell> mysqld_safe --defaults-file=/usr/local/mysql/my.cnf2

Another way to achieve a similar effect is to use environment variables to set the Unix socket file name and TCP/IP port number:

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> mysql_install_db --user=mysql
shell> mysqld_safe --datadir=/path/to/datadir &

This is a quick way of starting a second server to use for testing. The nice thing about this method is that the environment variable settings apply to any client programs that you invoke from the same shell. Thus, connections for those clients are automatically directed to the second server.

Section 2.12, “Environment Variables”, includes a list of other environment variables you can use to affect MySQL programs.

On Unix, the mysqld_multi script provides another way to start multiple servers. See Section 4.3.4, “mysqld_multi — Manage Multiple MySQL Servers”.

5.6.4 Using Client Programs in a Multiple-Server Environment

To connect with a client program to a MySQL server that is listening to different network interfaces from those compiled into your client, you can use one of the following methods:

  • Start the client with --host=host_name --port=port_number to connect using TCP/IP to a remote server, with --host=127.0.0.1 --port=port_number to connect using TCP/IP to a local server, or with --host=localhost --socket=file_name to connect to a local server using a Unix socket file or a Windows named pipe.

  • Start the client with --protocol=TCP to connect using TCP/IP, --protocol=SOCKET to connect using a Unix socket file, --protocol=PIPE to connect using a named pipe, or --protocol=MEMORY to connect using shared memory. For TCP/IP connections, you may also need to specify --host and --port options. For the other types of connections, you may need to specify a --socket option to specify a Unix socket file or Windows named-pipe name, or a --shared-memory-base-name option to specify the shared-memory name. Shared-memory connections are supported only on Windows.

  • On Unix, set the MYSQL_UNIX_PORT and MYSQL_TCP_PORT environment variables to point to the Unix socket file and TCP/IP port number before you start your clients. If you normally use a specific socket file or port number, you can place commands to set these environment variables in your .login file so that they apply each time you log in. See Section 2.12, “Environment Variables”.

  • Specify the default Unix socket file and TCP/IP port number in the [client] group of an option file. For example, you can use C:\my.cnf on Windows, or the .my.cnf file in your home directory on Unix. See Section 4.2.6, “Using Option Files”.

  • In a C program, you can specify the socket file or port number arguments in the mysql_real_connect() call. You can also have the program read option files by calling mysql_options(). See Section 23.8.7, “C API Function Descriptions”.

  • If you are using the Perl DBD::mysql module, you can read options from MySQL option files. For example:

    $dsn = "DBI:mysql:test;mysql_read_default_group=client;"
            . "mysql_read_default_file=/usr/local/mysql/data/my.cnf";
    $dbh = DBI->connect($dsn, $user, $password);
    

    See Section 23.10, “MySQL Perl API”.

    Other programming interfaces may provide similar capabilities for reading option files.

5.7 Tracing mysqld Using DTrace

The DTrace probes in the MySQL server are designed to provide information about the execution of queries within MySQL and the different areas of the system being utilized during that process. The organization and triggering of the probes means that the execution of an entire query can be monitored with one level of probes (query-start and query-done) but by monitoring other probes you can get successively more detailed information about the execution of the query in terms of the locks used, sort methods and even row-by-row and storage-engine level execution information.

The DTrace probes are organized so that you can follow the entire query process, from the point of connection from a client, through the query execution, row-level operations, and back out again. You can think of the probes as being fired within a specific sequence during a typical client connect/execute/disconnect sequence, as shown in the following figure.

Figure 5.1 The MySQL Architecture Using Pluggable Storage Engines

DTrace Probe Structure in mysqld

Global information is provided in the arguments to the DTrace probes at various levels. Global information, that is, the connection ID and user/host and where relevant the query string, is provided at key levels (connection-start, command-start, query-start, and query-exec-start). As you go deeper into the probes, it is assumed either you are only interested in the individual executions (row-level probes provide information on the database and table name only), or that you will combine the row-level probes with the notional parent probes to provide the information about a specific query. Examples of this will be given as the format and arguments of each probe are provided.

MySQL includes support for DTrace probes on these platforms:

  • Solaris 10 Update 5 (Solaris 5/08) on SPARC, x86 and x86_64 platforms

  • OS X 10.4 and higher

Enabling the probes should be automatic on these platforms. To explicitly enable or disable the probes during building, use the -DENABLE_DTRACE=1 or -DENABLE_DTRACE=0 option to CMake.

If a non-Solaris platform includes DTrace support, building mysqld on that platform will include DTrace support.

Additional Resources

5.7.1 mysqld DTrace Probe Reference

MySQL supports the following static probes, organized into groups of functionality.

Table 5.5 MySQL DTrace Probes

GroupProbes
Connectionconnection-start, connection-done
Commandcommand-start, command-done
Queryquery-start, query-done
Query Parsingquery-parse-start, query-parse-done
Query Cachequery-cache-hit, query-cache-miss
Query Executionquery-exec-start, query-exec-done
Row Levelinsert-row-start, insert-row-done
 update-row-start, update-row-done
 delete-row-start, delete-row-done
Row Readsread-row-start, read-row-done
Index Readsindex-read-row-start, index-read-row-done
Lockhandler-rdlock-start, handler-rdlock-done
 handler-wrlock-start, handler-wrlock-done
 handler-unlock-start, handler-unlock-done
Filesortfilesort-start, filesort-done
Statementselect-start, select-done
 insert-start, insert-done
 insert-select-start, insert-select-done
 update-start, update-done
 multi-update-start, multi-update-done
 delete-start, delete-done
 multi-delete-start, multi-delete-done
Networknet-read-start, net-read-done, net-write-start, net-write-done
Keycachekeycache-read-start, keycache-read-block, keycache-read-done, keycache-read-hit, keycache-read-miss, keycache-write-start, keycache-write-block, keycache-write-done

Note

When extracting the argument data from the probes, each argument is available as argN, starting with arg0. To identify each argument within the definitions they are provided with a descriptive name, but you must access the information using the corresponding argN parameter.

5.7.1.1 Connection Probes

The connection-start and connection-done probes enclose a connection from a client, regardless of whether the connection is through a socket or network connection.

connection-start(connectionid, user, host)
connection-done(status, connectionid)
  • connection-start: Triggered after a connection and successful login/authentication have been completed by a client. The arguments contain the connection information:

    • connectionid: An unsigned long containing the connection ID. This is the same as the process ID shown as the Id value in the output from SHOW PROCESSLIST.

    • user: The username used when authenticating. The value will be blank for the anonymous user.

    • host: The host of the client connection. For a connection made using UNIX sockets, the value will be blank.

  • connection-done: Triggered just as the connection to the client has been closed. The arguments are:

    • status: The status of the connection when it was closed. A logout operation will have a value of 0; any other termination of the connection has a nonzero value.

    • connectionid: The connection ID of the connection that was closed.

The following D script will quantify and summarize the average duration of individual connections, and provide a count, dumping the information every 60 seconds:

#!/usr/sbin/dtrace -s


mysql*:::connection-start
{
  self->start = timestamp;
}

mysql*:::connection-done
/self->start/
{
  @ = quantize(((timestamp - self->start)/1000000));
  self->start = 0;
}

tick-60s
{
  printa(@);
}

When executed on a server with a large number of clients you might see output similar to this:

  1  57413                        :tick-60s

           value  ------------- Distribution ------------- count
              -1 |                                         0
               0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 30011
               1 |                                         59
               2 |                                         5
               4 |                                         20
               8 |                                         29
              16 |                                         18
              32 |                                         27
              64 |                                         30
             128 |                                         11
             256 |                                         10
             512 |                                         1
            1024 |                                         6
            2048 |                                         8
            4096 |                                         9
            8192 |                                         8
           16384 |                                         2
           32768 |                                         1
           65536 |                                         1
          131072 |                                         0
          262144 |                                         1
524288 |                                         0        

5.7.1.2 Command Probes

The command probes are executed before and after a client command is executed, including any SQL statement that might be executed during that period. Commands include operations such as the initialization of the DB, use of the COM_CHANGE_USER operation (supported by the MySQL protocol), and manipulation of prepared statements. Many of these commands are used only by the MySQL client API from various connectors such as PHP and Java.

command-start(connectionid, command, user, host)
command-done(status)
  • command-start: Triggered when a command is submitted to the server.

    • connectionid: The connection ID of the client executing the command.

    • command: An integer representing the command that was executed. Possible values are shown in the following table.

      ValueNameDescription
      00COM_SLEEPInternal thread state
      01COM_QUITClose connection
      02COM_INIT_DBSelect database (USE ...)
      03COM_QUERYExecute a query
      04COM_FIELD_LISTGet a list of fields
      05COM_CREATE_DBCreate a database (deprecated)
      06COM_DROP_DBDrop a database (deprecated)
      07COM_REFRESHRefresh connection
      08COM_SHUTDOWNShutdown server
      09COM_STATISTICSGet statistics
      10COM_PROCESS_INFOGet processes (SHOW PROCESSLIST)
      11COM_CONNECTInitialize connection
      12COM_PROCESS_KILLKill process
      13COM_DEBUGGet debug information
      14COM_PINGPing
      15COM_TIMEInternal thread state
      16COM_DELAYED_INSERTInternal thread state
      17COM_CHANGE_USERChange user
      18COM_BINLOG_DUMPUsed by a replication slave or mysqlbinlog to initiate a binary log read
      19COM_TABLE_DUMPUsed by a replication slave to get the master table information
      20COM_CONNECT_OUTUsed by a replication slave to log a connection to the server
      21COM_REGISTER_SLAVEUsed by a replication slave during registration
      22COM_STMT_PREPAREPrepare a statement
      23COM_STMT_EXECUTEExecute a statement
      24COM_STMT_SEND_LONG_DATAUsed by a client when requesting extended data
      25COM_STMT_CLOSEClose a prepared statement
      26COM_STMT_RESETReset a prepared statement
      27COM_SET_OPTIONSet a server option
      28COM_STMT_FETCHFetch a prepared statement
    • user: The user executing the command.

    • host: The client host.

  • command-done: Triggered when the command execution completes. The status argument contains 0 if the command executed successfully, or 1 if the statement was terminated before normal completion.

The command-start and command-done probes are best used when combined with the statement probes to get an idea of overall execution time.

5.7.1.3 Query Probes

The query-start and query-done probes are triggered when a specific query is received by the server and when the query has been completed and the information has been successfully sent to the client.

query-start(query, connectionid, database, user, host)
query-done(status)
  • query-start: Triggered after the query string has been received from the client. The arguments are:

    • query: The full text of the submitted query.

    • connectionid: The connection ID of the client that submitted the query. The connection ID equals the connection ID returned when the client first connects and the Id value in the output from SHOW PROCESSLIST.

    • database: The database name on which the query is being executed.

    • user: The username used to connect to the server.

    • host: The hostname of the client.

  • query-done: Triggered once the query has been executed and the information has been returned to the client. The probe includes a single argument, status, which returns 0 when the query is successfully executed and 1 if there was an error.

You can get a simple report of the execution time for each query using the following D script:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-20s %-20s %-40s %-9s\n", "Who", "Database", "Query", "Time(ms)");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->connid = arg1;
   self->db    = copyinstr(arg2);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->querystart = timestamp;
}

mysql*:::query-done
{
   printf("%-20s %-20s %-40s %-9d\n",self->who,self->db,self->query,
          (timestamp - self->querystart) / 1000000);
}

When executing the above script you should get a basic idea of the execution time of your queries:

shell> ./query.d
Who                  Database             Query                                    Time(ms)
root@localhost       test                 select * from t1 order by i limit 10     0
root@localhost       test                 set global query_cache_size=0            0
root@localhost       test                 select * from t1 order by i limit 10     776
root@localhost       test                 select * from t1 order by i limit 10     773
root@localhost       test                 select * from t1 order by i desc limit 10 795 

5.7.1.4 Query Parsing Probes

The query parsing probes are triggered before the original SQL statement is parsed and when the parsing of the statement and determination of the execution model required to process the statement has been completed:

query-parse-start(query)
query-parse-done(status)
  • query-parse-start: Triggered just before the statement is parsed by the MySQL query parser. The single argument, query, is a string containing the full text of the original query.

  • query-parse-done: Triggered when the parsing of the original statement has been completed. The status is an integer describing the status of the operation. A 0 indicates that the query was successfully parsed. A 1 indicates that the parsing of the query failed.

For example, you could monitor the execution time for parsing a given query using the following D script:

#!/usr/sbin/dtrace -s

#pragma D option quiet

mysql*:::query-parse-start
{
   self->parsestart = timestamp;
   self->parsequery = copyinstr(arg0);
}

mysql*:::query-parse-done
/arg0 == 0/
{
   printf("Parsing %s: %d microseconds\n", self->parsequery,((timestamp - self->parsestart)/1000));
}

mysql*:::query-parse-done
/arg0 != 0/
{
   printf("Error parsing %s: %d microseconds\n", self->parsequery,((timestamp - self->parsestart)/1000));
}

In the above script a predicate is used on query-parse-done so that different output is generated based on the status value of the probe.

When running the script and monitoring the execution:

shell> ./query-parsing.d
Error parsing select from t1 join (t2) on (t1.i = t2.i) order by t1.s,t1.i limit 10: 36 ms
Parsing select * from t1 join (t2) on (t1.i = t2.i) order by t1.s,t1.i limit 10: 176 ms

5.7.1.5 Query Cache Probes

The query cache probes are fired when executing any query. The query-cache-hit query is triggered when a query exists in the query cache and can be used to return the query cache information. The arguments contain the original query text and the number of rows returned from the query cache for the query. If the query is not within the query cache, or the query cache is not enabled, then the query-cache-miss probe is triggered instead.

query-cache-hit(query, rows)
query-cache-miss(query)
  • query-cache-hit: Triggered when the query has been found within the query cache. The first argument, query, contains the original text of the query. The second argument, rows, is an integer containing the number of rows in the cached query.

  • query-cache-miss: Triggered when the query is not found within the query cache. The first argument, query, contains the original text of the query.

The query cache probes are best combined with a probe on the main query so that you can determine the differences in times between using or not using the query cache for specified queries. For example, in the following D script, the query and query cache information are combined into the information output during monitoring:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-20s %-20s %-40s %2s %-9s\n", "Who", "Database", "Query", "QC", "Time(ms)");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->connid = arg1;
   self->db    = copyinstr(arg2);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->querystart = timestamp;
   self->qc = 0;
}

mysql*:::query-cache-hit
{
   self->qc = 1;
}

mysql*:::query-cache-miss
{
   self->qc = 0;
}

mysql*:::query-done
{
   printf("%-20s %-20s %-40s %-2s %-9d\n",self->who,self->db,self->query,(self->qc ? "Y" : "N"),
          (timestamp - self->querystart) / 1000000);
}

When executing the script you can see the effects of the query cache. Initially the query cache is disabled. If you set the query cache size and then execute the query multiple times you should see that the query cache is being used to return the query data:

shell> ./query-cache.d
root@localhost       test                 select * from t1 order by i limit 10     N  1072
root@localhost                            set global query_cache_size=262144       N  0
root@localhost       test                 select * from t1 order by i limit 10     N  781
root@localhost       test                 select * from t1 order by i limit 10     Y  0 

5.7.1.6 Query Execution Probes

The query execution probe is triggered when the actual execution of the query starts, after the parsing and checking the query cache but before any privilege checks or optimization. By comparing the difference between the start and done probes you can monitor the time actually spent servicing the query (instead of just handling the parsing and other elements of the query).

query-exec-start(query, connectionid, database, user, host, exec_type)
query-exec-done(status)
Note

The information provided in the arguments for query-start and query-exec-start are almost identical and designed so that you can choose to monitor either the entire query process (using query-start) or only the execution (using query-exec-start) while exposing the core information about the user, client, and query being executed.

  • query-exec-start: Triggered when the execution of a individual query is started. The arguments are:

    • query: The full text of the submitted query.

    • connectionid: The connection ID of the client that submitted the query. The connection ID equals the connection ID returned when the client first connects and the Id value in the output from SHOW PROCESSLIST.

    • database: The database name on which the query is being executed.

    • user: The username used to connect to the server.

    • host: The hostname of the client.

    • exec_type: The type of execution. Execution types are determined based on the contents of the query and where it was submitted. The values for each type are shown in the following table.

      ValueDescription
      0Executed query from sql_parse, top-level query.
      1Executed prepared statement
      2Executed cursor statement
      3Executed query in stored procedure
  • query-exec-done: Triggered when the execution of the query has completed. The probe includes a single argument, status, which returns 0 when the query is successfully executed and 1 if there was an error.

5.7.1.7 Row-Level Probes

The *row-{start,done} probes are triggered each time a row operation is pushed down to a storage engine. For example, if you execute an INSERT statement with 100 rows of data, then the insert-row-start and insert-row-done probes will be triggered 100 times each, for each row insert.

insert-row-start(database, table)
insert-row-done(status)

update-row-start(database, table)
update-row-done(status)

delete-row-start(database, table)
delete-row-done(status)
  • insert-row-start: Triggered before a row is inserted into a table.

  • insert-row-done: Triggered after a row is inserted into a table.

  • update-row-start: Triggered before a row is updated in a table.

  • update-row-done: Triggered before a row is updated in a table.

  • delete-row-start: Triggered before a row is deleted from a table.

  • delete-row-done: Triggered before a row is deleted from a table.

The arguments supported by the probes are consistent for the corresponding start and done probes in each case:

  • database: The database name.

  • table: The table name.

  • status: The status; 0 for success or 1 for failure.

Because the row-level probes are triggered for each individual row access, these probes can be triggered many thousands of times each second, which may have a detrimental effect on both the monitoring script and MySQL. The DTrace environment should limit the triggering on these probes to prevent the performance being adversely affected. Either use the probes sparingly, or use counter or aggregation functions to report on these probes and then provide a summary when the script terminates or as part of a query-done or query-exec-done probes.

The following example script summarizes the duration of each row operation within a larger query:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-2s %-10s %-10s %9s %9s %-s \n",
          "St", "Who", "DB", "ConnID", "Dur ms", "Query");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->db    = copyinstr(arg2);
   self->connid = arg1;
   self->querystart = timestamp;
   self->rowdur = 0;
}

mysql*:::query-done
{
   this->elapsed = (timestamp - self->querystart) /1000000;
   printf("%2d %-10s %-10s %9d %9d %s\n",
          arg0, self->who, self->db,
          self->connid, this->elapsed, self->query);
}

mysql*:::query-done
/ self->rowdur /
{
   printf("%34s %9d %s\n", "", (self->rowdur/1000000), "-> Row ops");
}

mysql*:::insert-row-start
{
   self->rowstart = timestamp;
}

mysql*:::delete-row-start
{
   self->rowstart = timestamp;
}

mysql*:::update-row-start
{
   self->rowstart = timestamp;
}

mysql*:::insert-row-done
{
   self->rowdur += (timestamp-self->rowstart);
}

mysql*:::delete-row-done
{
   self->rowdur += (timestamp-self->rowstart);
}

mysql*:::update-row-done
{
   self->rowdur += (timestamp-self->rowstart);
}

Running the above script with a query that inserts data into a table, you can monitor the exact time spent performing the raw row insertion:

St Who        DB            ConnID    Dur ms Query
 0 @localhost test              13     20767 insert into t1(select * from t2)
                                        4827 -> Row ops

5.7.1.8 Read Row Probes

The read row probes are triggered at a storage engine level each time a row read operation occurs. These probes are specified within each storage engine (as opposed to the *row-start probes which are in the storage engine interface). These probes can therefore be used to monitor individual storage engine row-level operations and performance. Because these probes are triggered around the storage engine row read interface, they may be hit a significant number of times during a basic query.

read-row-start(database, table, scan_flag)
read-row-done(status)
  • read-row-start: Triggered when a row is read by the storage engine from the specified database and table. The scan_flag is set to 1 (true) when the read is part of a table scan (that is, a sequential read), or 0 (false) when the read is of a specific record.

  • read-row-done: Triggered when a row read operation within a storage engine completes. The status returns 0 on success, or a positive value on failure.

5.7.1.9 Index Probes

The index probes are triggered each time a row is read using one of the indexes for the specified table. The probe is triggered within the corresponding storage engine for the table.

index-read-row-start(database, table)
index-read-row-done(status)
  • index-read-row-start: Triggered when a row is read by the storage engine from the specified database and table.

  • index-read-row-done: Triggered when an indexed row read operation within a storage engine completes. The status returns 0 on success, or a positive value on failure.

5.7.1.10 Lock Probes

The lock probes are called whenever an external lock is requested by MySQL for a table using the corresponding lock mechanism on the table as defined by the table's engine type. There are three different types of lock, the read lock, write lock, and unlock operations. Using the probes you can determine the duration of the external locking routine (that is, the time taken by the storage engine to implement the lock, including any time waiting for another lock to become free) and the total duration of the lock/unlock process.

handler-rdlock-start(database, table)
handler-rdlock-done(status)

handler-wrlock-start(database, table)
handler-wrlock-done(status)

handler-unlock-start(database, table)
handler-unlock-done(status)
  • handler-rdlock-start: Triggered when a read lock is requested on the specified database and table.

  • handler-wrlock-start: Triggered when a write lock is requested on the specified database and table.

  • handler-unlock-start: Triggered when an unlock request is made on the specified database and table.

  • handler-rdlock-done: Triggered when a read lock request completes. The status is 0 if the lock operation succeeded, or >0 on failure.

  • handler-wrlock-done: Triggered when a write lock request completes. The status is 0 if the lock operation succeeded, or >0 on failure.

  • handler-unlock-done: Triggered when an unlock request completes. The status is 0 if the unlock operation succeeded, or >0 on failure.

You can use arrays to monitor the locking and unlocking of individual tables and then calculate the duration of the entire table lock using the following script:

#!/usr/sbin/dtrace -s

#pragma D option quiet

mysql*:::handler-rdlock-start
{
   self->rdlockstart = timestamp;
   this->lockref = strjoin(copyinstr(arg0),strjoin("@",copyinstr(arg1)));
   self->lockmap[this->lockref] = self->rdlockstart;
   printf("Start: Lock->Read   %s.%s\n",copyinstr(arg0),copyinstr(arg1));
}

mysql*:::handler-wrlock-start
{
   self->wrlockstart = timestamp;
   this->lockref = strjoin(copyinstr(arg0),strjoin("@",copyinstr(arg1)));
   self->lockmap[this->lockref] = self->rdlockstart;
   printf("Start: Lock->Write  %s.%s\n",copyinstr(arg0),copyinstr(arg1));
}

mysql*:::handler-unlock-start
{
   self->unlockstart = timestamp;
   this->lockref = strjoin(copyinstr(arg0),strjoin("@",copyinstr(arg1)));
   printf("Start: Lock->Unlock %s.%s (%d ms lock duration)\n",
          copyinstr(arg0),copyinstr(arg1),
          (timestamp - self->lockmap[this->lockref])/1000000);
}

mysql*:::handler-rdlock-done
{
   printf("End:   Lock->Read   %d ms\n",
          (timestamp - self->rdlockstart)/1000000);
}

mysql*:::handler-wrlock-done
{
   printf("End:   Lock->Write  %d ms\n",
          (timestamp - self->wrlockstart)/1000000);
}

mysql*:::handler-unlock-done
{
   printf("End:   Lock->Unlock %d ms\n",
          (timestamp - self->unlockstart)/1000000);
}

When executed, you should get information both about the duration of the locking process itself, and of the locks on a specific table:

Start: Lock->Read   test.t2
End:   Lock->Read   0 ms
Start: Lock->Unlock test.t2 (25743 ms lock duration)
End:   Lock->Unlock 0 ms
Start: Lock->Read   test.t2
End:   Lock->Read   0 ms
Start: Lock->Unlock test.t2 (1 ms lock duration)
End:   Lock->Unlock 0 ms
Start: Lock->Read   test.t2
End:   Lock->Read   0 ms
Start: Lock->Unlock test.t2 (1 ms lock duration)
End:   Lock->Unlock 0 ms
Start: Lock->Read   test.t2
End:   Lock->Read   0 ms

5.7.1.11 Filesort Probes

The filesort probes are triggered whenever a filesort operation is applied to a table. For more information on filesort and the conditions under which it occurs, see Section 8.2.1.11, “ORDER BY Optimization”.

filesort-start(database, table)
filesort-done(status, rows)
  • filesort-start: Triggered when the filesort operation starts on a table. The two arguments to the probe, database and table, will identify the table being sorted.

  • filesort-done: Triggered when the filesort operation completes. Two arguments are supplied, the status (0 for success, 1 for failure), and the number of rows sorted during the filesort process.

An example of this is in the following script, which tracks the duration of the filesort process in addition to the duration of the main query:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-2s %-10s %-10s %9s %18s %-s \n",
          "St", "Who", "DB", "ConnID", "Dur microsec", "Query");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->db    = copyinstr(arg2);
   self->connid = arg1;
   self->querystart = timestamp;
   self->filesort = 0;
   self->fsdb = "";
   self->fstable = "";
}

mysql*:::filesort-start
{
  self->filesort = timestamp;
  self->fsdb = copyinstr(arg0);
  self->fstable = copyinstr(arg1);
}

mysql*:::filesort-done
{
   this->elapsed = (timestamp - self->filesort) /1000;
   printf("%2d %-10s %-10s %9d %18d Filesort on %s\n",
          arg0, self->who, self->fsdb,
          self->connid, this->elapsed, self->fstable);
}

mysql*:::query-done
{
   this->elapsed = (timestamp - self->querystart) /1000;
   printf("%2d %-10s %-10s %9d %18d %s\n",
          arg0, self->who, self->db,
          self->connid, this->elapsed, self->query);
}

Executing a query on a large table with an ORDER BY clause that triggers a filesort, and then creating an index on the table and then repeating the same query, you can see the difference in execution speed:

St Who        DB            ConnID       Dur microsec Query
 0 @localhost test              14           11335469 Filesort on t1
 0 @localhost test              14           11335787 select * from t1 order by i limit 100
 0 @localhost test              14          466734378 create index t1a on t1 (i)
 0 @localhost test              14              26472 select * from t1 order by i limit 100

5.7.1.12 Statement Probes

The individual statement probes are provided to give specific information about different statement types. For the start probes the string of the query is provided as a the only argument. Depending on the statement type, the information provided by the corresponding done probe will differ. For all done probes the status of the operation (0 for success, >0 for failure) is provided. For SELECT, INSERT, INSERT ... (SELECT FROM ...), DELETE, and DELETE FROM t1,t2 operations the number of rows affected is returned.

For UPDATE and UPDATE t1,t2 ... statements the number of rows matched and the number of rows actually changed is provided. This is because the number of rows actually matched by the corresponding WHERE clause, and the number of rows changed can differ. MySQL does not update the value of a row if the value already matches the new setting.

select-start(query)
select-done(status,rows)

insert-start(query)
insert-done(status,rows)

insert-select-start(query)
insert-select-done(status,rows)

update-start(query)
update-done(status,rowsmatched,rowschanged)

multi-update-start(query)
multi-update-done(status,rowsmatched,rowschanged)

delete-start(query)
delete-done(status,rows)

multi-delete-start(query)
multi-delete-done(status,rows)
  • select-start: Triggered before a SELECT statement.

  • select-done: Triggered at the end of a SELECT statement.

  • insert-start: Triggered before a INSERT statement.

  • insert-done: Triggered at the end of an INSERT statement.

  • insert-select-start: Triggered before an INSERT ... SELECT statement.

  • insert-select-done: Triggered at the end of an INSERT ... SELECT statement.

  • update-start: Triggered before an UPDATE statement.

  • update-done: Triggered at the end of an UPDATE statement.

  • multi-update-start: Triggered before an UPDATE statement involving multiple tables.

  • multi-update-done: Triggered at the end of an UPDATE statement involving multiple tables.

  • delete-start: Triggered before a DELETE statement.

  • delete-done: Triggered at the end of a DELETE statement.

  • multi-delete-start: Triggered before a DELETE statement involving multiple tables.

  • multi-delete-done: Triggered at the end of a DELETE statement involving multiple tables.

The arguments for the statement probes are:

  • query: The query string.

  • status: The status of the query. 0 for success, and >0 for failure.

  • rows: The number of rows affected by the statement. This returns the number rows found for SELECT, the number of rows deleted for DELETE, and the number of rows successfully inserted for INSERT.

  • rowsmatched: The number of rows matched by the WHERE clause of an UPDATE operation.

  • rowschanged: The number of rows actually changed during an UPDATE operation.

You use these probes to monitor the execution of these statement types without having to monitor the user or client executing the statements. A simple example of this is to track the execution times:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-60s %-8s %-8s %-8s\n", "Query", "RowsU", "RowsM", "Dur (ms)");
}

mysql*:::update-start, mysql*:::insert-start,
mysql*:::delete-start, mysql*:::multi-delete-start,
mysql*:::multi-delete-done, mysql*:::select-start,
mysql*:::insert-select-start, mysql*:::multi-update-start
{
    self->query = copyinstr(arg0);
    self->querystart = timestamp;
}

mysql*:::insert-done, mysql*:::select-done,
mysql*:::delete-done, mysql*:::multi-delete-done, mysql*:::insert-select-done
/ self->querystart /
{
    this->elapsed = ((timestamp - self->querystart)/1000000);
    printf("%-60s %-8d %-8d %d\n",
           self->query,
           0,
           arg1,
           this->elapsed);
    self->querystart = 0;
}

mysql*:::update-done, mysql*:::multi-update-done
/ self->querystart /
{
    this->elapsed = ((timestamp - self->querystart)/1000000);
    printf("%-60s %-8d %-8d %d\n",
           self->query,
           arg1,
           arg2,
           this->elapsed);
    self->querystart = 0;
}

When executed you can see the basic execution times and rows matches:

Query                                                        RowsU    RowsM    Dur (ms)
select * from t2                                             0        275      0
insert into t2 (select * from t2)                            0        275      9
update t2 set i=5 where i > 75                               110      110      8
update t2 set i=5 where i < 25                               254      134      12
delete from t2 where i < 5                                   0        0        0

Another alternative is to use the aggregation functions in DTrace to aggregate the execution time of individual statements together:

#!/usr/sbin/dtrace -s

#pragma D option quiet


mysql*:::update-start, mysql*:::insert-start,
mysql*:::delete-start, mysql*:::multi-delete-start,
mysql*:::multi-delete-done, mysql*:::select-start,
mysql*:::insert-select-start, mysql*:::multi-update-start
{
    self->querystart = timestamp;
}

mysql*:::select-done
{
        @statements["select"] = sum(((timestamp - self->querystart)/1000000));
}

mysql*:::insert-done, mysql*:::insert-select-done
{
        @statements["insert"] = sum(((timestamp - self->querystart)/1000000));
}

mysql*:::update-done, mysql*:::multi-update-done
{
        @statements["update"] = sum(((timestamp - self->querystart)/1000000));
}

mysql*:::delete-done, mysql*:::multi-delete-done
{
        @statements["delete"] = sum(((timestamp - self->querystart)/1000000));
}

tick-30s
{
        printa(@statements);
}

The script just shown aggregates the times spent doing each operation, which could be used to help benchmark a standard suite of tests.

 delete                                                            0
  update                                                            0
  insert                                                           23
  select                                                         2484

  delete                                                            0
  update                                                            0
  insert                                                           39
  select                                                        10744

  delete                                                            0
  update                                                           26
  insert                                                           56
  select                                                        10944

  delete                                                            0
  update                                                           26
  insert                                                         2287
  select                                                        15985

5.7.1.13 Network Probes

The network probes monitor the transfer of information from the MySQL server and clients of all types over the network. The probes are defined as follows:

net-read-start()
net-read-done(status, bytes)
net-write-start(bytes)
net-write-done(status)
  • net-read-start: Triggered when a network read operation is started.

  • net-read-done: Triggered when the network read operation completes. The status is an integer representing the return status for the operation, 0 for success and 1 for failure. The bytes argument is an integer specifying the number of bytes read during the process.

  • net-start-bytes: Triggered when data is written to a network socket. The single argument, bytes, specifies the number of bytes written to the network socket.

  • net-write-done: Triggered when the network write operation has completed. The single argument, status, is an integer representing the return status for the operation, 0 for success and 1 for failure.

You can use the network probes to monitor the time spent reading from and writing to network clients during execution. The following D script provides an example of this. Both the cumulative time for the read or write is calculated, and the number of bytes. Note that the dynamic variable size has been increased (using the dynvarsize option) to cope with the rapid firing of the individual probes for the network reads/writes.

#!/usr/sbin/dtrace -s

#pragma D option quiet
#pragma D option dynvarsize=4m

dtrace:::BEGIN
{
   printf("%-2s %-30s %-10s %9s %18s %-s \n",
          "St", "Who", "DB", "ConnID", "Dur microsec", "Query");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->db    = copyinstr(arg2);
   self->connid = arg1;
   self->querystart = timestamp;
   self->netwrite = 0;
   self->netwritecum = 0;
   self->netwritebase = 0;
   self->netread = 0;
   self->netreadcum = 0;
   self->netreadbase = 0;
}

mysql*:::net-write-start
{
   self->netwrite += arg0;
   self->netwritebase = timestamp;
}

mysql*:::net-write-done
{
   self->netwritecum += (timestamp - self->netwritebase);
   self->netwritebase = 0;
}

mysql*:::net-read-start
{
   self->netreadbase = timestamp;
}

mysql*:::net-read-done
{
   self->netread += arg1;
   self->netreadcum += (timestamp - self->netreadbase);
   self->netreadbase = 0;
}

mysql*:::query-done
{
   this->elapsed = (timestamp - self->querystart) /1000000;
   printf("%2d %-30s %-10s %9d %18d %s\n",
          arg0, self->who, self->db,
          self->connid, this->elapsed, self->query);
   printf("Net read: %d bytes (%d ms) write: %d bytes (%d ms)\n",
               self->netread, (self->netreadcum/1000000),
               self->netwrite, (self->netwritecum/1000000));
}

When executing the above script on a machine with a remote client, you can see that approximately a third of the time spent executing the query is related to writing the query results back to the client.

St Who                            DB            ConnID       Dur microsec Query
 0 root@::ffff:192.168.0.108      test              31               3495 select * from t1 limit 1000000
Net read: 0 bytes (0 ms) write: 10000075 bytes (1220 ms)

5.7.1.14 Keycache Probes

The keycache probes are triggered when using the index key cache used with the MyISAM storage engine. Probes exist to monitor when data is read into the keycache, cached key data is written from the cache into a cached file, or when accessing the keycache.

Keycache usage indicates when data is read or written from the index files into the cache, and can be used to monitor how efficient the memory allocated to the keycache is being used. A high number of keycache reads across a range of queries may indicate that the keycache is too small for size of data being accessed.

keycache-read-start(filepath, bytes, mem_used, mem_free)
keycache-read-block(bytes)
keycache-read-hit()
keycache-read-miss()
keycache-read-done(mem_used, mem_free)
keycache-write-start(filepath, bytes, mem_used, mem_free)
keycache-write-block(bytes)
keycache-write-done(mem_used, mem_free)

When reading data from the index files into the keycache, the process first initializes the read operation (indicated by keycache-read-start), then loads blocks of data (keycache-read-block), and then the read block is either matches the data being identified (keycache-read-hit) or more data needs to be read (keycache-read-miss). Once the read operation has completed, reading stops with the keycache-read-done.

Data will be read from the index file into the keycache only when the specified key is not already within the keycache.

  • keycache-read-start: Triggered when the keycache read operation is started. Data is read from the specified filepath, reading the specified number of bytes. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.

  • keycache-read-block: Triggered when the keycache reads a block of data, of the specified number of bytes, from the index file into the keycache.

  • keycache-read-hit: Triggered when the block of data read from the index file matches the key data requested.

  • keycache-read-miss: Triggered when the block of data read from the index file does not match the key data needed.

  • keycache-read-done: Triggered when the keycache read operation has completed. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.

Keycache writes occur when the index information is updated during an INSERT, UPDATE, or DELETE operation, and the cached key information is flushed back to the index file.

  • keycache-write-start: Triggered when the keycache write operation is started. Data is written to the specified filepath, reading the specified number of bytes. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.

  • keycache-write-block: Triggered when the keycache writes a block of data, of the specified number of bytes, to the index file from the keycache.

  • keycache-write-done: Triggered when the keycache write operation has completed. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.