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
- 5.5 MySQL Server Plugins
- 5.6 Running Multiple MySQL Instances on One Machine
- 5.7 Tracing mysqld Using DTrace
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:
Server configuration
The data directory, particularly the
mysqlsystem databaseThe server log files
Management of multiple servers on a single machine
For additional information on administrative topics, see also:
- 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
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.
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”.
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
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.
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.
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
[
groups, where xxxxx_SERVER]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:
Options that affect security: See Section 6.1.4, “Security-Related mysqld Options and Variables”.
SSL-related options: See Section 6.4.5, “Command Options for Secure Connections”.
Binary log control options: See Section 5.4.4, “The Binary Log”.
Replication-related options: See Section 17.1.3, “Replication and Binary Logging Options and Variables”.
Options for loading plugins such as pluggable storage engines: See Section 5.5.2, “Installing and Uninstalling Plugins”.
Options specific to particular storage engines: See Section 14.17, “InnoDB Startup Options and System Variables”, Section 15.3.1, “MyISAM Startup Options”, and Section 18.3.3.8.1, “MySQL Server Options for MySQL Cluster”.
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
--.
For example, var_name=value--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-
at server startup.
var_name=value
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 --helpDisplay a short help message and exit. Use both the
--verboseand--helpoptions to see the full message.-
Command-Line Format --allow-suspicious-udfsPermitted Values Type booleanDefault FALSEThis option controls whether user-defined functions that have only an
xxxsymbol 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”. -
Command-Line Format --ansiUse standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the
--sql-modeoption instead. See Section 1.7, “MySQL Standards Compliance”, and Section 5.1.7, “Server SQL Modes”. --basedir=,dir_name-bdir_nameCommand-Line Format --basedir=dir_nameSystem Variable Name basedirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe path to the MySQL installation directory. All paths are usually resolved relative to this directory.
-
Command-Line Format --big-tablesSystem Variable (<= 5.5.2) Name big_tablesVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name big_tablesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault OFFEnable 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.
-
Command-Line Format --bind-address=addrPermitted Values Type stringDefault 0.0.0.0The 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=option at server startup, whereaddraddris an IPv4 or IPv6 address or a host name. (IPv6 addresses are not supported before MySQL 5.5.3.) Ifaddris 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=127.0.0.1or--host=::ffff:127.0.0.1.If the address is a “regular” IPv4 or IPv6 address (such as
127.0.0.1or::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.usergrant 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 themysql.usertable so you can still connect to the server to shut it down. --binlog-format={ROW|STATEMENT|MIXED}Command-Line Format --binlog-format=formatSystem Variable Name binlog_formatVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type enumerationDefault STATEMENTValid Values ROWSTATEMENTMIXEDPermitted Values (>= 5.5.15-ndb-7.2.1, <= 5.5.30-ndb-7.2.12) Type enumerationDefault STATEMENTValid Values ROWSTATEMENTMIXEDPermitted Values (>= 5.5.31-ndb-7.2.13) Type enumerationDefault MIXEDValid Values ROWSTATEMENTMIXEDSpecify 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_formatglobal system variable is set, and a warning is logged instead of an error. (Bug #42928)-
Command-Line Format --bootstrapThis 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-fileoption. For more information, see the description of that option. -
Command-Line Format --character-sets-dir=dir_nameSystem Variable Name character_sets_dirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe directory where character sets are installed. See Section 10.5, “Character Set Configuration”.
--character-set-client-handshakeCommand-Line Format --character-set-client-handshakePermitted Values Type booleanDefault TRUEDo 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_nameCommand-Line Format --character-set-filesystem=nameSystem Variable Name character_set_filesystemVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault binaryThe file system character set. This option sets the
character_set_filesystemsystem variable.--character-set-server=,charset_name-Ccharset_nameCommand-Line Format --character-set-serverSystem Variable Name character_set_serverVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault latin1Use
charset_nameas 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-serverto specify the collation.--chroot=,dir_name-rdir_nameCommand-Line Format --chroot=dir_namePermitted Values Type directory namePut 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 limitsLOAD DATA INFILEandSELECT ... INTO OUTFILE.--collation-server=collation_nameCommand-Line Format --collation-serverSystem Variable Name collation_serverVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault latin1_swedish_ciUse
collation_nameas the default server collation. See Section 10.5, “Character Set Configuration”.-
Command-Line Format --consolePlatform Specific Windows (Windows only.) Write error log messages to
stderrandstdout. mysqld does not close the console window if this option is used.As of MySQL 5.5.3, if both
--log-errorand--consoleare specified,--log-errortakes precedence. The server writes to the log file, but not to the console. Before 5.5.3, whichever option is given last takes precedence. -
Command-Line Format --core-filePermitted Values Type booleanDefault OFFWrite a core file if mysqld dies. The name and location of the core file is system dependent. On Linux, a core file named
core.is written to the current working directory of the process, which for mysqld is the data directory.pidpidrepresents the process ID of the server process. On OS X, a core file namedcore.is written to thepid/coresdirectory. 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-sizeoption 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--useroption. 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-hdir_nameCommand-Line Format --datadir=dir_nameSystem Variable Name datadirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe path to the data directory.
--debug[=,debug_options]-# [debug_options]Command-Line Format --debug[=debug_options]System Variable Name debugVariable Scope Global, Session Dynamic Variable Yes Permitted Values (Unix) Type stringDefault d:t:i:o,/tmp/mysqld.tracePermitted Values (Windows) Type stringDefault d:t:i:O,\mysqld.traceIf MySQL is configured with the
-DWITH_DEBUG=1CMake option, you can use this option to get a trace file of what mysqld is doing. A typicaldebug_optionsstring isd:t:o,. The default isfile_named:t:i:o,/tmp/mysqld.traceon Unix andd:t:i:O,\mysqld.traceon Windows.Using
-DWITH_DEBUG=1to 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=+Psets the value toP:T.For more information, see Section 24.5.3, “The DBUG Package”.
-
Command-Line Format --debug-sync-timeout[=#]Permitted Values Type integerControls 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=1CMake 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_nameDeprecated 5.0.0 Removed 5.5.3 Command-Line Format --default-character-set=namePermitted Values Type stringUse
charset_nameas the default character set. This option is deprecated in favor of--character-set-server. See Section 10.5, “Character Set Configuration”.--default-character-setwas removed in MySQL 5.5.3.--default-collation=collation_nameDeprecated 4.1.3 Command-Line Format --default-collation=namePermitted Values Type stringUse
collation_nameas the default collation. This option is deprecated in favor of--collation-server. See Section 10.5, “Character Set Configuration”.--default-collationwas removed in MySQL 5.5.3.-
Command-Line Format --default-storage-engine=nameSystem Variable (<= 5.5.2) Name storage_engineVariable Scope Global, Session Dynamic Variable Yes System Variable (>= 5.5.3) Name default_storage_engineVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.4) Type enumerationDefault MyISAMPermitted Values (>= 5.5.5) Type enumerationDefault InnoDBSet 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.
-
Command-Line Format --default-time-zone=namePermitted Values Type stringSet the default server time zone. This option sets the global
time_zonesystem variable. If this option is not given, the default time zone is the same as the system time zone (given by the value of thesystem_time_zonesystem variable. --defaults-extra-file=file_nameRead 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_namemust 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.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_namemust 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.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=_otheroption is given, mysqld also reads the[mysqld_other]group.--delay-key-write[={OFF|ON|ALL}]Command-Line Format --delay-key-write[=name]System Variable Name delay_key_writeVariable Scope Global Dynamic Variable Yes Permitted Values Type enumerationDefault ONValid Values ONOFFALLSpecify how to use delayed key writes. Delayed key writing causes key buffers not to be flushed between writes for
MyISAMtables.OFFdisables delayed key writes.ONenables delayed key writes for those tables that were created with theDELAY_KEY_WRITEoption.ALLdelays key writes for allMyISAMtables. See Section 8.12.2, “Tuning Server Parameters”, and Section 15.3.1, “MyISAM Startup Options”.NoteIf you set this variable to
ALL, you should not useMyISAMtables from within another program (such as another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.-
Command-Line Format --des-key-file=file_nameRead the default DES keys from this file. These keys are used by the
DES_ENCRYPT()andDES_DECRYPT()functions. This option is deprecated and was removed in MySQL 5.5.3. Use
--external-lockinginstead.-
Command-Line Format --enable-named-pipePlatform Specific Windows Enable support for named pipes. This option applies only on Windows.
-
Deprecated 5.1.54 Removed 5.5.7 Command-Line Format --enable-pstackPermitted Values Type booleanDefault FALSEThis option is nonfunctional before MySQL 5.5.7 and removed in 5.5.7.
--engine-condition-pushdown={ON|OFF}Deprecated 5.5.3, by optimizer_switch Command-Line Format --engine-condition-pushdownSystem Variable Name engine_condition_pushdownVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault ONSets the
engine_condition_pushdownsystem variable. For more information, see Section 8.2.1.5, “Engine Condition Pushdown Optimization”.-
Command-Line Format --event-scheduler[=value]System Variable Name event_schedulerVariable Scope Global Dynamic Variable Yes Permitted Values Type enumerationDefault OFFValid Values ONOFFDISABLEDEnable or disable, and start or stop, the event scheduler.
For detailed information, see The
--event-schedulerOption. --exit-info[=,flags]-T [flags]Command-Line Format --exit-info[=flags]Permitted Values Type integerThis 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!
-
Command-Line Format --external-lockingPermitted Values Type booleanDefault FALSEEnable external locking (system locking), which is disabled by default. If you use this option on a system on which
lockddoes 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
MyISAMtable access. For more information, including conditions under which it can and cannot be used, see Section 8.11.5, “External Locking”. -
Command-Line Format --flushSystem Variable Name flushVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFFlush (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”.
-
Command-Line Format --gdbPermitted Values Type booleanDefault FALSEInstall an interrupt handler for
SIGINT(needed to stop mysqld with^Cto set breakpoints) and disable stack tracing and core file handling. See Section 24.5, “Debugging and Porting MySQL”. -
Command-Line Format --general-logSystem Variable Name general_logVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFSpecify the initial general query log state. With no argument or an argument of 1, the
--general-logoption enables the log. If omitted or given with an argument of 0, the option disables the log. -
Command-Line Format --init-file=file_nameSystem Variable Name init_fileVariable Scope Global Dynamic Variable No Permitted Values Type file nameRead 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
--bootstrapoption, 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 asCREATE USERorGRANT). --innodb-xxxSet an option for the
InnoDBstorage engine. TheInnoDBoptions are listed in Section 14.17, “InnoDB Startup Options and System Variables”.-
Command-Line Format --install [service_name]Platform Specific Windows (Windows only) Install the server as a Windows service that starts automatically during Windows startup. The default service name is
MySQLif noservice_namevalue is given. For more information, see Section 2.3.7.7, “Starting MySQL as a Windows Service”.NoteIf the server is started with the
--defaults-fileand--installoptions,--installmust be first. --install-manual [service_name]Command-Line Format --install-manual [service_name]Platform Specific Windows (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
MySQLif noservice_namevalue is given. For more information, see Section 2.3.7.7, “Starting MySQL as a Windows Service”.NoteIf the server is started with the
--defaults-fileand--install-manualoptions,--install-manualmust be first.--language=lang_name, -Llang_nameDeprecated 5.5.0, by lc-messages-dir Command-Line Format --language=nameSystem Variable Name languageVariable Scope Global Dynamic Variable No Permitted Values Type directory nameDefault /usr/local/mysql/share/mysql/english/The language to use for error messages.
lang_namecan 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-dirand--lc-messagesshould be used rather than--language, which is deprecated and handled as an alias for--lc-messages-dir.-
Command-Line Format --large-pagesSystem Variable Name large_pagesVariable Scope Global Dynamic Variable No Platform Specific Linux Permitted Values (Linux) Type booleanDefault FALSESome 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-pagesoption.--large-pagesis disabled by default. -
Command-Line Format --lc-messages=nameSystem Variable Name lc_messagesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault en_USThe 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-dirto produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”. -
Command-Line Format --lc-messages-dir=dir_nameSystem Variable Name lc_messages_dirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe directory where error messages are located. The server uses the value together with the value of
--lc-messagesto produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”. -
Command-Line Format --local-service(Windows only) A
--local-serviceoption following the service name causes the server to run using theLocalServiceWindows account that has limited system privileges. This account is available only for Windows XP or newer. If both--defaults-fileand--local-serviceare 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]Deprecated 5.1.29, by general-log Command-Line Format --log[=file_name]System Variable Name logVariable Scope Global Dynamic Variable Yes Permitted Values Type file nameThis 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-outputoption. If you omit the file name, MySQL useshost_name.logThe
--logoption is deprecated and is removed (along with thelogsystem variable) in MySQL 5.6. Instead, use the--general_logoption to enable the general query log and the--general_log_file=option to set the general query log file name.file_name-
Command-Line Format --log-error[=file_name]System Variable Name log_errorVariable Scope Global Dynamic Variable No Permitted Values Type file nameLog 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 ishost_name.errhost_name.err--pid-fileoption is specified. In that case, the default name is the PID file base name with a suffix of.errin the data directory. -
Command-Line Format --log-isam[=file_name]Permitted Values Type file nameLog all
MyISAMchanges to this file (used only when debuggingMyISAM). -
Deprecated 4.1.0 Command-Line Format --log-long-formatLog 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-indexesoption is available for the purpose of logging queries that do not use indexes to the slow query log.--log-long-formatwas removed in MySQL 5.5.3. -
Command-Line Format --log-output=nameSystem Variable Name log_outputVariable Scope Global Dynamic Variable Yes Permitted Values Type setDefault FILEValid Values TABLEFILENONEThis 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, orNONE.TABLEselect logging to thegeneral_logandslow_logtables in themysqldatabase as a destination.FILEselects logging to log files as a destination.NONEdisables logging. IfNONEis present in the option value, it takes precedence over any other words that are present.TABLEandFILEcan 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_logand--slow_query_logoptions. ForFILElogging, the--general_log_fileand-slow_query_log_fileoptions 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-indexesCommand-Line Format --log-queries-not-using-indexesSystem Variable Name log_queries_not_using_indexesVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFIf 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.
-
Command-Line Format --log-short-formatPermitted Values Type booleanDefault FALSELog less information to the slow query log, if it has been activated.
-
Command-Line Format --log-slow-admin-statementsPermitted Values Type booleanDefault OFFInclude 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, andREPAIR TABLE. --log-slow-queries[=file_name]Deprecated 5.1.29, by slow-query-log Command-Line Format --log-slow-queries[=name]System Variable Name log_slow_queriesVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanThis option enables logging to the slow query log, which contains entries for all queries that have taken more than
long_query_timeseconds to execute. See the descriptions of the--log-long-formatand--log-short-formatoptions for details. The log output destination can be selected with the--log-outputoption. If you omit the file name, MySQL useshost_name-slow.logThe
--log-slow-queriesoption is deprecated and is removed (along with thelog_slow_queriessystem variable) in MySQL 5.6. Instead, use the--slow_query_logoption to enable the slow query log and the--slow_query_log_file=option to set the slow query log file name.file_name-
Command-Line Format --log-tc=file_namePermitted Values Type file nameDefault tc.logThe 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. -
Command-Line Format --log-tc-size=#Permitted Values (32-bit platforms) Type integerDefault 24576Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 24576Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 24576Max Value 18446744073709551615The 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 Variable Name log_warningsVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 1Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 1Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 1Min Value 0Max Value 18446744073709551615Print 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 alevelvalue 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-warningsenabled, 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-warningsis greater than 0.-
Command-Line Format --low-priority-updatesSystem Variable Name low_priority_updatesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault FALSEGive 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 bySET LOW_PRIORITY_UPDATES=1to 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=numberCommand-Line Format --min-examined-row-limit=#System Variable Name min_examined_row_limitVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 0Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 0Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 0Min Value 0Max Value 18446744073709551615When this option is set, queries which examine fewer than
numberrows are not written to the slow query log. The default is 0.-
Command-Line Format --memlockPermitted Values Type booleanDefault FALSELock 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.
--memlockworks on systems that support themlockall()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 notmlockall()(and thus this option) is supported by checking to see whether or not it is defined in the systemmman.hfile, like this:shell>
grep mlockall /usr/include/sys/mman.hIf
mlockall()is supported, you should see in the output of the previous command something like the following:extern int mlockall (int __flags) __THROW;
ImportantUse 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
rootby changing thelimits.conffile. 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. -
Command-Line Format --myisam-block-size=#Permitted Values Type integerDefault 1024Min Value 1024Max Value 16384The block size to be used for
MyISAMindex 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]...]]Introduced 5.5.3 Command-Line Format --myisam-recover-options[=name]Permitted Values Type enumerationDefault OFFValid Values OFFDEFAULTBACKUPFORCEQUICKSet the
MyISAMstorage engine recovery mode. The option value is any combination of the values ofOFF,DEFAULT,BACKUP,FORCE, orQUICK. If you specify multiple values, separate them by commas. Specifying the option with no argument is the same as specifyingDEFAULT, and specifying with an explicit value of""disables recovery (same as a value ofOFF). If recovery is enabled, each time mysqld opens aMyISAMtable, 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.
Option Description OFFNo recovery. DEFAULTRecovery without backup, forcing, or quick checking. BACKUPIf the data file was changed during recovery, save a backup of the tbl_name.MYDtbl_name-datetime.BAKFORCERun recovery even if we would lose more than one row from the .MYDfile.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
OFFis available as of MySQL 5.5.3.Do not read any option files. If program startup fails due to reading unknown options from an option file,
--no-defaultscan be used to prevent them from being read.-
Command-Line Format --old-alter-tableSystem Variable Name old_alter_tableVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault OFFWhen this option is given, the server does not use the optimized method of processing an
ALTER TABLEoperation. 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 ofALTER TABLE, see Section 13.1.7, “ALTER TABLE Syntax”. -
Command-Line Format --old-style-user-limitsPermitted Values Type booleanDefault FALSEEnable 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
usertable.) See Section 6.3.4, “Setting Account Resource Limits”. -
Command-Line Format --one-threadOnly 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-threadsinstead. -
Command-Line Format --open-files-limit=#System Variable Name open_files_limitVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 0Min Value 0Max Value platform dependentChanges 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 withsetrlimit(). 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_connectionsandtable_open_cacheto estimate whether more descriptors will be needed.On Unix, the value cannot be set less than ulimit -n.
-
Command-Line Format --partitionDisabled by skip-partitionPermitted Values Type booleanDefault ONEnables or disables user-defined partitioning support in the MySQL Server.
-
Command-Line Format --pid-file=file_nameSystem Variable Name pid_fileVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe 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.
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
--pluginprefix. Thus, the--innodb_file_per_tableoption forInnoDBcan be specified as--plugin-innodb_file_per_table.For boolean options that can be enabled or disabled, the
--skipprefix and other alternative formats are supported as well (see Section 4.2.5, “Program Option Modifiers”). For example,--skip-plugin-innodb_file_per_tabledisablesinnodb_file_per_table.The rationale for the
--pluginprefix 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--pluginprefix for plugin options is recommended to avoid any question of ambiguity.-
Command-Line Format --plugin-load=plugin_listPermitted Values Type stringThis option tells the server to load the named plugins at startup. All plugins to load must be named in the same
--plugin-loadoption. If multiple--plugin-loadoptions are given, only the last one is used.The option value is a semicolon-separated list of
name=plugin_libraryandplugin_libraryvalues. Eachnameis the name of a plugin to load, andplugin_libraryis 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 theplugin_dirsystem variable.For example, if plugins named
myplug1andmyplug2have library filesmyplug1.soandmyplug2.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-loadis used again. This is in contrast toINSTALL PLUGIN, which adds an entry to themysql.pluginstable 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.pluginssystem table. If the server is started with the--skip-grant-tablesoption, it does not consult themysql.pluginstable and does not load plugins listed there.--plugin-loadenables plugins to be loaded even when--skip-grant-tablesis given.--plugin-loadalso 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-Pport_numCommand-Line Format --port=#System Variable Name portVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 3306Min Value 0Max Value 65535The 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
rootsystem user.-
Command-Line Format --port-open-timeout=#Permitted Values Type integerDefault 0On 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 the program name and all options that it gets from option files.
-
Command-Line Format --remove [service_name]Platform Specific Windows (Windows only) Remove a MySQL Windows service. The default service name is
MySQLif noservice_namevalue is given. For more information, see Section 2.3.7.7, “Starting MySQL as a Windows Service”. -
Deprecated 5.5.26 Command-Line Format --safe-modeSkip some optimization stages. This option is deprecated and is removed in MySQL 5.6.
-
Deprecated 4.0.2 Removed 5.5.3 Command-Line Format --safe-show-databasePermitted Values Type booleanThis option is deprecated and does not do anything because there is a
SHOW DATABASESprivilege 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-databasewas removed in MySQL 5.5.3. -
Command-Line Format --safe-user-createPermitted Values Type booleanDefault FALSEIf this option is enabled, a user cannot create new MySQL users by using the
GRANTstatement unless the user has theINSERTprivilege for themysql.usertable 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
GRANTstatement to give privileges to other users. -
Command-Line Format --secure-authSystem Variable Name secure_authVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFThis 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-authoption, which prevents connections to a server if the server requires a password in old format for the client account.NotePasswords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.
-
Command-Line Format --secure-file-priv=dir_nameSystem Variable Name secure_file_privVariable Scope Global Dynamic Variable No Permitted Values Type stringDefault emptyValid Values emptydirnameThis option limits the effect of the
LOAD DATAandSELECT ... INTO OUTFILEstatements and theLOAD_FILE()function to work only with files in the specified directory. -
Command-Line Format --shared_memory[={0,1}]System Variable Name shared_memoryVariable Scope Global Dynamic Variable No Platform Specific Windows Permitted Values Type booleanDefault FALSEEnable shared-memory connections by local clients. This option is available only on Windows.
--shared-memory-base-name=nameCommand-Line Format --shared_memory_base_name=nameSystem Variable Name shared_memory_base_nameVariable Scope Global Dynamic Variable No Platform Specific Windows Permitted Values Type stringDefault MYSQLThe 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.Turn off the ability to select and insert at the same time on
MyISAMtables. (This is to be used only if you think you have found a bug in this feature.) See Section 8.11.3, “Concurrent Inserts”.-
Command-Line Format --skip-event-scheduler--disable-event-schedulerTurns the Event Scheduler
OFF. This is not the same as disabling the Event Scheduler, which requires setting--event-scheduler=DISABLED; see The--event-schedulerOption, for more information. 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 PRIVILEGESstatement 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 PLUGINstatement. To cause plugins to be loaded anyway, use the--plugin-loadoption.FLUSH PRIVILEGESmight be executed implicitly by other actions performed after startup. For example, mysql_upgrade flushes the privileges during the upgrade procedure.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”.
Disable the
InnoDBstorage engine. In this case, if the default storage engine isInnoDB, the server will not start unless you also use--default-storage-engineto set the default to some other engine.Do not resolve host names when checking client connections. Use only IP addresses. If you use this option, all
Hostcolumn values in the grant tables must be IP addresses orlocalhost. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.Depending on the network configuration of your system and the
Hostvalues for your accounts, clients may need to connect using an explicit--hostoption, such as--host=localhost,--host=127.0.0.1, or--host=::1.An attempt to connect to the host
127.0.0.1normally resolves to thelocalhostaccount. However, this fails if the server is run with the--skip-name-resolveoption, so make sure that an account exists that can accept a connection. For example, to be able to connect asrootusing--host=127.0.0.1or--host=::1, create these accounts:CREATE USER 'root'@'127.0.0.1' IDENTIFIED BY '
root-password'; CREATE USER 'root'@'::1' IDENTIFIED BY 'root-password';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”.
-
Command-Line Format --skip-partition--disable-partitionDisables user-defined partitioning. Partitioned tables can be seen using
SHOW TABLESor by querying theINFORMATION_SCHEMA.TABLEStable, but cannot be created or modified, nor can data in such tables be accessed. All partition-specific columns in theINFORMATION_SCHEMA.PARTITIONStable displayNULL.Since
DROP TABLEremoves table definition (.frm) files, this statement works on partitioned tables even when partitioning is disabled using the option. The statement, however, does not remove.parfiles 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.parfiles manually. Options that begin with
--sslspecify 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”.-
Command-Line Format --standalonePlatform Specific Windows Available on Windows only; instructs the MySQL server not to run as a service.
-
Introduced 5.5.3 Command-Line Format --super-large-pagesPlatform Specific Solaris Permitted Values (Solaris) Type booleanDefault FALSEStandard 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-pagesor--skip-super-large-pagesoption. --symbolic-links,--skip-symbolic-linksCommand-Line Format --symbolic-linksEnable or disable symbolic link support. This option has different effects on Windows and Unix:
On Windows, enabling symbolic links enables you to establish a symbolic link to a database directory by creating a
db_name.symOn Unix, enabling symbolic links means that you can link a
MyISAMindex file or data file to another directory with theINDEX DIRECTORYorDATA DIRECTORYoptions of theCREATE TABLEstatement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See Section 8.12.4.2, “Using Symbolic Links for MyISAM Tables on Unix”.
-
Removed 5.5.6 Command-Line Format --skip-safemallocPreviously, 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-safemallocoption.safemalloc, along with this option, was removed in MySQL 5.5.6. -
Command-Line Format --skip-show-databaseSystem Variable Name skip_show_databaseVariable Scope Global Dynamic Variable No This option sets the
skip_show_databasesystem variable that controls who is permitted to use theSHOW DATABASESstatement. See Section 5.1.4, “Server System Variables”. -
Command-Line Format --skip-stack-traceDo 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”.
-
Deprecated 5.1.29 Command-Line Format --skip-thread-priorityDisable using thread priorities for faster response time. This option is deprecated and is removed in MySQL 5.6.
-
Command-Line Format --slow-query-logSystem Variable Name slow_query_logVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFSpecify the initial slow query log state. With no argument or an argument of 1, the
--slow-query-logoption enables the log. If omitted or given with an argument of 0, the option disables the log. -
Introduced 5.5.20 Command-Line Format --slow-start-timeout=#Permitted Values (Windows) Type integerDefault 15000This 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.
-
Command-Line Format --socket={file_name|pipe_name}System Variable Name socketVariable Scope Global Dynamic Variable No Permitted Values Type stringDefault /tmp/mysql.sockOn 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 isMySQL(not case sensitive). --sql-mode=value[,value[,value...]]Command-Line Format --sql-mode=nameSystem Variable Name sql_modeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type setDefault ''Valid Values ALLOW_INVALID_DATESANSI_QUOTESERROR_FOR_DIVISION_BY_ZEROHIGH_NOT_PRECEDENCEIGNORE_SPACENO_AUTO_CREATE_USERNO_AUTO_VALUE_ON_ZERONO_BACKSLASH_ESCAPESNO_DIR_IN_CREATENO_ENGINE_SUBSTITUTIONNO_FIELD_OPTIONSNO_KEY_OPTIONSNO_TABLE_OPTIONSNO_UNSIGNED_SUBTRACTIONNO_ZERO_DATENO_ZERO_IN_DATEONLY_FULL_GROUP_BYPAD_CHAR_TO_FULL_LENGTHPIPES_AS_CONCATREAL_AS_FLOATSTRICT_ALL_TABLESSTRICT_TRANS_TABLESSet the SQL mode. See Section 5.1.7, “Server SQL Modes”.
NoteMySQL 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.
-
Command-Line Format --sysdate-is-nowPermitted Values Type booleanDefault FALSESYSDATE()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 ofNOW(). This option causesSYSDATE()to be an alias forNOW(). For information about the implications for binary logging and replication, see the description forSYSDATE()in Section 12.7, “Date and Time Functions” and forSET TIMESTAMPin Section 5.1.4, “Server System Variables”. --tc-heuristic-recover={COMMIT|ROLLBACK}Command-Line Format --tc-heuristic-recover=namePermitted Values Type enumerationDefault COMMITValid Values COMMITROLLBACKThe type of decision to use in the heuristic recovery process. This option is unused.
-
Command-Line Format --temp-poolPermitted Values Type booleanDefault TRUEThis 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.
-
Command-Line Format --transaction-isolation=namePermitted Values Type enumerationDefault REPEATABLE-READValid Values READ-UNCOMMITTEDREAD-COMMITTEDREPEATABLE-READSERIALIZABLESets the default transaction isolation level. The
levelvalue can beREAD-UNCOMMITTED,READ-COMMITTED,REPEATABLE-READ, orSERIALIZABLE. See Section 13.3.6, “SET TRANSACTION Syntax”.The default transaction isolation level can also be set at runtime using the
SET TRANSACTIONstatement or by setting thetx_isolationsystem variable. --tmpdir=,dir_name-tdir_nameCommand-Line Format --tmpdir=dir_nameSystem Variable Name tmpdirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe path of the directory to use for creating temporary files. It might be useful if your default
/tmpdirectory 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--tmpdirto 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 orLOAD DATA INFILEoperations. 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=namePermitted Values Type stringRun the mysqld server as the user having the name
user_nameor the numeric user IDuser_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 asroot. See Section 6.1.1, “Security Guidelines”.To avoid a possible security hole where a user adds a
--user=rootoption to amy.cnffile (thus causing the server to run asroot), mysqld uses only the first--useroption specified and produces a warning if there are multiple--useroptions. Options in/etc/my.cnfand$MYSQL_HOME/my.cnfare processed before command-line options, so it is recommended that you put a--useroption in/etc/my.cnfand specify a value other thanroot. The option in/etc/my.cnfis found before any other--useroptions, which ensures that the server runs as a user other thanroot, and that a warning results if any other--useroption is found.Use this option with the
--helpoption for detailed help.--version,-VDisplay version information and exit.
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 VARIABLESstatement.
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
For additional system variable information, see these sections:
Section 5.1.5, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
Section 5.1.5.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
Information on tuning system variables can be found in Section 8.12.2, “Tuning Server Parameters”.
Section 14.17, “InnoDB Startup Options and System Variables”, lists
InnoDBsystem variables.Section 18.3.3.8.2, “MySQL Cluster System Variables”, lists system variables which are specific to MySQL Cluster.
For information on server system variables specific to replication, see Section 17.1.3, “Replication and Binary Logging Options and Variables”.
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_levelIntroduced 5.5.16 Command-Line Format --authentication_windows_log_levelPermitted Values Type integerDefault 0Min Value 0Max Value 4This variable is available only if the
authentication_windowsWindows 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.
Value Description 0 No logging 1 Log only error messages 2 Log level 1 messages and warning messages 3 Log level 2 messages and information notes 4 Log level 3 messages and debug messages This variable was added in MySQL 5.5.16.
authentication_windows_use_principal_nameIntroduced 5.5.16 Command-Line Format --authentication_windows_use_principal_namePermitted Values Type booleanDefault ONThis variable is available only if the
authentication_windowsWindows 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 formuser_id@computer_nameThis 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
0x00byte in the first challenge, the client does not specifytargetName, 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.
-
Command-Line Format --autocommit[=#](>= 5.5.8)System Variable (<= 5.5.2) Name autocommitVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name autocommitVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault ONThe autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use
COMMITto accept a transaction orROLLBACKto cancel it. Ifautocommitis 0 and you change it to 1, MySQL performs an automaticCOMMITof any open transaction. Another way to begin a transaction is to use aSTART TRANSACTIONorBEGINstatement. See Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.By default, client connections begin with
autocommitset to 1. To cause clients to begin with a default of 0, set the globalautocommitvalue by starting the server with the--autocommit=0option. To set the variable using an option file, include these lines:[mysqld] autocommit=0
Before MySQL 5.5.8, the global
autocommitvalue cannot be set at startup. As a workaround, set theinit_connectsystem variable:SET GLOBAL init_connect='SET autocommit=0';
The
init_connectvariable 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_connectis not executed for users that have theSUPERprivilege (unlike the effect of setting the globalautocommitvalue at startup). -
System Variable Name automatic_sp_privilegesVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault TRUEWhen this variable has a value of 1 (the default), the server automatically grants the
EXECUTEandALTER ROUTINEprivileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (TheALTER ROUTINEprivilege is required to drop the routine.) The server also automatically drops those privileges from the creator when the routine is dropped. Ifautomatic_sp_privilegesis 0, the server does not automatically add or drop these privileges.The creator of a routine is the account used to execute the
CREATEstatement for it. This might not be the same as the account named as theDEFINERin the routine definition.See also Section 20.2.2, “Stored Routines and MySQL Privileges”.
-
System Variable Name back_logVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 50Min Value 1Max Value 65535The 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_logvalue 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_logcannot be set higher than your operating system limit. -
Command-Line Format --basedir=dir_nameSystem Variable Name basedirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe MySQL installation base directory. This variable can be set with the
--basediroption. Relative path names for other variables usually are resolved relative to the base directory. -
Command-Line Format --big-tablesSystem Variable (<= 5.5.2) Name big_tablesVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name big_tablesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault OFFIf set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error
The tabledoes not occur fortbl_nameis fullSELECToperations 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.NoteThis variable was formerly named
sql_big_tables. -
Command-Line Format --bulk_insert_buffer_size=#System Variable Name bulk_insert_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 8388608Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 8388608Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 8388608Min Value 0Max Value 18446744073709551615MyISAMuses a special tree-like cache to make bulk inserts faster forINSERT ... SELECT,INSERT ... VALUES (...), (...), ..., andLOAD DATA INFILEwhen 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. -
System Variable Name character_set_clientVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringThe 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-setoption 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
sjiswhen connecting to a server not configured withsjissupport.mysqld was started with the
--skip-character-set-client-handshakeoption, 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, andutf32cannot be used as a client character set, which means that they also do not work forSET NAMESorSET CHARACTER SET. -
System Variable Name character_set_connectionVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringThe character set used for literals that do not have a character set introducer and for number-to-string conversion.
-
System Variable Name character_set_databaseVariable Scope Global, Session Dynamic Variable Yes Footnote This option is dynamic, but only the server should set this information. You should not set the value of this variable manually. Permitted Values Type stringThe 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. -
Command-Line Format --character-set-filesystem=nameSystem Variable Name character_set_filesystemVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault binaryThe file system character set. This variable is used to interpret string literals that refer to file names, such as in the
LOAD DATA INFILEandSELECT ... INTO OUTFILEstatements and theLOAD_FILE()function. Such file names are converted fromcharacter_set_clienttocharacter_set_filesystembefore the file opening attempt occurs. The default value isbinary, 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, setcharacter_set_filesystemto'utf8'. -
System Variable Name character_set_resultsVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringThe character set used for returning query results such as result sets or error messages to the client.
-
Command-Line Format --character-set-serverSystem Variable Name character_set_serverVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault latin1The server's default character set.
-
System Variable Name character_set_systemVariable Scope Global Dynamic Variable No Permitted Values Type stringDefault utf8The character set used by the server for storing identifiers. The value is always
utf8. -
Command-Line Format --character-sets-dir=dir_nameSystem Variable Name character_sets_dirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe directory where character sets are installed.
-
System Variable Name collation_connectionVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringThe collation of the connection character set.
-
System Variable Name collation_databaseVariable Scope Global, Session Dynamic Variable Yes Footnote This option is dynamic, but only the server should set this information. You should not set the value of this variable manually. Permitted Values Type stringThe 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. -
Command-Line Format --collation-serverSystem Variable Name collation_serverVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault latin1_swedish_ciThe server's default collation.
-
Command-Line Format --completion_type=#System Variable Name completion_typeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.2) Type integerDefault 0Valid Values 012Permitted Values (>= 5.5.3) Type enumerationDefault NO_CHAINValid Values NO_CHAINCHAINRELEASE012The 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.
Value Description NO_CHAIN(or 0)COMMITandROLLBACKare unaffected. This is the default value.CHAIN(or 1)COMMITandROLLBACKare equivalent toCOMMIT AND CHAINandROLLBACK AND CHAIN, respectively. (A new transaction starts immediately with the same isolation level as the just-terminated transaction.)RELEASE(or 2)COMMITandROLLBACKare equivalent toCOMMIT RELEASEandROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)completion_typeaffects transactions that begin withSTART TRANSACTIONorBEGINand end withCOMMITorROLLBACK. 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 forXA COMMIT,XA ROLLBACK, or whenautocommit=1. -
Command-Line Format --concurrent_insert[=#]System Variable Name concurrent_insertVariable Scope Global Dynamic Variable Yes Permitted Values (<= 5.5.2) Type integerDefault 1Valid Values 012Permitted Values (>= 5.5.3) Type enumerationDefault AUTOValid Values NEVERAUTOALWAYS012If
AUTO(the default), MySQL permitsINSERTandSELECTstatements to run concurrently forMyISAMtables that have no free blocks in the middle of the data file. If you start mysqld with--skip-new, this variable is set toNEVER.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.
Value Description NEVER(or 0)Disables concurrent inserts AUTO(or 1)(Default) Enables concurrent insert for MyISAMtables that do not have holesALWAYS(or 2)Enables concurrent inserts for all MyISAMtables, 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”.
-
Command-Line Format --connect_timeout=#System Variable Name connect_timeoutVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 10Min Value 2Max Value 31536000The 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_timeoutvalue might help if clients frequently encounter errors of the formLost connection to MySQL server at '.XXX', system error:errno -
Command-Line Format --datadir=dir_nameSystem Variable Name datadirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe MySQL data directory. This variable can be set with the
--datadiroption. This variable is unused.
This variable is unused.
-
Command-Line Format --debug[=debug_options]System Variable Name debugVariable Scope Global, Session Dynamic Variable Yes Permitted Values (Unix) Type stringDefault d:t:i:o,/tmp/mysqld.tracePermitted Values (Windows) Type stringDefault d:t:i:O,\mysqld.traceThis 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
--debugoption given at server startup. The global and session values may be set at runtime; theSUPERprivilege 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”.
-
System Variable (<= 5.5.2) Name debug_syncVariable Scope Global, Session Dynamic Variable Yes System Variable (>= 5.5.3) Name debug_syncVariable Scope Session Dynamic Variable Yes Permitted Values Type stringThis variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the
-DENABLE_DEBUG_SYNC=1CMake 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_syncisOFF. If the server is started with--debug-sync-timeout=, whereNNis a timeout value greater than 0, Debug Sync is enabled and the value ofdebug_syncisON - current signalfollowed by the signal name. Also,Nbecomes 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
SUPERprivilege to control synchronization points.For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.
-
Command-Line Format --default-storage-engine=nameSystem Variable (<= 5.5.2) Name storage_engineVariable Scope Global, Session Dynamic Variable Yes System Variable (>= 5.5.3) Name default_storage_engineVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.4) Type enumerationDefault MyISAMPermitted Values (>= 5.5.5) Type enumerationDefault InnoDBThe default storage engine. To set the storage engine at server startup, use the
--default-storage-engineoption. See Section 5.1.3, “Server Command Options”.To see which storage engines are available and enabled, use the
SHOW ENGINESstatement or query theINFORMATION_SCHEMAENGINEStable.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. -
Command-Line Format --default_week_format=#System Variable Name default_week_formatVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 7The default mode value to use for the
WEEK()function. See Section 12.7, “Date and Time Functions”. -
Command-Line Format --delay-key-write[=name]System Variable Name delay_key_writeVariable Scope Global Dynamic Variable Yes Permitted Values Type enumerationDefault ONValid Values ONOFFALLThis option applies only to
MyISAMtables. It can have one of the following values to affect handling of theDELAY_KEY_WRITEtable option that can be used inCREATE TABLEstatements.Option Description OFFDELAY_KEY_WRITEis ignored.ONMySQL honors any DELAY_KEY_WRITEoption specified inCREATE TABLEstatements. This is the default value.ALLAll new opened tables are treated as if they were created with the DELAY_KEY_WRITEoption enabled.If
DELAY_KEY_WRITEis 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 allMyISAMtables by starting the server with the--myisam-recover-optionsoption (for example,--myisam-recover-options=BACKUP,FORCE). See Section 5.1.3, “Server Command Options”, and Section 15.3.1, “MyISAM Startup Options”.WarningIf you enable external locking with
--external-locking, there is no protection against index corruption for tables that use delayed key writes. -
Command-Line Format --delayed_insert_limit=#System Variable Name delayed_insert_limitVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 100Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 100Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 100Min Value 1Max Value 18446744073709551615After inserting
delayed_insert_limitdelayed rows, theINSERT DELAYEDhandler thread checks whether there are anySELECTstatements pending. If so, it permits them to execute before continuing to insert delayed rows. -
Command-Line Format --delayed_insert_timeout=#System Variable Name delayed_insert_timeoutVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 300How many seconds an
INSERT DELAYEDhandler thread should wait forINSERTstatements before terminating. -
Command-Line Format --delayed_queue_size=#System Variable Name delayed_queue_sizeVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 1000Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 1000Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 1000Min Value 1Max Value 18446744073709551615This is a per-table limit on the number of rows to queue when handling
INSERT DELAYEDstatements. If the queue becomes full, any client that issues anINSERT DELAYEDstatement waits until there is room in the queue again. -
Command-Line Format --div_precision_increment=#System Variable Name div_precision_incrementVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 4Min Value 0Max Value 30This 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 | +----------------+ -
Deprecated 5.5.3, by optimizer_switch Command-Line Format --engine-condition-pushdownSystem Variable Name engine_condition_pushdownVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault ONThe 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
NDBCLUSTERstorage engine. Enabling this optimization on a MySQL Server acting as a MySQL Cluster SQL node causesWHEREconditions 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_pushdownvariable controls whether engine condition pushdown is enabled. By default, this variable isON(1). Setting it toOFF(0) disables pushdown.This variable is deprecated as of MySQL 5.5.3 and is removed in MySQL 5.6. Use the
engine_condition_pushdownflag of theoptimizer_switchvariable instead. See Section 8.9.2, “Controlling Switchable Optimizations”. 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”.
-
Command-Line Format --event-scheduler[=value]System Variable Name event_schedulerVariable Scope Global Dynamic Variable Yes Permitted Values Type enumerationDefault OFFValid Values ONOFFDISABLEDThis variable indicates the status of the Event Scheduler; possible values are
ON,OFF, andDISABLED, with the default beingOFF. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in the Overview section of the Events chapter. -
Command-Line Format --expire_logs_days=#System Variable Name expire_logs_daysVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 99The 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 LOGSstatement. See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”. -
Introduced 5.5.7 System Variable Name external_userVariable Scope Session Dynamic Variable No Permitted Values Type stringThe 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.
-
Command-Line Format --flushSystem Variable Name flushVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFIf
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 toONif you start mysqld with the--flushoption. -
Command-Line Format --flush_time=#System Variable Name flush_timeVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Permitted Values (Windows) Type integerDefault 1800Min Value 0If this is set to a nonzero value, all tables are closed every
flush_timeseconds to free up resources and synchronize unflushed data to disk. This option is best used only on systems with minimal resources. If set to 1 (the default), foreign key constraints for
InnoDBtables 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, anALTER TABLEoperation 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
InnoDBtables 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_checksto 0 also affects data definition statements:DROP SCHEMAdrops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, andDROP TABLEdrops tables that have foreign keys that are referred to by other tables.NoteSetting
foreign_key_checksto 1 does not trigger a scan of the existing table data. Therefore, rows added to the table whileforeign_key_checks=0will not be verified for consistency.WarningWith
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).-
Command-Line Format --ft_boolean_syntax=nameSystem Variable Name ft_boolean_syntaxVariable Scope Global Dynamic Variable Yes Permitted Values Type stringDefault + -><()~*:""&|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.
-
Command-Line Format --ft_max_word_len=#System Variable Name ft_max_word_lenVariable Scope Global Dynamic Variable No Permitted Values Type integerMin Value 10The maximum length of the word to be included in a
FULLTEXTindex.NoteFULLTEXTindexes must be rebuilt after changing this variable. UseREPAIR TABLE.tbl_nameQUICK -
Command-Line Format --ft_min_word_len=#System Variable Name ft_min_word_lenVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 4Min Value 1The minimum length of the word to be included in a
FULLTEXTindex.NoteFULLTEXTindexes must be rebuilt after changing this variable. UseREPAIR TABLE.tbl_nameQUICK -
Command-Line Format --ft_query_expansion_limit=#System Variable Name ft_query_expansion_limitVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 20Min Value 0Max Value 1000The number of top matches to use for full-text searches performed using
WITH QUERY EXPANSION. -
Command-Line Format --ft_stopword_file=file_nameSystem Variable Name ft_stopword_fileVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe 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.cfile). Setting this variable to the empty string ('') disables stopword filtering. See also Section 12.9.4, “Full-Text Stopwords”.NoteFULLTEXTindexes must be rebuilt after changing this variable or the contents of the stopword file. UseREPAIR TABLE.tbl_nameQUICK -
Command-Line Format --general-logSystem Variable Name general_logVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFWhether the general query log is enabled. The value can be 0 (or
OFF) to disable the log or 1 (orON) to enable the log. The default value depends on whether the--general_logoption is given. The destination for log output is controlled by thelog_outputsystem variable; if that value isNONE, no log entries are written even if the log is enabled. -
Command-Line Format --general-log-file=file_nameSystem Variable Name general_log_fileVariable Scope Global Dynamic Variable Yes Permitted Values Type file nameDefault host_name.logThe name of the general query log file. The default value is
host_name.log--general_log_fileoption. -
Command-Line Format --group_concat_max_len=#System Variable Name group_concat_max_lenVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 1024Min Value 4Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 1024Min Value 4Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 1024Min Value 4Max Value 18446744073709551615The maximum permitted result length in bytes for the
GROUP_CONCAT()function. The default is 1024. YESif thezlibcompression library is available to the server,NOif not. If not, theCOMPRESS()andUNCOMPRESS()functions cannot be used.YESif thecrypt()system call is available to the server,NOif not. If not, theENCRYPT()function cannot be used.YESif mysqld supportsCSVtables,NOif not.This variable is deprecated and is removed in MySQL 5.6. Use
SHOW ENGINESinstead.YESif mysqld supports dynamic loading of plugins,NOif not. If the value isNO, you cannot use options such as--plugin-loadto load plugins at server startup, or theINSTALL PLUGINstatement to load plugins at runtime.YESif the server supports spatial data types,NOif not.YESif mysqld supportsInnoDBtables.DISABLEDif--skip-innodbis used.This variable is deprecated and is removed in MySQL 5.6. Use
SHOW ENGINESinstead.This variable is an alias for
have_ssl.YESif mysqld supports partitioning.This variable is deprecated and is removed in MySQL 5.6. Use
SHOW PLUGINSinstead. For more information, see Chapter 19, Partitioning.YESif statement profiling capability is present,NOif not. If present, theprofilingsystem variable controls whether this capability is enabled or disabled. See Section 13.7.5.32, “SHOW PROFILES Syntax”.YESif mysqld supports the query cache,NOif not.YESifRTREEindexes are available,NOif not. (These are used for spatial indexes inMyISAMtables.)YESif mysqld supports SSL connections,NOif not.DISABLEDindicates that the server was compiled with SSL support, but was not started with the appropriate--ssl-options. For more information, see Section 6.4.2, “Building MySQL with Support for Secure Connections”.xxxYESif symbolic link support is enabled,NOif not. This is required on Unix for support of theDATA DIRECTORYandINDEX DIRECTORYtable options, and on Windows for support of data directory symlinks. If the server is started with the--skip-symbolic-linksoption, the value isDISABLED.-
System Variable Name hostnameVariable Scope Global Dynamic Variable No Permitted Values Type stringThe server sets this variable to the server host name at startup.
This variable is a synonym for the
last_insert_idvariable. It exists for compatibility with other database systems. You can read its value withSELECT @@identity, and set it usingSET identity.-
Command-Line Format --init-connect=nameSystem Variable Name init_connectVariable Scope Global Dynamic Variable Yes Permitted Values Type stringA 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
autocommitsystem variable to specify that autocommit should be disabled by default, but as a workaroundinit_connectcan be used to achieve the same effect:SET GLOBAL init_connect='SET autocommit=0';
The
init_connectvariable 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_connectis not executed for users that have theSUPERprivilege. This is done so that an erroneous value forinit_connectdoes 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 executinginit_connectfor users that have theSUPERprivilege enables them to open a connection and fix theinit_connectvalue.The server discards any result sets produced by statements in the value of of
init_connect. -
Command-Line Format --init-file=file_nameSystem Variable Name init_fileVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe name of the file specified with the
--init-fileoption 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_xxxInnoDBsystem 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 forInnoDBtables, and are especially important now thatInnoDBis the default storage engine.The value to be used by the following
INSERTorALTER TABLEstatement when inserting anAUTO_INCREMENTvalue. This is mainly used with the binary log.-
Command-Line Format --interactive_timeout=#System Variable Name interactive_timeoutVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 28800Min Value 1The 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_INTERACTIVEoption tomysql_real_connect(). See alsowait_timeout. -
Command-Line Format --join_buffer_size=#System Variable Name join_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (Windows, >= 5.5.3) Type integerDefault 131072Min Value 128Max Value 4294967295Permitted Values (Windows, 32-bit platforms, <= 5.5.2) Type integerDefault 131072Min Value 8200Max Value 4294967295Permitted Values (Windows, 64-bit platforms, <= 5.5.2) Type integerDefault 131072Min Value 8228Max Value 4294967295Permitted Values (Other, 32-bit platforms, <= 5.5.2) Type integerDefault 131072Min Value 8200Max Value 4294967295Permitted Values (Other, 32-bit platforms, >= 5.5.3) Type integerDefault 131072Min Value 128Max Value 4294967295Permitted Values (Other, 64-bit platforms, <= 5.5.2) Type integerDefault 131072Min Value 8228Max Value 18446744073709547520Permitted Values (Other, 64-bit platforms, >= 5.5.3) Type integerDefault 131072Min Value 128Max Value 18446744073709547520The 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_sizeto 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_sizeis 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”.
-
Command-Line Format --keep_files_on_create=#System Variable Name keep_files_on_createVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault OFFIf a
MyISAMtable is created with noDATA DIRECTORYoption, the.MYDfile is created in the database directory. By default, ifMyISAMfinds an existing.MYDfile in this case, it overwrites it. The same applies to.MYIfiles for tables created with noINDEX DIRECTORYoption. To suppress this behavior, set thekeep_files_on_createvariable toON(1), in which caseMyISAMwill not overwrite existing files and returns an error instead. The default value isOFF(0).If a
MyISAMtable is created with aDATA DIRECTORYorINDEX DIRECTORYoption and an existing.MYDor.MYIfile is found, MyISAM always returns an error. It will not overwrite a file in the specified directory. -
Command-Line Format --key_buffer_size=#System Variable Name key_buffer_sizeVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 8388608Min Value 8Max Value 4294967295Permitted Values (64-bit platforms) Type integerDefault 8388608Min Value 8Max Value OS_PER_PROCESS_LIMITIndex blocks for
MyISAMtables are buffered and are shared by all threads.key_buffer_sizeis 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_sizeis 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
MyISAMstorage 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 toMyISAM.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 STATUSstatement and examining theKey_read_requests,Key_reads,Key_write_requests, andKey_writesstatus variables. (See Section 13.7.5, “SHOW Syntax”.) TheKey_reads/Key_read_requestsratio should normally be less than 0.01. TheKey_writes/Key_write_requestsratio 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 theDELAY_KEY_WRITEtable option.The fraction of the key buffer in use can be determined using
key_buffer_sizein conjunction with theKey_blocks_unusedstatus variable and the buffer block size, which is available from thekey_cache_block_sizesystem 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
MyISAMkey caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 8.10.2, “The MyISAM Key Cache”. -
Command-Line Format --key_cache_age_threshold=#System Variable Name key_cache_age_thresholdVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 300Min Value 100Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 300Min Value 100Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 300Min Value 100Max Value 18446744073709551615This 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”.
-
Command-Line Format --key_cache_block_size=#System Variable Name key_cache_block_sizeVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 1024Min Value 512Max Value 16384The size in bytes of blocks in the key cache. The default value is 1024. See Section 8.10.2, “The MyISAM Key Cache”.
-
Command-Line Format --key_cache_division_limit=#System Variable Name key_cache_division_limitVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 100Min Value 1Max Value 100The 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”.
-
Deprecated 5.5.0, by lc-messages-dir Command-Line Format --language=nameSystem Variable Name languageVariable Scope Global Dynamic Variable No Permitted Values Type directory nameDefault /usr/local/mysql/share/mysql/english/The directory where error messages are located. See Section 10.2, “Setting the Error Message Language”.
The
languagesystem variable is removed as of MySQL 5.5.0 (although the--languagecommand-line option can still be used). Similar information is available from thelc_messages_dirandlc_messagesvariables. -
System Variable Name large_files_supportVariable Scope Global Dynamic Variable No Whether mysqld was compiled with options for large file support.
-
Command-Line Format --large-pagesSystem Variable Name large_pagesVariable Scope Global Dynamic Variable No Platform Specific Linux Permitted Values (Linux) Type booleanDefault FALSEWhether large page support is enabled (via the
--large-pagesoption). See Section 8.12.5.2, “Enabling Large Page Support”. -
System Variable Name large_page_sizeVariable Scope Global Dynamic Variable No Permitted Values (Linux) Type integerDefault 0If 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”.
The value to be returned from
LAST_INSERT_ID(). This is stored in the binary log when you useLAST_INSERT_ID()in a statement that updates a table. Setting this variable does not update the value returned by themysql_insert_id()C API function.-
Command-Line Format --lc-messages=nameSystem Variable Name lc_messagesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringDefault en_USThe 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 oflc_messages_dirto produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”. -
Command-Line Format --lc-messages-dir=dir_nameSystem Variable Name lc_messages_dirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe directory where error messages are located. The server uses the value together with the value of
lc_messagesto produce the location for the error message file. See Section 10.2, “Setting the Error Message Language”. -
System Variable Name lc_time_namesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringThis 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()andMONTHNAME()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”. -
System Variable Name licenseVariable Scope Global Dynamic Variable No Permitted Values Type stringDefault GPLThe type of license the server has.
-
System Variable Name local_infileVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault trueWhether
LOCALis supported forLOAD DATA INFILEstatements. If this variable is disabled, clients cannot useLOCALinLOAD DATAstatements. While the default for this variable istrue, whetherLOAD DATA INFILE LOCALis 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. -
Introduced 5.5.3 Command-Line Format --lock_wait_timeout=#System Variable Name lock_wait_timeoutVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 31536000Min Value 1Max Value 31536000This 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, andHANDLERstatements.This timeout does not apply to implicit accesses to system tables in the
mysqldatabase, such as grant tables modified byGRANTorREVOKEstatements or table logging statements. The timeout does apply to system tables accessed directly, such as withSELECTorUPDATE.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_timeoutvalue before reporting a timeout error. When lock timeout occurs,ER_LOCK_WAIT_TIMEOUTis reported.lock_wait_timeoutdoes 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.
-
System Variable Name locked_in_memoryVariable Scope Global Dynamic Variable No -
Deprecated 5.1.29, by general-log Command-Line Format --log[=file_name]System Variable Name logVariable Scope Global Dynamic Variable Yes Permitted Values Type file nameWhether 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_loginstead. log_bin_trust_function_creatorsCommand-Line Format --log-bin-trust-function-creatorsSystem Variable Name log_bin_trust_function_creatorsVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault FALSEThis 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
SUPERprivilege in addition to theCREATE ROUTINEorALTER ROUTINEprivilege. A setting of 0 also enforces the restriction that a function must be declared with theDETERMINISTICcharacteristic, or with theREADS SQL DATAorNO SQLcharacteristic. 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_creatorsThis is the old name for
log_bin_trust_function_creators. This variable is deprecated and was removed in MySQL 5.5.3.-
Command-Line Format --log-error[=file_name]System Variable Name log_errorVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe 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”.
-
Command-Line Format --log-output=nameSystem Variable Name log_outputVariable Scope Global Dynamic Variable Yes Permitted Values Type setDefault FILEValid Values TABLEFILENONEThe 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), orNONE(do not log to tables or files). The default value isFILE.NONE, if present, takes precedence over any other specifiers. If the value isNONElog entries are not written even if the logs are enabled. If the logs are not enabled, no logging occurs even if the value oflog_outputis notNONE. For more information, see Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”. -
Command-Line Format --log-queries-not-using-indexesSystem Variable Name log_queries_not_using_indexesVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFWhether queries that do not use indexes are logged to the slow query log. See Section 5.4.5, “The Slow Query Log”.
-
Deprecated 5.1.29, by slow-query-log Command-Line Format --log-slow-queries[=name]System Variable Name log_slow_queriesVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanWhether slow queries should be logged. “Slow” is determined by the value of the
long_query_timevariable. See Section 5.4.5, “The Slow Query Log”.This variable is deprecated and is removed in MySQL 5.6. Use
slow_query_loginstead. -
Command-Line Format --log-warnings[=#]System Variable Name log_warningsVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 1Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 1Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 1Min Value 0Max Value 18446744073709551615Whether 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.
-
Command-Line Format --long_query_time=#System Variable Name long_query_timeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type numericDefault 10Min Value 0If a query takes longer than this many seconds, the server increments the
Slow_queriesstatus 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 oflong_query_timeare 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”. -
Command-Line Format --low-priority-updatesSystem Variable Name low_priority_updatesVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault FALSEIf set to
1, allINSERT,UPDATE,DELETE, andLOCK TABLE WRITEstatements wait until there is no pendingSELECTorLOCK TABLE READon the affected table. This affects only storage engines that use only table-level locking (such asMyISAM,MEMORY, andMERGE). This variable previously was namedsql_low_priority_updates. -
System Variable Name lower_case_file_systemVariable Scope Global Dynamic Variable No Permitted Values Type booleanThis variable describes the case sensitivity of file names on the file system where the data directory is located.
OFFmeans file names are case sensitive,ONmeans 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. -
Command-Line Format --lower_case_table_names[=#]System Variable Name lower_case_table_namesVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 0Min Value 0Max Value 2If 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_namesto 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 anINSERT INTO ... SELECT ... FROMoperation with the wrongtbl_nametbl_nameletter case. WithMyISAM, 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=0on a case-insensitive file system.If you are using
InnoDBtables, 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_nameson 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”. -
Command-Line Format --max_allowed_packet=#System Variable Name max_allowed_packetVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 1048576Min Value 1024Max Value 1073741824The maximum size of one packet or any generated/intermediate string.
The packet message buffer is initialized to
net_buffer_lengthbytes, but can grow up tomax_allowed_packetbytes when needed. This value by default is small, to catch large (possibly incorrect) packets.You must increase this value if you are using large
BLOBcolumns or long strings. It should be as big as the largestBLOByou want to use. The protocol limit formax_allowed_packetis 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_packetvariable, you should also change the buffer size on the client side if your client program permits it. The defaultmax_allowed_packetvalue 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 settingmax_allowed_packeton 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_packetvalue. (The global value could be less than the session value if the global value is changed after the client connects.) -
Command-Line Format --max_connect_errors=#System Variable Name max_connect_errorsVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 10Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 10Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 10Min Value 1Max Value 18446744073709551615If 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 HOSTSstatement or execute a mysqladmin flush-hosts command. If a connection is established successfully within fewer thanmax_connect_errorsattempts 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. -
Command-Line Format --max_connections=#System Variable Name max_connectionsVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 151Min Value 1Max Value 100000The 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.
-
Command-Line Format --max_delayed_threads=#System Variable Name max_delayed_threadsVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 20Min Value 0Max Value 16384Do not start more than this number of threads to handle
INSERT DELAYEDstatements. If you try to insert data into a new table after allINSERT DELAYEDthreads are in use, the row is inserted as if theDELAYEDattribute was not specified. If you set this to 0, MySQL never creates a thread to handleDELAYEDrows; in effect, this disablesDELAYEDentirely.For the
SESSIONvalue of this variable, the only valid values are 0 or theGLOBALvalue. -
Command-Line Format --max_error_count=#System Variable Name max_error_countVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 64Min Value 0Max Value 65535The maximum number of error, warning, and note messages to be stored for display by the
SHOW ERRORSandSHOW WARNINGSstatements. -
Command-Line Format --max_heap_table_size=#System Variable Name max_heap_table_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 16777216Min Value 16384Max Value 4294967295Permitted Values (64-bit platforms) Type integerDefault 16777216Min Value 16384Max Value 1844674407370954752This variable sets the maximum size to which user-created
MEMORYtables are permitted to grow. The value of the variable is used to calculateMEMORYtableMAX_ROWSvalues. Setting this variable has no effect on any existingMEMORYtable, unless the table is re-created with a statement such asCREATE TABLEor altered withALTER TABLEorTRUNCATE TABLE. A server restart also sets the maximum size of existingMEMORYtables to the globalmax_heap_table_sizevalue.This variable is also used in conjunction with
tmp_table_sizeto limit the size of internal in-memory tables. See Section 8.4.4, “Internal Temporary Table Use in MySQL”.max_heap_table_sizeis not replicated. See Section 17.4.1.23, “Replication and MEMORY Tables”, and Section 17.4.1.38, “Replication and Variables”, for more information. -
System Variable Name max_insert_delayed_threadsVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerThis variable is a synonym for
max_delayed_threads. -
Command-Line Format --max_join_size=#System Variable Name max_join_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 18446744073709551615Min Value 1Max Value 18446744073709551615Do not permit statements that probably need to examine more than
max_join_sizerows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more thanmax_join_sizedisk 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 aWHEREclause, that take a long time, or that return millions of rows.Setting this variable to a value other than
DEFAULTresets the value ofsql_big_selectsto0. If you set thesql_big_selectsvalue again, themax_join_sizevariable 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. -
Command-Line Format --max_length_for_sort_data=#System Variable Name max_length_for_sort_dataVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 1024Min Value 4Max Value 8388608The cutoff on the size of index values that determines which
filesortalgorithm to use. See Section 8.2.1.11, “ORDER BY Optimization”. -
Introduced 5.5.11 Deprecated 5.5.11 Command-Line Format --max_long_data_size=#System Variable Name max_long_data_sizeVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 1048576Min Value 1024Max Value 4294967295The 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 themax_allowed_packetsystem variable. This variable is deprecated. In MySQL 5.6, it is removed and the maximum parameter size is controlled bymax_allowed_packet. -
Command-Line Format --max_prepared_stmt_count=#System Variable Name max_prepared_stmt_countVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 16382Min Value 0Max Value 1048576This 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.
-
Command-Line Format --max_relay_log_size=#System Variable Name max_relay_log_sizeVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 1073741824If 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_sizeis 0, the server usesmax_binlog_sizefor both the binary log and the relay log. Ifmax_relay_log_sizeis greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must setmax_relay_log_sizeto between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 17.2.1, “Replication Implementation Details”. -
Command-Line Format --max_seeks_for_key=#System Variable Name max_seeks_for_keyVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 4294967295Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 18446744073709547520Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 18446744073709551615Min Value 1Max Value 18446744073709551615Limit 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.
-
Command-Line Format --max_sort_length=#System Variable Name max_sort_lengthVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 1024Min Value 4Max Value 8388608The number of bytes to use when sorting data values. The server uses only the first
max_sort_lengthbytes of each value and ignores the rest. Consequently, values that differ only after the firstmax_sort_lengthbytes compare as equal forGROUP BY,ORDER BY, andDISTINCToperations.Increasing the value of
max_sort_lengthmay require increasing the value ofsort_buffer_sizeas well. For details, see Section 8.2.1.11, “ORDER BY Optimization” -
Command-Line Format --max_sp_recursion_depth[=#]System Variable Name max_sp_recursion_depthVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 0Max Value 255The 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 ofthread_stackat server startup. This variable is unused.
-
Command-Line Format --max_user_connections=#System Variable Name max_user_connectionsVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 4294967295The 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_CONNECTIONSresource limit, the sessionmax_user_connectionsvalue is set to that limit.Otherwise, the session
max_user_connectionsvalue is set to the global value.
Account resource limits are specified using the
GRANTstatement. See Section 6.3.4, “Setting Account Resource Limits”, and Section 13.7.1.3, “GRANT Syntax”. -
Command-Line Format --max_write_lock_count=#System Variable Name max_write_lock_countVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 4294967295Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 18446744073709547520Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 18446744073709551615Min Value 1Max Value 18446744073709551615After this many write locks, permit some pending read lock requests to be processed in between.
-
Introduced 5.5.19 System Variable Name metadata_locks_cache_sizeVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 1024Min Value 1Max Value 1048576The 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.
-
Command-Line Format --min-examined-row-limit=#System Variable Name min_examined_row_limitVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 0Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 0Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 0Min Value 0Max Value 18446744073709551615Queries that examine fewer than this number of rows are not logged to the slow query log.
-
Command-Line Format --multi_range_count=#System Variable Name multi_range_countVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 256Min Value 1Max Value 4294967295The 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
NDBCLUSTERtable 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.
-
Command-Line Format --myisam_data_pointer_size=#System Variable Name myisam_data_pointer_sizeVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 6Min Value 2Max Value 7The default pointer size in bytes, to be used by
CREATE TABLEforMyISAMtables when noMAX_ROWSoption 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”. -
Command-Line Format --myisam_max_sort_file_size=#System Variable Name myisam_max_sort_file_sizeVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 2147483648Permitted Values (64-bit platforms) Type integerDefault 9223372036854775807The maximum size of the temporary file that MySQL is permitted to use while re-creating a
MyISAMindex (duringREPAIR TABLE,ALTER TABLE, orLOAD 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
MyISAMindex 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. -
Introduced 5.5.1 Command-Line Format --myisam_mmap_size=#System Variable Name myisam_mmap_sizeVariable Scope Global Dynamic Variable No Permitted Values (32-bit platforms) Type integerDefault 4294967295Min Value 7Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 18446744073709547520Min Value 7Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 18446744073709551615Min Value 7Max Value 18446744073709551615The maximum amount of memory to use for memory mapping compressed
MyISAMfiles. If many compressedMyISAMtables are used, the value can be decreased to reduce the likelihood of memory-swapping problems. This variable was added in MySQL 5.5.1. -
System Variable Name myisam_recover_optionsVariable Scope Global Dynamic Variable No The value of the
--myisam-recover-optionsoption. See Section 5.1.3, “Server Command Options”. -
Command-Line Format --myisam_repair_threads=#System Variable Name myisam_repair_threadsVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 1Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 1Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 1Min Value 1Max Value 18446744073709551615If this value is greater than 1,
MyISAMtable indexes are created in parallel (each index in its own thread) during theRepair by sortingprocess. The default value is 1.NoteMulti-threaded repair is still beta-quality code.
-
Command-Line Format --myisam_sort_buffer_size=#System Variable Name myisam_sort_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (Windows, 32-bit platforms) Type integerDefault 8388608Min Value 4096Max Value 4294967295Permitted Values (Windows, 64-bit platforms, <= 5.5.21) Type integerDefault 8388608Min Value 4096Max Value 4294967295Permitted Values (Windows, 64-bit platforms, >= 5.5.22) Type integerDefault 8388608Min Value 4096Max Value 18446744073709551615Permitted Values (Other, 32-bit platforms) Type integerDefault 8388608Min Value 4096Max Value 4294967295Permitted Values (Other, 64-bit platforms, <= 5.5.2) Type integerDefault 8388608Min Value 4096Max Value 18446744073709547520Permitted Values (Other, 64-bit platforms, >= 5.5.3) Type integerDefault 8388608Min Value 4096Max Value 18446744073709551615The size of the buffer that is allocated when sorting
MyISAMindexes during aREPAIR TABLEor when creating indexes withCREATE INDEXorALTER TABLE.The maximum permissible setting for
myisam_sort_buffer_sizeis 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). -
Command-Line Format --myisam_stats_method=nameSystem Variable Name myisam_stats_methodVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type enumerationDefault nulls_unequalValid Values nulls_equalnulls_unequalnulls_ignoredHow the server treats
NULLvalues when collecting statistics about the distribution of index values forMyISAMtables. This variable has three possible values,nulls_equal,nulls_unequal, andnulls_ignored. Fornulls_equal, allNULLindex values are considered equal and form a single value group that has a size equal to the number ofNULLvalues. Fornulls_unequal,NULLvalues are considered unequal, and eachNULLforms a distinct value group of size 1. Fornulls_ignored,NULLvalues 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”.
-
Command-Line Format --myisam_use_mmapSystem Variable Name myisam_use_mmapVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFUse memory mapping for reading and writing
MyISAMtables. -
System Variable Name named_pipeVariable Scope Global Dynamic Variable No Platform Specific Windows Permitted Values (Windows) Type booleanDefault OFF(Windows only.) Indicates whether the server supports connections over named pipes.
-
Command-Line Format --net_buffer_length=#System Variable Name net_buffer_lengthVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 16384Min Value 1024Max Value 1048576Each client thread is associated with a connection buffer and result buffer. Both begin with a size given by
net_buffer_lengthbut are dynamically enlarged up tomax_allowed_packetbytes as needed. The result buffer shrinks tonet_buffer_lengthafter 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_lengthcan be set is 1MB.The session value of this variable is read only.
-
Command-Line Format --net_read_timeout=#System Variable Name net_read_timeoutVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 30Min Value 1The 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_timeoutis the timeout value controlling when to abort. When the server is writing to the client,net_write_timeoutis the timeout value controlling when to abort. See alsoslave_net_timeout. -
Command-Line Format --net_retry_count=#System Variable Name net_retry_countVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 10Min Value 1Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 10Min Value 1Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 10Min Value 1Max Value 18446744073709551615If 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.
-
Command-Line Format --net_write_timeout=#System Variable Name net_write_timeoutVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 60Min Value 1The number of seconds to wait for a block to be written to a connection before aborting the write. See also
net_read_timeout. -
Command-Line Format --newSystem Variable Name newVariable Scope Global, Session Dynamic Variable Yes Disabled by skip-newPermitted Values Type booleanDefault FALSEThis 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
ONmakes it possible to employ partitioning types other thanKEYorLINEAR KEYwithNDBtables. This feature is experimental only, and not supported in production. For additional information, see User-defined partitioning and the NDB storage engine (MySQL Cluster). -
Command-Line Format --oldSystem Variable Name oldVariable Scope Global Dynamic Variable No oldis 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
oldis enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with noFORclause apply only to how indexes are used for row retrieval and not to resolution ofORDER BYorGROUP BYclauses. (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. -
Command-Line Format --old-alter-tableSystem Variable Name old_alter_tableVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault OFFWhen this variable is enabled, the server does not use the optimized method of processing an
ALTER TABLEoperation. 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 ofALTER TABLE, see Section 13.1.7, “ALTER TABLE Syntax”. -
System Variable Name old_passwordsVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 0This variable controls the password hashing method used by the
PASSWORD()function. It also influences password hashing performed byCREATE USERandGRANTstatements that specify a password using anIDENTIFIED BYclause.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.Value Password Hashing Method Associated Authentication Plugin 0orOFFMySQL 4.1 native hashing mysql_native_password1orONPre-4.1 (“old”) hashing mysql_old_passwordIf
old_passwords=1,PASSWORD(returns the same value asstr)OLD_PASSWORD(. The latter function is not affected by the value ofstr)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”.
NotePasswords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.
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”.
-
Command-Line Format --open-files-limit=#System Variable Name open_files_limitVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 0Min Value 0Max Value platform dependentThe 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-limitoption to mysqld or mysqld_safe. The value is 0 on systems where MySQL cannot change the number of open files. -
Command-Line Format --optimizer_prune_level[=#]System Variable Name optimizer_prune_levelVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 1Controls 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.
-
Command-Line Format --optimizer_search_depth[=#]System Variable Name optimizer_search_depthVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 62Min Value 0Max Value 63The 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.
-
Command-Line Format --optimizer_switch=valueSystem Variable Name optimizer_switchVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.2) Type setValid Values index_merge={on|off}index_merge_intersection={on|off}index_merge_sort_union={on|off}index_merge_union={on|off}Permitted Values (>= 5.5.3) Type setValid Values engine_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_switchsystem variable enables control over optimizer behavior. The value of this variable is a set of flags, each of which has a value ofonoroffto 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=onFor 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_xxxPerformance Schema system variables are listed in Section 22.9, “Performance Schema System Variables”. These variables may be used to configure Performance Schema operation.
-
Command-Line Format --pid-file=file_nameSystem Variable Name pid_fileVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe path name of the process ID (PID) file. This variable can be set with the
--pid-fileoption. -
Command-Line Format --plugin_dir=dir_nameSystem Variable Name plugin_dirVariable Scope Global Dynamic Variable No Permitted Values (>= 5.5.5) Type directory nameDefault BASEDIR/lib/pluginPermitted Values (Windows, <= 5.5.4) Type directory nameDefault BASEDIR/lib/pluginPermitted Values (Other, <= 5.5.4) Type directory nameDefault BASEDIR/lib/mysql/pluginThe 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 makingplugin_dirread only to the server or by setting--secure-file-privto a directory whereSELECTwrites can be made safely. -
Command-Line Format --port=#System Variable Name portVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 3306Min Value 0Max Value 65535The number of the port on which the server listens for TCP/IP connections. This variable can be set with the
--portoption. -
Command-Line Format --preload_buffer_size=#System Variable Name preload_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 32768Min Value 1024Max Value 1073741824The size of the buffer that is allocated when preloading indexes.
If set to 0 or
OFF(the default), statement profiling is disabled. If set to 1 orON, statement profiling is enabled and theSHOW PROFILEandSHOW PROFILESstatements provide access to profiling information. See Section 13.7.5.32, “SHOW PROFILES Syntax”.The number of statements for which to maintain profiling information if
profilingis 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”.-
System Variable Name protocol_versionVariable Scope Global Dynamic Variable No Permitted Values Type integerThe version of the client/server protocol used by the MySQL server.
-
Introduced 5.5.7 System Variable Name proxy_userVariable Scope Session Dynamic Variable No Permitted Values Type stringIf 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.
-
Introduced 5.5.30 System Variable Name pseudo_slave_modeVariable Scope Session Dynamic Variable Yes Permitted Values Type integerThis variable is for internal server use. It was added in MySQL 5.5.30.
-
System Variable Name pseudo_thread_idVariable Scope Session Dynamic Variable Yes Permitted Values Type integerThis variable is for internal server use.
-
Command-Line Format --query_alloc_block_size=#System Variable Name query_alloc_block_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 8192Min Value 1024Max Value 4294967295Block Size 1024Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 8192Min Value 1024Max Value 18446744073709547520Block Size 1024Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 8192Min Value 1024Max Value 18446744073709551615Block Size 1024The 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.
-
Command-Line Format --query_cache_limit=#System Variable Name query_cache_limitVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 1048576Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 1048576Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 1048576Min Value 0Max Value 18446744073709551615Do not cache results that are larger than this number of bytes. The default value is 1MB.
-
Command-Line Format --query_cache_min_res_unit=#System Variable Name query_cache_min_res_unitVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 4096Min Value 512Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 4096Min Value 512Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 4096Min Value 512Max Value 18446744073709551615The 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”.
-
Command-Line Format --query_cache_size=#System Variable Name query_cache_sizeVariable Scope Global Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 0Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 0Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 0Min Value 0Max Value 18446744073709551615The 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=0if 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_sizebytes of memory are allocated even ifquery_cache_typeis 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_sizetoo small, a warning will occur, as described in Section 8.10.3.3, “Query Cache Configuration”. -
Command-Line Format --query_cache_type=#System Variable Name query_cache_typeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type enumerationDefault 1Valid Values 012Set the query cache type. Setting the
GLOBALvalue sets the type for all clients that connect thereafter. Individual clients can set theSESSIONvalue to affect their own use of the query cache. Possible values are shown in the following table.Option Description 0orOFFDo 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_sizeto 0.1orONCache all cacheable query results except for those that begin with SELECT SQL_NO_CACHE.2orDEMANDCache results only for cacheable queries that begin with SELECT SQL_CACHE.This variable defaults to
ON.If the server is started with
query_cache_typeset 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. -
Command-Line Format --query_cache_wlock_invalidateSystem Variable Name query_cache_wlock_invalidateVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault FALSENormally, when one client acquires a
WRITElock on aMyISAMtable, 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 aWRITElock 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. -
Command-Line Format --query_prealloc_size=#System Variable Name query_prealloc_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 8192Min Value 8192Max Value 4294967295Block Size 1024Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 8192Min Value 8192Max Value 18446744073709547520Block Size 1024Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 8192Min Value 8192Max Value 18446744073709551615Block Size 1024The 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_sizevalue might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations. The
rand_seed1andrand_seed2variables exist as session variables only, and can be set but not read. The variables—but not their values—are shown in the output ofSHOW VARIABLES.The purpose of these variables is to support replication of the
RAND()function. For statements that invokeRAND(), 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 variablesrand_seed1andrand_seed2so thatRAND()on the slave generates the same value as on the master.See the description for
rand_seed1.-
Command-Line Format --range_alloc_block_size=#System Variable Name range_alloc_block_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (32-bit platforms) Type integerDefault 4096Min Value 4096Max Value 4294967295Block Size 1024Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 4096Min Value 4096Max Value 18446744073709551615Block Size 1024The size of blocks that are allocated when doing range optimization.
-
Command-Line Format --read_buffer_size=#System Variable Name read_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 131072Min Value 8200Max Value 2147479552Each thread that does a sequential scan for a
MyISAMtable 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
MEMORYtables.The maximum permissible setting for
read_buffer_sizeis 2GB.For more information about memory use during different operations, see Section 8.12.5.1, “How MySQL Uses Memory”.
-
Command-Line Format --read_onlySystem Variable Name read_onlyVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFWhen the
read_onlysystem variable is enabled, the server permits no client updates except from users who have theSUPERprivilege. This variable is disabled by default.Even with
read_onlyenabled, the server permits these operations:Updates performed by slave threads, if the server is a replication slave. In replication setups, it can be useful to enable
read_onlyon slave servers to ensure that slaves accept updates only from the master server and not from clients.Use of
ANALYZE TABLEorOPTIMIZE TABLEstatements. The purpose of read-only mode is to prevent changes to table structure or contents. Analysis and optimization do not qualify as such changes. This means, for example, that consistency checks on read-only replication slaves can be performed with mysqlcheck --all-databases --analyze.Operations on
TEMPORARYtables.Inserts into the log tables (
mysql.general_logandmysql.slow_log; see Section 5.4.1, “Selecting General Query and Slow Query Log Output Destinations”).
read_onlyexists only as aGLOBALvariable, so changes to its value require theSUPERprivilege. Changes toread_onlyon 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_onlyis pending, requests by other clients for table locks or to begin transactions also block untilread_onlyhas 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_onlycan be enabled while you hold a global read lock (acquired withFLUSH TABLES WITH READ LOCK) because that does not involve table locks.
-
Command-Line Format --read_rnd_buffer_size=#System Variable Name read_rnd_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.2) Type integerDefault 262144Min Value 8200Max Value 2147483647Permitted Values (>= 5.5.3) Type integerDefault 262144Min Value 1Max Value 2147483647When reading rows from a
MyISAMtable 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 improveORDER BYperformance 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_sizeis 2GB.For more information about memory use during different operations, see Section 8.12.5.1, “How MySQL Uses Memory”.
-
Command-Line Format --relay_log_purgeSystem Variable Name relay_log_purgeVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault TRUEDisables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (
ON). -
Command-Line Format --relay_log_space_limit=#System Variable Name relay_log_space_limitVariable Scope Global Dynamic Variable No Permitted Values (32-bit platforms) Type integerDefault 0Min Value 0Max Value 4294967295Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 0Min Value 0Max Value 18446744073709547520Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 0Min Value 0Max Value 18446744073709551615The maximum amount of space to use for all relay logs.
-
Command-Line Format --report-host=host_nameSystem Variable Name report_hostVariable Scope Global Dynamic Variable No Permitted Values Type stringThe value of the
--report-hostoption. -
Command-Line Format --report-password=nameSystem Variable Name report_passwordVariable Scope Global Dynamic Variable No Permitted Values Type stringThe value of the
--report-passwordoption. Not the same as the password used for the MySQL replication user account. -
Command-Line Format --report-port=#System Variable Name report_portVariable Scope Global Dynamic Variable No Permitted Values (<= 5.5.22) Type integerDefault 3306Min Value 0Max Value 65535Permitted Values (>= 5.5.23) Type integerDefault 0Min Value 0Max Value 65535The value of the
--report-portoption. -
Command-Line Format --report-user=nameSystem Variable Name report_userVariable Scope Global Dynamic Variable No Permitted Values Type stringThe value of the
--report-useroption. Not the same as the name for the MySQL replication user account. -
System Variable Name rpl_semi_sync_master_enabledVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFControls whether semisynchronous replication is enabled on the master. To enable or disable the plugin, set this variable to
ONorOFF(or 1 or 0), respectively. The default isOFF.This variable is available only if the master-side semisynchronous replication plugin is installed.
-
System Variable Name rpl_semi_sync_master_timeoutVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 10000A 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_levelSystem Variable Name rpl_semi_sync_master_trace_levelVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 32The 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_slaveSystem Variable Name rpl_semi_sync_master_wait_no_slaveVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault ONWith 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.
-
System Variable Name rpl_semi_sync_slave_enabledVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFControls whether semisynchronous replication is enabled on the slave. To enable or disable the plugin, set this variable to
ONorOFF(or 1 or 0), respectively. The default isOFF.This variable is available only if the slave-side semisynchronous replication plugin is installed.
rpl_semi_sync_slave_trace_levelSystem Variable Name rpl_semi_sync_slave_trace_levelVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 32The semisynchronous replication debug trace level on the slave. See
rpl_semi_sync_master_trace_levelfor the permissible values.This variable is available only if the slave-side semisynchronous replication plugin is installed.
-
Command-Line Format --secure-authSystem Variable Name secure_authVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFIf 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”.
NotePasswords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided.
-
Command-Line Format --secure-file-priv=dir_nameSystem Variable Name secure_file_privVariable Scope Global Dynamic Variable No Permitted Values Type stringDefault emptyValid Values emptydirnameThis variable is used to limit the effect of data import and export operations, such as those performed by the
LOAD DATAandSELECT ... INTO OUTFILEstatements and theLOAD_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. -
Command-Line Format --server-id=#System Variable Name server_idVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 4294967295The server ID, used in replication to give each master and slave a unique identity. This variable is set by the
--server-idoption. 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. -
Command-Line Format --shared_memory[={0,1}]System Variable Name shared_memoryVariable Scope Global Dynamic Variable No Platform Specific Windows Permitted Values Type booleanDefault FALSE(Windows only.) Whether the server permits shared-memory connections.
-
Command-Line Format --shared_memory_base_name=nameSystem Variable Name shared_memory_base_nameVariable Scope Global Dynamic Variable No Platform Specific Windows Permitted Values Type stringDefault MYSQL(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. -
Command-Line Format --skip-external-lockingSystem Variable Name skip_external_lockingVariable Scope Global Dynamic Variable No Permitted Values Type booleanDefault ONThis is
OFFif mysqld uses external locking (system locking),ONif external locking is disabled. This affects onlyMyISAMtable access.This variable is set by the
--external-lockingor--skip-external-lockingoption. External locking is disabled by default.External locking affects only
MyISAMtable access. For more information, including conditions under which it can and cannot be used, see Section 8.11.5, “External Locking”. -
Command-Line Format --skip-name-resolveSystem Variable Name skip_name_resolveVariable Scope Global Dynamic Variable No Permitted Values Type booleanDefault OFFThis variable is set from the value of the
--skip-name-resolveoption. If it isOFF, mysqld resolves host names when checking client connections. If it isON, mysqld uses only IP numbers; in this case, allHostcolumn values in the grant tables must be IP addresses orlocalhost. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.This variable was added in MySQL 5.5.5.
-
Command-Line Format --skip-networkingSystem Variable Name skip_networkingVariable Scope Global Dynamic Variable No This is
ONif 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 toONwith the--skip-networkingoption. -
Command-Line Format --skip-show-databaseSystem Variable Name skip_show_databaseVariable Scope Global Dynamic Variable No This prevents people from using the
SHOW DATABASESstatement if they do not have theSHOW DATABASESprivilege. This can improve security if you have concerns about users being able to see databases belonging to other users. Its effect depends on theSHOW DATABASESprivilege: If the variable value isON, theSHOW DATABASESstatement is permitted only to users who have theSHOW DATABASESprivilege, and the statement displays all database names. If the value isOFF,SHOW DATABASESis permitted to all users, but displays the names of only those databases for which the user has theSHOW DATABASESor other privilege. (Note that any global privilege is considered a privilege for the database.) -
Command-Line Format --slow_launch_time=#System Variable Name slow_launch_timeVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 2If creating a thread takes longer than this many seconds, the server increments the
Slow_launch_threadsstatus variable. -
Command-Line Format --slow-query-logSystem Variable Name slow_query_logVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFWhether the slow query log is enabled. The value can be 0 (or
OFF) to disable the log or 1 (orON) to enable the log. The default value depends on whether the--slow_query_logoption is given. The destination for log output is controlled by thelog_outputsystem variable; if that value isNONE, no log entries are written even if the log is enabled.“Slow” is determined by the value of the
long_query_timevariable. See Section 5.4.5, “The Slow Query Log”. -
Command-Line Format --slow-query-log-file=file_nameSystem Variable Name slow_query_log_fileVariable Scope Global Dynamic Variable Yes Permitted Values Type file nameDefault host_name-slow.logThe name of the slow query log file. The default value is
host_name-slow.log--slow_query_log_fileoption. -
Command-Line Format --socket={file_name|pipe_name}System Variable Name socketVariable Scope Global Dynamic Variable No Permitted Values Type stringDefault /tmp/mysql.sockOn 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/mysqlfor 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). -
Command-Line Format --sort_buffer_size=#System Variable Name sort_buffer_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (Windows) Type integerDefault 2097144Min Value 32768Max Value 4294967295Permitted Values (Other, 32-bit platforms) Type integerDefault 2097144Min Value 32768Max Value 4294967295Permitted Values (Other, 64-bit platforms) Type integerDefault 2097144Min Value 32768Max Value 18446744073709551615Each session that must perform a sort allocates a buffer of this size.
sort_buffer_sizeis not specific to any storage engine and applies in a general manner for optimization. At minimum thesort_buffer_sizevalue must be large enough to accommodate fifteen tuples in the sort buffer. Also, increasing the value ofmax_sort_lengthmay require increasing the value ofsort_buffer_size. For more information, see Section 8.2.1.11, “ORDER BY Optimization”If you see many
Sort_merge_passesper second inSHOW GLOBAL STATUSoutput, you can consider increasing thesort_buffer_sizevalue to speed upORDER BYorGROUP BYoperations 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_sizeis 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). -
System Variable (<= 5.5.2) Name sql_auto_is_nullVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name sql_auto_is_nullVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.2) Type booleanDefault 1Permitted Values (>= 5.5.3) Type booleanDefault 0If this variable is set to 1, then after a statement that successfully inserts an automatically generated
AUTO_INCREMENTvalue, you can find that value by issuing a statement of the following form:SELECT * FROM
tbl_nameWHEREauto_colIS NULLIf 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 noAUTO_INCREMENTvalue was successfully inserted, theSELECTstatement returns no row.The behavior of retrieving an
AUTO_INCREMENTvalue by using anIS NULLcomparison is used by some ODBC programs, such as Access. See Obtaining Auto-Increment Values. This behavior can be disabled by settingsql_auto_is_nullto 0.The default value of
sql_auto_is_nullis 0 as of MySQL 5.5.3, and 1 for earlier versions. -
System Variable (<= 5.5.2) Name sql_big_selectsVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name sql_big_selectsVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 1If set to 0, MySQL aborts
SELECTstatements 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 ofmax_join_size). This is useful when an inadvisableWHEREstatement has been issued. The default value for a new connection is 1, which permits allSELECTstatements.If you set the
max_join_sizesystem variable to a value other thanDEFAULT,sql_big_selectsis set to 0. -
System Variable (<= 5.5.2) Name sql_buffer_resultVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name sql_buffer_resultVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 0If set to 1,
sql_buffer_resultforces results fromSELECTstatements 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. -
System Variable (<= 5.5.2) Name sql_log_binVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3, <= 5.5.40) Name sql_log_binVariable Scope Global, Session Dynamic Variable Yes System Variable (>= 5.5.41) Name sql_log_binVariable Scope Session Dynamic Variable Yes Permitted Values Type booleanThis 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
SUPERprivilege to set this variable.Beginning with MySQL 5.5.5, it is no longer possible to set
@@session.sql_log_binwithin a transaction or subquery. (Bug #53437) -
System Variable (<= 5.5.2) Name sql_log_offVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name sql_log_offVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 0This 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
SUPERprivilege to set this option. The default value is 0. -
Deprecated 5.0.0, by sql_log_bin Removed 5.5.3 System Variable Name sql_log_updateVariable Scope Session Dynamic Variable Yes Permitted Values Type booleanThis variable is deprecated, and is mapped to
sql_log_bin. It was removed in MySQL 5.5.3. -
Command-Line Format --sql-mode=nameSystem Variable Name sql_modeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type setDefault ''Valid Values ALLOW_INVALID_DATESANSI_QUOTESERROR_FOR_DIVISION_BY_ZEROHIGH_NOT_PRECEDENCEIGNORE_SPACENO_AUTO_CREATE_USERNO_AUTO_VALUE_ON_ZERONO_BACKSLASH_ESCAPESNO_DIR_IN_CREATENO_ENGINE_SUBSTITUTIONNO_FIELD_OPTIONSNO_KEY_OPTIONSNO_TABLE_OPTIONSNO_UNSIGNED_SUBTRACTIONNO_ZERO_DATENO_ZERO_IN_DATEONLY_FULL_GROUP_BYPAD_CHAR_TO_FULL_LENGTHPIPES_AS_CONCATREAL_AS_FLOATSTRICT_ALL_TABLESSTRICT_TRANS_TABLESThe current server SQL mode, which can be set dynamically. For details, see Section 5.1.7, “Server SQL Modes”.
NoteMySQL 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.
If set to 1 (the default), warnings of
Notelevel incrementwarning_countand the server records them. If set to 0,Notewarnings do not incrementwarning_countand 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.If set to 1 (the default), the server quotes identifiers for
SHOW CREATE TABLEandSHOW CREATE DATABASEstatements. 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”.If set to 1, MySQL aborts
UPDATEorDELETEstatements that do not use a key in theWHEREclause or aLIMITclause. (Specifically,UPDATEstatements must have aWHEREclause that uses a key or aLIMITclause, or both.DELETEstatements must have both.) This makes it possible to catchUPDATEorDELETEstatements where keys are not used properly and that would probably change or delete a large number of rows. The default value is 0.-
System Variable Name sql_select_limitVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerThe maximum number of rows to return from
SELECTstatements. 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 ofDEFAULT.If a
SELECThas aLIMITclause, theLIMITtakes precedence over the value ofsql_select_limit. This variable controls whether single-row
INSERTstatements produce an information string if warnings occur. The default is 0. Set the value to 1 to produce an information string.-
Command-Line Format --ssl-ca=file_nameSystem Variable Name ssl_caVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe path to a file with a list of trusted SSL CAs.
-
Command-Line Format --ssl-capath=dir_nameSystem Variable Name ssl_capathVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe path to a directory that contains trusted SSL CA certificates in PEM format.
-
Command-Line Format --ssl-cert=file_nameSystem Variable Name ssl_certVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe name of the SSL certificate file to use for establishing a secure connection.
-
Command-Line Format --ssl-cipher=nameSystem Variable Name ssl_cipherVariable Scope Global Dynamic Variable No Permitted Values Type stringA list of permissible ciphers to use for SSL encryption.
-
Command-Line Format --ssl-key=file_nameSystem Variable Name ssl_keyVariable Scope Global Dynamic Variable No Permitted Values Type file nameThe name of the SSL key file to use for establishing a secure connection.
-
System Variable Name storage_engineVariable Scope Global, Session Dynamic Variable Yes Permitted Values (<= 5.5.4) Type enumerationDefault MyISAMPermitted Values (>= 5.5.5) Type enumerationDefault InnoDBThe default storage engine (table type). To set the storage engine at server startup, use the
--default-storage-engineoption. See Section 5.1.3, “Server Command Options”.This variable is deprecated as of MySQL 5.5.3. Use
default_storage_engineinstead. -
Introduced 5.5.21 Command-Line Format --stored-program-cache=#System Variable Name stored_program_cacheVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 256Min Value 256Max Value 524288Sets 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.
-
Command-Line Format --sync-frmSystem Variable Name sync_frmVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault TRUEIf this variable is set to 1, when any nontemporary table is created its
.frmfile is synchronized to disk (usingfdatasync()). This is slower but safer in case of a crash. The default is 1. -
System Variable Name system_time_zoneVariable Scope Global Dynamic Variable No Permitted Values Type stringThe 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 theTZenvironment variable. It also can be specified using the--timezoneoption of the mysqld_safe script.The
system_time_zonevariable differs fromtime_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”. -
System Variable Name table_definition_cacheVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 400Min Value 400Max Value 524288The number of table definitions (from
.frmfiles) 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. -
Removed 5.5.3 Command-Line Format --table_lock_wait_timeout=#System Variable Name table_lock_wait_timeoutVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 50Min Value 1Max Value 1073741824This variable is unused. It was removed in 5.5.3.
-
System Variable Name table_open_cacheVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 400Min Value 1Max Value 524288The 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_tablesstatus variable. See Section 5.1.6, “Server Status Variables”. If the value ofOpened_tablesis large and you do not useFLUSH TABLESoften (which just forces all tables to be closed and reopened), then you should increase the value of thetable_open_cachevariable. For more information about the table cache, see Section 8.4.3.1, “How MySQL Opens and Closes Tables”. This variable was removed in MySQL 5.5.3. Use
storage_engineinstead.-
Command-Line Format --thread_cache_size=#System Variable Name thread_cache_sizeVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 16384How 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_sizethreads 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 setthread_cache_sizehigh enough so that most new connections use cached threads. By examining the difference between theConnectionsandThreads_createdstatus variables, you can see how efficient the thread cache is. For details, see Section 5.1.6, “Server Status Variables”. -
Command-Line Format --thread_concurrency=#System Variable Name thread_concurrencyVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 10Min Value 1Max Value 512This 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. -
Command-Line Format --thread_handling=nameSystem Variable Name thread_handlingVariable Scope Global Dynamic Variable No Permitted Values (<= 5.5.13) Type enumerationDefault one-thread-per-connectionValid Values no-threadsone-thread-per-connectionPermitted Values (>= 5.5.16) Type enumerationDefault one-thread-per-connectionValid Values no-threadsone-thread-per-connectiondynamically-loadedThe 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) andone-thread-per-connection(the server uses one thread to handle each client connection; this is the default).no-threadsis 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_handlingvalue todynamically-loaded. See Section 5.5.4.2, “Thread Pool Installation”. -
Introduced 5.5.16 Command-Line Format --thread_pool_algorithm=#System Variable Name thread_pool_algorithmVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 0Min Value 0Max Value 1This 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_connectionIntroduced 5.5.16 Command-Line Format --thread_pool_high_priority_connection=#System Variable Name thread_pool_high_priority_connectionVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 1This 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_threadsIntroduced 5.5.16 Command-Line Format --thread_pool_max_unused_threads=#System Variable Name thread_pool_max_unused_threadsVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 0Min Value 0Max Value 4096The 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
NwhereNis greater than 0 means 1 consumer thread andN−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”
-
Introduced 5.5.16 Command-Line Format --thread_pool_prio_kickup_timer=#System Variable Name thread_pool_prio_kickup_timerVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 1000Min Value 0Max Value 4294967294This 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”
-
Introduced 5.5.16 Command-Line Format --thread_pool_size=#System Variable Name thread_pool_sizeVariable Scope Global Dynamic Variable No Permitted Values Type integerDefault 16Min Value 1Max Value 64The 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”
-
Introduced 5.5.16 Command-Line Format --thread_pool_stall_limit=#System Variable Name thread_pool_stall_limitVariable Scope Global Dynamic Variable Yes Permitted Values Type integerDefault 6Min Value 4Max Value 600This 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”
-
Command-Line Format --thread_stack=#System Variable Name thread_stackVariable Scope Global Dynamic Variable No Permitted Values (32-bit platforms) Type integerDefault 196608Min Value 131072Max Value 4294967295Block Size 1024Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 262144Min Value 131072Max Value 18446744073709547520Block Size 1024Permitted Values (64-bit platforms, >= 5.5.3) Type integerDefault 262144Min Value 131072Max Value 18446744073709551615Block Size 1024The stack size for each thread. Many of the limits detected by the
crash-metest 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. This variable is unused.
-
System Variable Name time_zoneVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type stringThe 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 ofsystem_time_zone”). The value can be specified explicitly at server startup with the--default-time-zoneoption. See Section 10.6, “MySQL Server Time Zone Support”. -
Deprecated 5.5.39 Command-Line Format --timed_mutexesSystem Variable Name timed_mutexesVariable Scope Global Dynamic Variable Yes Permitted Values Type booleanDefault OFFThis variable is deprecated; it has no use. It will be removed in a future MySQL release.
-
System Variable Name timestampVariable Scope Session Dynamic Variable Yes Permitted Values Type numericSet the time for this client. This is used to get the original timestamp if you use the binary log to restore rows.
timestamp_valueshould be a Unix epoch timestamp (a value like that returned byUNIX_TIMESTAMP(), not a value in'YYYY-MM-DD hh:mm:ss'format) orDEFAULT.Setting
timestampto a constant value causes it to retain that value until it is changed again. SettingtimestamptoDEFAULTcauses its value to be the current date and time as of the time it is accessed.SET timestampaffects the value returned byNOW()but not bySYSDATE(). This means that timestamp settings in the binary log have no effect on invocations ofSYSDATE(). The server can be started with the--sysdate-is-nowoption to causeSYSDATE()to be an alias forNOW(), in which caseSET timestampaffects both functions. -
Command-Line Format --tmp_table_size=#System Variable Name tmp_table_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type integerDefault 16777216Min Value 1024Max Value 18446744073709551615The maximum size of internal in-memory temporary tables. This variable does not apply to user-created
MEMORYtables.The actual limit is determined from whichever of the values of
tmp_table_sizeandmax_heap_table_sizeis smaller. If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-diskMyISAMtable. Increase the value oftmp_table_size(andmax_heap_table_sizeif necessary) if you do many advancedGROUP BYqueries 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_tablesandCreated_tmp_tablesvariables.See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.
-
Command-Line Format --tmpdir=dir_nameSystem Variable Name tmpdirVariable Scope Global Dynamic Variable No Permitted Values Type directory nameThe 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
tmpdirto 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 orLOAD DATA INFILEoperations. If files in the temporary file directory are lost when the server restarts, replication fails. You can set the slave's temporary directory using theslave_load_tmpdirvariable. In that case, the slave will not use the generaltmpdirvalue and you can settmpdirto a nonpermanent location. -
Command-Line Format --transaction_alloc_block_size=#System Variable Name transaction_alloc_block_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (>= 5.5.43) Type integerDefault 8192Min Value 1024Max Value 131072Block Size 1024Permitted Values (32-bit platforms, <= 5.5.42) Type integerDefault 8192Min Value 1024Max Value 4294967295Block Size 1024Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 8192Min Value 1024Max Value 18446744073709547520Block Size 1024Permitted Values (64-bit platforms, >= 5.5.3, <= 5.5.42) Type integerDefault 8192Min Value 1024Max Value 18446744073709551615Block Size 1024The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of
transaction_prealloc_size. -
Command-Line Format --transaction_prealloc_size=#System Variable Name transaction_prealloc_sizeVariable Scope Global, Session Dynamic Variable Yes Permitted Values (>= 5.5.43) Type integerDefault 4096Min Value 1024Max Value 131072Block Size 1024Permitted Values (32-bit platforms, <= 5.5.42) Type integerDefault 4096Min Value 1024Max Value 4294967295Block Size 1024Permitted Values (64-bit platforms, <= 5.5.2) Type integerDefault 4096Min Value 1024Max Value 18446744073709547520Block Size 1024Permitted Values (64-bit platforms, >= 5.5.3, <= 5.5.42) Type integerDefault 4096Min Value 1024Max Value 18446744073709551615Block Size 1024There 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 bytransaction_alloc_block_sizebytes. When the transaction ends, the pool is truncated totransaction_prealloc_sizebytes.By making
transaction_prealloc_sizesufficiently large to contain all statements within a single transaction, you can avoid manymalloc()calls. -
System Variable Name tx_isolationVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type enumerationDefault REPEATABLE-READValid Values READ-UNCOMMITTEDREAD-COMMITTEDREPEATABLE-READSERIALIZABLEThe default transaction isolation level. Defaults to
REPEATABLE-READ.This variable can be set directly, or indirectly using the
SET TRANSACTIONstatement. See Section 13.3.6, “SET TRANSACTION Syntax”. If you settx_isolationdirectly 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-isolationserver option. -
System Variable (<= 5.5.2) Name unique_checksVariable Scope Session Dynamic Variable Yes System Variable (>= 5.5.3) Name unique_checksVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 1If set to 1 (the default), uniqueness checks for secondary indexes in
InnoDBtables 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 toInnoDB.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.
-
Command-Line Format --updatable_views_with_limit=#System Variable Name updatable_views_with_limitVariable Scope Global, Session Dynamic Variable Yes Permitted Values Type booleanDefault 1This 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
LIMITclause. (Such updates often are generated by GUI tools.) An update is anUPDATEorDELETEstatement. Primary key here means aPRIMARY KEY, or aUNIQUEindex in which no column can containNULL.The variable can have two values:
1orYES: Issue a warning only (not an error message). This is the default value.0orNO: Prohibit the update.
The version number for the server. The value might also include a suffix indicating server build or configuration information.
-logindicates that one or more of the general log, slow query log, or binary log are enabled.-debugindicates that the server was built with debugging support enabled.-
System Variable Name version_commentVariable Scope Global Dynamic Variable No Permitted Values Type stringThe CMake configuration program has a
COMPILATION_COMMENToption 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”. -
System Variable Name version_compile_machineVariable Scope Global Dynamic Variable No Permitted Values Type stringThe type of machine or architecture on which MySQL was built.
-
System Variable Name version_compile_osVariable Scope Global Dynamic Variable No Permitted Values Type stringThe type of operating system on which MySQL was built.
-
Command-Line Format --wait_timeout=#System Variable Name wait_timeoutVariable Scope Global, Session Dynamic Variable Yes Permitted Values (Windows) Type integerDefault 28800Min Value 1Max Value 2147483Permitted Values (Other) Type integerDefault 28800Min Value 1Max Value 31536000The number of seconds the server waits for activity on a noninteractive connection before closing it.
On thread startup, the session
wait_timeoutvalue is initialized from the globalwait_timeoutvalue or from the globalinteractive_timeoutvalue, depending on the type of client (as defined by theCLIENT_INTERACTIVEconnect option tomysql_real_connect()). See alsointeractive_timeout. 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”.
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_modevalue, which is initialized when the client connects to the value of the globalsql_modevalue.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-
at server startup. For example, to prevent the value of
var_name=valuequery_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
GLOBALkeyword or the@@global.qualifier:SET GLOBAL max_connections = 1000; SET @@global.max_connections = 1000;
The
SUPERprivilege is required to set global variables.To indicate that a variable is a session variable, precede its name by the
SESSIONkeyword 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
SUPERprivilege (such assql_log_bin). A client can change its own session variables, but not those of any other client.LOCALand@@local.are synonyms forSESSIONand@@session..If no modifier is present,
SETchanges 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 GLOBALAn error occurs if you:
Use
SET GLOBAL(or@@global.) when setting a variable that has only a session valueOmit
GLOBAL(or@@global.) orPERSIST(or@@persist.) when setting a variable that has only a global valueUse
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
@@ (that is,
when you do not specify var_name@@global. or
@@session.), MySQL returns the session value if
it exists and the global value otherwise. This differs from
SET @@, which always refers to
the session value.
var_name =
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=16Mshell>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;
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.
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, sodefaultis 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-cacheis not legal, but`hot-cache`is.global,session, andlocalare not legal instance names. This avoids a conflict with notation such as@@global.for referring to nonstructured system variables.var_name
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.
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
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
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”.
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”.
The number of failed attempts to connect to the MySQL server. See Section B.5.2.11, “Communication Errors and Aborted Connections”.
The number of transactions that used the binary log cache but that exceeded the value of
binlog_cache_sizeand 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_usestatus variable.The number of transactions that used the binary log cache.
The number of nontransaction statements that used the binary log statement cache but that exceeded the value of
binlog_stmt_cache_sizeand used a temporary file to store those statements.The number of nontransactional statements that used the binary log statement cache.
The number of bytes received from all clients.
The number of bytes sent to all clients.
The
Com_statement counter variables indicate the number of times eachxxxxxxstatement has been executed. There is one status variable for each type of statement. For example,Com_deleteandCom_updatecountDELETEandUPDATEstatements, respectively.Com_delete_multiandCom_update_multiare similar but apply toDELETEandUPDATEstatements that use multiple-table syntax.If a query result is returned from query cache, the server increments the
Qcache_hitsstatus variable, notCom_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_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.xxxThe
Com_stmt_status variables are as follows:xxxCom_stmt_prepareCom_stmt_executeCom_stmt_fetchCom_stmt_send_long_dataCom_stmt_resetCom_stmt_close
Those variables stand for prepared statement commands. Their names refer to the
COM_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,xxxCom_stmt_prepare,Com_stmt_executeandCom_stmt_closealso increase forPREPARE,EXECUTE, orDEALLOCATE PREPARE, respectively. Additionally, the values of the older statement counter variablesCom_prepare_sql,Com_execute_sql, andCom_dealloc_sqlincrease for thePREPARE,EXECUTE, andDEALLOCATE PREPAREstatements.Com_stmt_fetchstands for the total number of network round-trips issued when fetching from cursors.Com_stmt_reprepareindicates 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 incrementsCom_stmt_reprepare, and alsoCom_stmt_prepare.Whether the client connection uses compression in the client/server protocol.
The number of connection attempts (successful or not) to the MySQL server.
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_sizeandmax_heap_table_sizevalues. IfCreated_tmp_disk_tablesis large, you may want to increase thetmp_table_sizeormax_heap_table_sizevalue 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_tablesandCreated_tmp_tablesvariables.See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.
How many temporary files mysqld has created.
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_tablesandCreated_tmp_tablesvariables.See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.
Each invocation of the
SHOW STATUSstatement uses an internal temporary table and increments the globalCreated_tmp_tablesvalue.The number of rows written with
INSERT DELAYEDfor which some error occurred (probablyduplicate key).The number of
INSERT DELAYEDhandler threads in use.The number of
INSERT DELAYEDrows written.The number of times the server flushes tables, whether because a user executed a
FLUSH TABLESstatement or due to internal server operation. It is also incremented by receipt of aCOM_REFRESHpacket. This is in contrast toCom_flush, which indicates how manyFLUSHstatements have been executed, whetherFLUSH TABLES,FLUSH LOGS, and so forth.The number of internal
COMMITstatements.The number of times that rows have been deleted from tables.
A counter for the prepare phase of two-phase commit operations.
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 thatcol1is indexed.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.
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 withORDER 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.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.
The number of requests to read the previous row in key order. This read method is mainly used to optimize
ORDER BY ... DESC.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.
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.
The number of requests for a storage engine to perform a rollback operation.
The number of requests for a storage engine to place a savepoint.
The number of requests for a storage engine to roll back to a savepoint.
The number of requests to update a row in a table.
The number of requests to insert a row in a table.
The total number of bytes in the
InnoDBbuffer pool containing data. The number includes both dirty and clean pages. For more accurate memory usage calculations than withInnodb_buffer_pool_pages_data, when compressed tables cause the buffer pool to hold pages of different sizes.The number of pages in the
InnoDBbuffer pool containing data. The number includes both dirty and clean pages. When using compressed tables, the reportedInnodb_buffer_pool_pages_datavalue may be larger thanInnodb_buffer_pool_pages_total(Bug #59550).Innodb_buffer_pool_bytes_dirtyThe total current number of bytes held in dirty pages in the
InnoDBbuffer pool. For more accurate memory usage calculations than withInnodb_buffer_pool_pages_dirty, when compressed tables cause the buffer pool to hold pages of different sizes.Innodb_buffer_pool_pages_dirtyThe number of pages currently dirty.
Innodb_buffer_pool_pages_flushedThe number of buffer pool page-flush requests.
The number of free pages.
Innodb_buffer_pool_pages_latchedThe number of latched pages in
InnoDBbuffer 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 theUNIV_DEBUGsystem is defined at server build time.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_total−Innodb_buffer_pool_pages_free−Innodb_buffer_pool_pages_data. When using compressed tables,Innodb_buffer_pool_pages_miscmay report an out-of-bounds value (Bug #59550).Innodb_buffer_pool_pages_totalThe total size of the
InnoDBbuffer pool, in pages. When using compressed tables, the reportedInnodb_buffer_pool_pages_datavalue may be larger thanInnodb_buffer_pool_pages_total(Bug #59550)The number of pages read into the
InnoDBbuffer pool by the read-ahead background thread.Innodb_buffer_pool_read_ahead_evictedThe number of pages read into the
InnoDBbuffer pool by the read-ahead background thread that were subsequently evicted without having been accessed by queries.Innodb_buffer_pool_read_ahead_rndThe 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_requestsThe number of logical read requests.
The number of logical reads that
InnoDBcould not satisfy from the buffer pool, and had to read directly from the disk.Normally, writes to the
InnoDBbuffer 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_requestsThe number writes done to the
InnoDBbuffer pool.The number of
fsync()operations so far.The current number of pending
fsync()operations.The current number of pending reads.
The current number of pending writes.
The amount of data read since the server was started (in bytes).
The total number of data reads (OS file reads).
The total number of data writes.
The amount of data written so far, in bytes.
The number of pages that have been written for doublewrite operations. See Section 14.15.1, “InnoDB Disk I/O”.
The number of doublewrite operations that have been performed. See Section 14.15.1, “InnoDB Disk I/O”.
Indicates whether the server was built with atomic instructions.
The number of times that the log buffer was too small and a wait was required for it to be flushed before continuing.
The number of log write requests.
The number of physical writes to the log file.
The number of
fsync()writes done to the log file.The number of pending log file
fsync()operations.The number of pending log file writes.
The number of bytes written to the log file.
The compiled-in
InnoDBpage size (default 16KB). Many values are counted in pages; the page size enables them to be easily converted to bytes.The number of pages created.
The number of pages read from the
InnoDBbuffer pool by operations onInnoDBtables.The number of pages written.
The number of row locks currently being waited for.
The total time spent in acquiring row locks, in milliseconds.
The average time to acquire a row lock, in milliseconds.
The maximum time to acquire a row lock, in milliseconds.
The number of times a row lock had to be waited for.
The number of rows deleted from
InnoDBtables.The number of rows inserted into
InnoDBtables.The number of rows read from
InnoDBtables.The number of rows updated in
InnoDBtables.Innodb_truncated_status_writesThe number of times output from the
SHOW ENGINE INNODB STATUSis truncated. Monitoring applications that parse the output from this command can test this value before and after issuing theSHOW ENGINEcommand, to confirm if the output is complete or not.The number of key blocks in the key cache that have changed but have not yet been flushed to disk.
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_sizein Section 5.1.4, “Server System Variables”.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.
The number of requests to read a key block from the cache.
The number of physical reads of a key block from disk. If
Key_readsis large, then yourkey_buffer_sizevalue is probably too small. The cache miss rate can be calculated asKey_reads/Key_read_requests.The number of requests to write a key block to the cache.
The number of physical writes of a key block to disk.
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_costhas session scope.The
Last_query_costvalue can be computed accurately only for simple “flat” queries, not complex queries such as those with subqueries orUNION. For the latter, the value is set to 0.The maximum number of connections that have been in use simultaneously since the server started.
The number of rows waiting to be written in
INSERT DELAYEDqueues.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.
The number of streams that are open (used mainly for logging).
The number of cached
.frmfiles.The number of tables that are open.
The number of files that have been opened with
my_open()(amysyslibrary function). Parts of the server that open files without using this function do not increment the count.The number of
.frmfiles that have been cached.The number of tables that have been opened. If
Opened_tablesis big, yourtable_open_cachevalue is probably too small.Performance_schema_xxxPerformance 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.
The current number of prepared statements. (The maximum number of statements is given by the
max_prepared_stmt_countsystem variable.)The number of free memory blocks in the query cache.
The amount of free memory for the query cache.
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.
The number of queries added to the query cache.
The number of queries that were deleted from the query cache because of low memory.
The number of noncached queries (not cacheable, or not cached due to the
query_cache_typesetting).The number of queries registered in the query cache.
The total number of blocks in the query cache.
The number of statements executed by the server. This variable includes statements executed within stored programs, unlike the
Questionsvariable. It does not countCOM_PINGorCOM_STATISTICScommands.The discussion at the beginning of this section indicates how to relate this statement-counting status variable to other such variables.
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
Queriesvariable. This variable does not countCOM_PING,COM_STATISTICS,COM_STMT_PREPARE,COM_STMT_CLOSE, orCOM_STMT_RESETcommands.The discussion at the beginning of this section indicates how to relate this statement-counting status variable to other such variables.
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_timeThe 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_timeThe 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_waitsThe total number of times the master waited for slave replies.
This variable is available only if the master-side semisynchronous replication plugin is installed.
The number of times the master turned off semisynchronous replication.
This variable is available only if the master-side semisynchronous replication plugin is installed.
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.
Whether semisynchronous replication currently is operational on the master. The value is
ONif the plugin has been enabled and a commit acknowledgment has occurred. It isOFFif 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_failuresThe 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_timeThe 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_timeThe total time in microseconds the master waited for transactions.
This variable is available only if the master-side semisynchronous replication plugin is installed.
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_backtraverseThe 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_sessionsThe number of sessions currently waiting for slave replies.
This variable is available only if the master-side semisynchronous replication plugin is installed.
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.
Whether semisynchronous replication currently is operational on the slave. This is
ONif the plugin has been enabled and the slave I/O thread is running,OFFotherwise.This variable is available only if the slave-side semisynchronous replication plugin is installed.
The status of fail-safe replication (not implemented). This variable is unused and is removed in MySQL 5.6.
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.
The number of joins that used a range search on a reference table.
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.
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.
The number of joins that did a full scan of the first table.
Shows the replication heartbeat interval (in seconds) on a replication slave.
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”.
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 TOstatement was issued.The total number of times since startup that the replication slave SQL thread has retried transactions.
This is
ONif 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 isOFF.The number of threads that have taken more than
slow_launch_timeseconds to create.The number of queries that have taken more than
long_query_timeseconds. 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”.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_sizesystem variable.The number of sorts that were done using ranges.
The number of sorted rows.
The number of sorts that were done by scanning the table.
The number of negotiates needed to establish the connection.
The number of accepted SSL connections.
The number of callback cache hits.
The current encryption cipher (empty for unencrypted connections).
The list of possible SSL ciphers (empty for non-SSL connections).
The number of SSL connection attempts to an SSL-enabled master.
The number of negotiates needed to establish the connection to an SSL-enabled master.
The SSL context verification depth (how many certificates in the chain are tested).
The SSL context verification mode.
The default SSL timeout.
The number of successful SSL connections to the server.
The number of successful slave connections to an SSL-enabled master.
The number of SSL session cache hits.
The number of SSL session cache misses.
The SSL session cache mode.
The number of SSL session cache overflows.
The SSL session cache size.
The number of SSL session cache timeouts.
How many SSL connections were reused from the cache.
Ssl_used_session_cache_entriesHow many SSL session cache entries were used.
The verification depth for replication SSL connections.
The verification mode for replication SSL connections.
The SSL protocol version of the connection; for example, TLSv1. If the connection is not encrypted, the value is empty.
The number of times that a request for a table lock could be granted immediately.
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.
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_usedandTc_log_page_sizeis 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-sizeoption. 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. (InnoDBis the only applicable engine.)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 forTc_log_max_pages_used.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-sizeoption). 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.)The number of threads in the thread cache.
The number of currently open connections.
The number of threads created to handle connections. If
Threads_createdis big, you may want to increase thethread_cache_sizevalue. The cache miss rate can be calculated asThreads_created/Connections.The number of threads that are not sleeping.
The number of seconds that the server has been up.
The number of seconds since the most recent
FLUSH STATUSstatement.
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.
The default SQL mode is empty (no modes set).
To set the SQL mode at server startup, use the
--sql-mode="
option on the command line, or
modes"sql-mode="
in an option file such as modes"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.
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;
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_mode
values are probably these:
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.
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.
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.
When this manual refers to “strict mode,” it means
a mode with either or both
STRICT_TRANS_TABLES or
STRICT_ALL_TABLES enabled.
The following list describes all supported SQL modes:
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
DATEandDATETIMEcolumns. It does not applyTIMESTAMPcolumns, 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, enableALLOW_INVALID_DATES.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. WithANSI_QUOTESenabled, you cannot use double quotation marks to quote literal strings, because it is interpreted as an identifier.The
ERROR_FOR_DIVISION_BY_ZEROmode affects handling of division by zero, which includesMOD(. For data-change operations (N,0)INSERT,UPDATE), its effect also depends on whether strict SQL mode is enabled.If this mode is not enabled, division by zero inserts
NULLand produces no warning.If this mode is enabled, division by zero inserts
NULLand produces a warning.If this mode and strict mode are enabled, division by zero produces an error, unless
IGNOREis given as well. ForINSERT IGNOREandUPDATE IGNORE, division by zero insertsNULLand produces a warning.
For
SELECT, division by zero returnsNULL. EnablingERROR_FOR_DIVISION_BY_ZEROcauses a warning to be produced as well, regardless of whether strict mode is enabled.The precedence of the
NOToperator is such that expressions such asNOT a BETWEEN b AND care parsed asNOT (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 theHIGH_NOT_PRECEDENCESQL 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;-> 1Permit 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 aCOUNT()function, the use ofcountas 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 syntaxThe table name should be quoted:
mysql>
CREATE TABLE `count` (i INT);Query OK, 0 rows affected (0.00 sec)The
IGNORE_SPACESQL 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 whetherIGNORE_SPACEis enabled.For further discussion of
IGNORE_SPACE, see Section 9.2.4, “Function Name Parsing and Resolution”.Prevent the
GRANTstatement from automatically creating new users if it would otherwise do so, unless authentication information is specified. The statement must specify a nonempty password usingIDENTIFIED BYor an authentication plugin usingIDENTIFIED WITH.NO_AUTO_VALUE_ON_ZEROaffects handling ofAUTO_INCREMENTcolumns. Normally, you generate the next sequence number for the column by inserting eitherNULLor0into it.NO_AUTO_VALUE_ON_ZEROsuppresses this behavior for0so that onlyNULLgenerates the next sequence number.This mode can be useful if
0has been stored in a table'sAUTO_INCREMENTcolumn. (Storing0is 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 the0values, resulting in a table with contents different from the one that was dumped. EnablingNO_AUTO_VALUE_ON_ZERObefore reloading the dump file solves this problem. mysqldump now automatically includes in its output a statement that enablesNO_AUTO_VALUE_ON_ZERO, to avoid this problem.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.When creating a table, ignore all
INDEX DIRECTORYandDATA DIRECTORYdirectives. This option is useful on slave replication servers.Control automatic substitution of the default storage engine when a statement such as
CREATE TABLEorALTER TABLEspecifies 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_SUBSTITUTIONdisabled, forCREATE TABLEthe default engine is used and a warning occurs if the desired engine is unavailable. ForALTER TABLE, a warning occurs and the table is not altered.With
NO_ENGINE_SUBSTITUTIONenabled, an error occurs and the table is not created or altered if the desired engine is unavailable.Do not print MySQL-specific column options in the output of
SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.Do not print MySQL-specific index options in the output of
SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.Do not print MySQL-specific table options (such as
ENGINE) in the output ofSHOW CREATE TABLE. This mode is used by mysqldump in portability mode.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_SUBTRACTIONSQL 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
UNSIGNEDinteger column, the result is clipped to the maximum value for the column type, or clipped to 0 ifNO_UNSIGNED_SUBTRACTIONis enabled. If strict SQL mode is enabled, an error occurs and the column remains unchanged.When
NO_UNSIGNED_SUBTRACTIONis enabled, the subtraction result is signed, even if any operand is unsigned. For example, compare the type of columnc2in tablet1with that of columnc2in tablet2: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 UNSIGNEDis not 100% usable in all contexts. See Section 12.10, “Cast Functions and Operators”.The
NO_ZERO_DATEmode 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, unlessIGNOREis given as well. ForINSERT IGNOREandUPDATE IGNORE,'0000-00-00'is permitted and inserts produce a warning.
The
NO_ZERO_IN_DATEmode 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 theNO_ZERO_DATEmode.) The effect ofNO_ZERO_IN_DATEalso 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
IGNOREis given as well. ForINSERT IGNOREandUPDATE IGNORE, dates with zero parts are inserted as'0000-00-00'and produce a warning.
Reject queries for which the select list or
HAVINGcondition refer to nonaggregated columns that are not named in theGROUP BYclause.A MySQL extension to standard SQL permits references in the
HAVINGclause to aliased expressions in the select list. EnablingONLY_FULL_GROUP_BYdisables this extension, thus requiring theHAVINGclause to be written using unaliased expressions.For additional discussion and examples, see Section 12.17.3, “MySQL Handling of GROUP BY”.
By default, trailing spaces are trimmed from
CHARcolumn values on retrieval. IfPAD_CHAR_TO_FULL_LENGTHis enabled, trimming does not occur and retrievedCHARvalues are padded to their full length. This mode does not apply toVARCHARcolumns, 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)Treat
||as a string concatenation operator (same asCONCAT()) rather than as a synonym forOR.Treat
REALas a synonym forFLOAT. By default, MySQL treatsREALas a synonym forDOUBLE.Enable strict SQL mode for all storage engines. Invalid data values are rejected. For details, see Strict SQL Mode.
Enable strict SQL mode for transactional storage engines, and when possible for nontransactional storage engines. For details, see Strict SQL Mode.
The following special modes are provided as shorthand for combinations of mode values from the preceding list.
Equivalent to
REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE.ANSImode also causes the server to return an error for queries where a set functionSwith an outer referenceS(outer_ref)SELECT * FROM t1 WHERE t1.a IN (SELECT MAX(t1.b) FROM t2 WHERE ...);
Here,
MAX(t1.b)cannot aggregated in the outer query because it appears in theWHEREclause of that query. Standard SQL requires an error in this situation. IfANSImode is not enabled, the server treatsS(outer_ref)S(const)Equivalent to
PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS.Equivalent to
PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS,NO_AUTO_CREATE_USER.Equivalent to
PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS.Equivalent to
MYSQL323,HIGH_NOT_PRECEDENCE. This meansHIGH_NOT_PRECEDENCEplus someSHOW CREATE TABLEbehaviors specific toMYSQL323:TIMESTAMPcolumn display does not includeDEFAULTorON UPDATEattributes that were introduced in MySQL 4.1.String column display does not include character set and collation attributes that were introduced in MySQL 4.1. For
CHARandVARCHARcolumns, if the collation is binary,BINARYis appended to the column type.The
ENGINE=table option displays asengine_nameTYPE=.engine+nameFor
MEMORYtables, the storage engine is displayed asHEAP.
Equivalent to
MYSQL40,HIGH_NOT_PRECEDENCE. This meansHIGH_NOT_PRECEDENCEplus some behaviors specific toMYSQL40. These are the same as forMYSQL323, except thatSHOW CREATE TABLEdoes not displayHEAPas the storage engine forMEMORYtables.Equivalent to
PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS,NO_AUTO_CREATE_USER.Equivalent to
PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS.Equivalent to
STRICT_TRANS_TABLES,STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER, andNO_ENGINE_SUBSTITUTION.
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_TABLESorSTRICT_TRANS_TABLESis 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.
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 ::1To use this capability, two things must be true:
Your system must be configured to support IPv6. See Section 5.1.8.1, “Verifying System Support for IPv6”.
The default MySQL server configuration permits only IPv4 connections, so the server must be configured for IPv6 connections. To permit IPv6 connections in addition to or instead of IPv4 connections, start the server with an appropriate
--bind-addressoption. See Section 5.1.4, “Server System Variables”.
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, andREVOKE. 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.
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:
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=
option at server startup, where addraddr
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=127.0.0.1or--host=::ffff:127.0.0.1.If the address is a “regular” IPv4 or IPv6 address (such as
127.0.0.1or::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.
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.
Start the MySQL server with an appropriate
--bind-addressoption 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”.As an administrator, connect to the server and create an account for a local user who will connect from the
::1local 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 USERstatement, you can issueGRANTstatements that give specific privileges to the account, although that is not necessary for the remaining steps in this procedure.Invoke the mysql client to connect to the server using the new account:
shell>
mysql -h ::1 -u ipv6user -pipv6passTry some simple statements that show connection information:
mysql>
STATUS... Connection: ::1 via TCP/IP ... mysql>SELECT CURRENT_USER();+----------------+ | CURRENT_USER() | +----------------+ | ipv6user@::1 | +----------------+
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”.
Start the MySQL server with an appropriate
--bind-addressoption 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”.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';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 -premoteipv6passTry 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 | +-----------------------------------+
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.
Create an account at Freenet6 by visiting this URL and signing up:
http://gogonet.gogo6.com
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
As
root, install gogoc:shell>
emerge gogocEdit
/etc/gogoc/gogoc.confto set theuseridandpasswordvalues. For example:userid=gogouser passwd=gogopass
Start gogoc:
shell>
/etc/init.d/gogoc startTo start gogoc each time your system boots, execute this command:
shell>
rc-update add gogoc defaultUse ping6 to try to ping a host:
shell>
ping6 ipv6.google.comTo see your IPv6 address:
shell>
ifconfig tun
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.
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”.
On Unix, signals can be sent to processes. mysqld responds to signals sent to it as follows:
SIGTERMcauses the server to shut down.SIGHUPcauses 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 theFLUSHstatement. 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
The server shutdown process takes place as follows:
The shutdown process is initiated.
This can occur initiated several ways. For example, a user with the
SHUTDOWNprivilege 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 aSIGTERMsignal. A server running as a service on Windows shuts down when the services manager tells it to.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
SIGTERMsignal, 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
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.
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 TABLEorOPTIMIZE TABLEoperations onMyISAMtables.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
UPDATEorINSERTmay 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 QUERYorKILL CONNECTIONstatement. 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.
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,
MyISAMflushes any pending index writes for a table.InnoDBflushes its buffer pool to disk (unlessinnodb_fast_shutdownis 2), writes the current LSN to the tablespace, and terminates its own internal threads.The server exits.
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:
The
mysqldirectory corresponds to themysqlsystem database, which contains information required by the MySQL server as it runs. See Section 5.3, “The mysql System Database”.The
performance_schemadirectory corresponds to the Performance Schema, which provides information used to inspect the internal execution of the server at runtime. See Chapter 22, MySQL Performance Schema.The
ndbinfodirectory corresponds to thendbinfodatabase that stores information specific to MySQL Cluster (present only for installations built to include MySQL Cluster). See Section 18.5.10, “The ndbinfo MySQL Cluster Information Database”.
Other subdirectories correspond to databases created by users or applications.
NoteINFORMATION_SCHEMAis a standard database, but its implementation uses no corresponding database directory.Log files written by the server. See Section 5.4, “MySQL Server Logs”.
InnoDBtablespace 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.
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.
These system tables contain grant information about user accounts and the privileges held by them:
For more information about the structure, contents, and purpose of the grant tables, see Section 6.2.2, “Grant Tables”.
These system tables contain information about stored programs, user-defined functions, and server-side plugins:
event: Information about Event Scheduler events. See Section 20.4, “Using the Event Scheduler”. The server loads events listed in this table during its startup sequence, unless started with the--skip-grant-tablesoption.func: Information about user-defined functions (UDFs). See Section 24.4, “Adding New Functions to MySQL”. The server loads UDFs listed in this table during its startup sequence, unless started with the--skip-grant-tablesoption.plugin: Information about server-side plugins. See Section 5.5.2, “Installing and Uninstalling Plugins”, and Section 24.2, “The MySQL Plugin API”. The server loads plugins listed in this table during its startup sequence, unless started with the--skip-grant-tablesoption.proc: Information about stored procedures and functions. See Section 20.2, “Using Stored Routines (Procedures and Functions)”.
The server uses these system tables for logging:
Log tables use the CSV storage engine.
For more information, see Section 5.4, “MySQL Server Logs”.
These system tables contain server-side help information:
For more information, see Section 5.1.9, “Server-Side Help”.
These system tables contain time zone information:
For more information, see Section 10.6, “MySQL Server Time Zone Support”.
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”.
Other system tables do not fall into the preceding categories:
servers: Used by theFEDERATEDstorage engine. See Section 15.9.2.2, “Creating a FEDERATED Table Using CREATE SERVER”.
MySQL Server has several logs that can help you find out what activity is taking place.
| Log Type | Information Written to Log |
|---|---|
| Error log | Problems encountered starting, running, or stopping mysqld |
| General query log | Established client connections and statements received from clients |
| Binary log | Statements that change data (also used for replication) |
| Relay log | Data changes received from a replication master server |
| Slow query log | Queries 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”.
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-outputis given with a value, the value should be a comma-separated list of one or more of the wordsTABLE(log to tables),FILE(log to files), orNONE(do not log to tables or files).NONE, if present, takes precedence over any other specifiers.If
--log-outputis omitted, the default logging destination isFILE.
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,FILEto select both log destinations and--general_logto enable the general query log.To write general and slow query log entries only to the log tables, use
--log-output=TABLEto select tables as the log destination and--general_logand--slow_query_logto enable both logs.To write slow query log entries only to the log file, use
--log-output=FILEto select files as the log destination and--slow_query_logto enable the slow query log. (In this case, because the default log destination isFILE, you could omit the--log-outputoption.)
Log control at runtime. The system variables associated with log tables and files enable runtime control over logging:
The global
log_outputsystem variable indicates the current logging destination. It can be modified at runtime to change the destination.The global
general_logandslow_query_logvariables 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_fileandslow_query_log_filevariables 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_offvariable toONorOFF.
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, andDROP TABLEare valid operations on a log table. ForALTER TABLEandDROP TABLE, the log table cannot be in use and must be disabled, as described later.By default, the log tables use the
CSVstorage engine that writes data in comma-separated values format. For users who have access to the.CSVfiles 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
MyISAMstorage engine. You cannot useALTER TABLEto alter a log table that is in use. The log must be disabled first. No engines other thanCSVorMyISAMare 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_logtable andslow_query_logsystem 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 TABLEis a valid operation on a log table. It can be used to expire log entries.RENAME TABLEis 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 TABLEis a valid operation on a log table.LOCK TABLEScannot be used on a log table.INSERT,DELETE, andUPDATEcannot be used on a log table. These operations are permitted only internally to the server itself.FLUSH TABLES WITH READ LOCKand the state of theread_onlysystem 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 TABLESorFLUSH LOGS, respectively.Partitioning of log tables is not permitted.
Before MySQL 5.5.25, mysqldump does not dump the
general_logorslow_query_logtables for dumps of themysqldatabase. 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.
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[=, 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.file_name]The default log file is
host_name.err--pid-fileoption is specified. In that case, the default name is the PID file base name with a suffix of.errin the data directory. This default is used if--log-erroris not given, or is given without naming a log file.With
--console, mysqld writes error messages to the console, unless--log-erroris also given. If both options are present,--consoleis ignored and has no effect. Their order does not matter:--log-errortakes 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[=, 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.file_name]NoteIt is common for Yum or APT package installations to configure the error log location to be under
/var/logwith an entry likelog-error=/var/log/mysqld.login 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--log-erroris 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.errshell>
mvshell>host_name.errhost_name.err-oldmysqladmin flush-logsshell>mvhost_name.err-oldbackup-directoryOn 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=
to mysqld_safe, and
mysqld_safe will arrange for
mysqld to write messages to a log file. To use
file_namesyslog 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”.
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=.
To specify the log destination, use
file_name--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
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>mvshell>host_name.loghost_name-old.logmysqladmin flush-logsshell>mvhost_name-old.logbackup-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”.
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[=
option. If no base_name]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=),
the extension is silently removed and ignored.
base_name.extension
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[=
option. You should not manually edit this file while
mysqld is running; doing so would confuse
mysqld.
file_name]
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 . In this case, this binary log is not
correct and replication should be restarted from a fresh snapshot
of the master's data.
file_name is shorter than
its expected size
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:
sql_mode(except that theNO_DIR_IN_CREATEmode is not replicated; see Section 17.4.1.38, “Replication and Variables”)
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.
You can select the binary logging format explicitly by starting
the MySQL server with
--binlog-format=.
The supported values for typetype are:
STATEMENTcauses logging to be statement based.ROWcauses logging to be row based.MIXEDcauses 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';
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
WHEREclause 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
NDBCLUSTERstorage engine is enabledIf 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.
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”.
When running in MIXED logging format, the
server automatically switches from statement-based to row-based
logging under the following conditions:
When a DML statement updates an
NDBCLUSTERtable.When a function contains
UUID().When one or more tables with
AUTO_INCREMENTcolumns are updated and a trigger or stored function is invoked. Like all other unsafe statements, this generates a warning ifbinlog_format = STATEMENT.For more information, see Section 17.4.1.1, “Replication and AUTO_INCREMENT”.
When any
INSERT DELAYEDis executed.When the body of a view requires row-based replication, the statement creating the view also uses it. For example, this occurs when the statement creating a view uses the
UUID()function.When a call to a UDF is involved.
If a statement is logged by row and the session that executed the statement has any temporary tables, logging by row is used for all subsequent statements (except for those accessing temporary tables) until all temporary tables in use by that session are dropped.
This is true whether or not any temporary tables are actually logged.
Temporary tables cannot be logged using row-based format; thus, once row-based logging is used, all subsequent statements using that table are unsafe. The server approximates this condition by treating all statements executed during the session as unsafe until the session no longer holds any temporary tables.
When
FOUND_ROWS()orROW_COUNT()is used. (Bug #12092, Bug #30244)When
USER(),CURRENT_USER(), orCURRENT_USERis used. (Bug #28086)When a statement refers to one or more system variables. (Bug #31168)
Exception. The following system variables, when used with session scope (only), do not cause the logging format to switch:
For information about determining system variable scope, see Section 5.1.5, “Using System Variables”.
For information about how replication treats
sql_mode, see Section 17.4.1.38, “Replication and Variables”.When one of the tables involved is a log table in the
mysqldatabase.When the
LOAD_FILE()function is used. (Bug #39701)
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 Engine | Row Logging Supported | Statement Logging Supported |
|---|---|---|
ARCHIVE | Yes | Yes |
BLACKHOLE | Yes | Yes |
CSV | Yes | Yes |
EXAMPLE | Yes | No |
FEDERATED | Yes | Yes |
HEAP | Yes | Yes |
InnoDB | Yes | Yes when the transaction isolation level is
REPEATABLE READ or
SERIALIZABLE; No
otherwise. |
MyISAM | Yes | Yes |
MERGE | Yes | Yes |
NDBCLUSTER | Yes | No |
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”.
| Condition | Action | ||||
|---|---|---|---|---|---|
| Type | binlog_format | SLC | RLC | Error / Warning | Logged as |
| * | * | No | No | Error: Cannot execute statement: Binary logging is impossible since at least one engine is involved that is both row-incapable and statement-incapable. | - |
| Safe | STATEMENT | Yes | No | - | STATEMENT |
| Safe | MIXED | Yes | No | - | STATEMENT |
| Safe | ROW | Yes | No | Error: 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. | - |
| Unsafe | STATEMENT | Yes | No | Warning: Unsafe statement binlogged in statement
format, since BINLOG_FORMAT =
STATEMENT | STATEMENT |
| Unsafe | MIXED | Yes | No | Error: 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. | - |
| Unsafe | ROW | Yes | No | Error: 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 Injection | STATEMENT | Yes | No | Error: 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 Injection | MIXED | Yes | No | Error: 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 Injection | ROW | Yes | No | Error: 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. | - |
| Safe | STATEMENT | No | Yes | Error: 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. | - |
| Safe | MIXED | No | Yes | - | ROW |
| Safe | ROW | No | Yes | - | ROW |
| Unsafe | STATEMENT | No | Yes | Error: 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. | - |
| Unsafe | MIXED | No | Yes | - | ROW |
| Unsafe | ROW | No | Yes | - | ROW |
| Row Injection | STATEMENT | No | Yes | Error: Cannot execute row injection: Binary
logging is not possible since BINLOG_FORMAT =
STATEMENT. | - |
| Row Injection | MIXED | No | Yes | - | ROW |
| Row Injection | ROW | No | Yes | - | ROW |
| Safe | STATEMENT | Yes | Yes | - | STATEMENT |
| Safe | MIXED | Yes | Yes | - | STATEMENT |
| Safe | ROW | Yes | Yes | - | ROW |
| Unsafe | STATEMENT | Yes | Yes | Warning: Unsafe statement binlogged in statement
format since BINLOG_FORMAT =
STATEMENT. | STATEMENT |
| Unsafe | MIXED | Yes | Yes | - | ROW |
| Unsafe | ROW | Yes | Yes | - | ROW |
| Row Injection | STATEMENT | Yes | Yes | Error: Cannot execute row injection: Binary
logging is not possible because BINLOG_FORMAT =
STATEMENT. | - |
| Row Injection | MIXED | Yes | Yes | - | ROW |
| Row Injection | ROW | Yes | Yes | - | 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.
| Condition | Action | ||||
|---|---|---|---|---|---|
| Safe/unsafe | binlog_format | SLR | RLR | Error/Warning | Logged as |
| Safe | STATEMENT | Yes | Yes | Error: not loggable | |
| Safe | STATEMENT | Yes | No | STATEMENT | |
| Safe | STATEMENT | No | Yes | Error: not loggable | |
| Safe | STATEMENT | No | No | STATEMENT | |
| Safe | MIXED | Yes | Yes | Error: not loggable | |
| Safe | MIXED | Yes | No | STATEMENT | |
| Safe | MIXED | No | Yes | ROW | |
| Safe | MIXED | No | No | STATEMENT | |
| Safe | ROW | Yes | Yes | Error: not loggable | |
| Safe | ROW | Yes | No | Error: not loggable | |
| Safe | ROW | No | Yes | ROW | |
| Safe | ROW | No | No | ROW | |
| Unsafe | STATEMENT | Yes | Yes | Error: not loggable | |
| Unsafe | STATEMENT | Yes | No | Warning: unsafe | STATEMENT |
| Unsafe | STATEMENT | No | Yes | Error: not loggable | |
| Unsafe | STATEMENT | No | No | Warning: unsafe | STATEMENT |
| Unsafe | MIXED | Yes | Yes | Error: not loggable | |
| Unsafe | MIXED | Yes | No | Error: not loggable | |
| Unsafe | MIXED | No | Yes | ROW | |
| Unsafe | MIXED | No | No | ROW | |
| Unsafe | ROW | Yes | Yes | Error: not loggable | |
| Unsafe | ROW | Yes | No | Error: not loggable | |
| Unsafe | ROW | No | Yes | ROW | |
| Unsafe | ROW | No | No | 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.
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:
Data manipulation statements that change data in
mysqldatabase tables directly are logged according to the setting of thebinlog_formatsystem variable. This pertains to statements such asINSERT,UPDATE,DELETE,REPLACE,DO,LOAD DATA INFILE,SELECT, andTRUNCATE TABLE.Statements that change the
mysqldatabase indirectly are logged as statements regardless of the value ofbinlog_format. This pertains to statements such asGRANT,REVOKE,SET PASSWORD,RENAME USER,CREATE(all forms exceptCREATE TABLE ... SELECT),ALTER(all forms), andDROP(all forms).
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.
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=.
To specify the log destination, use
file_name--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
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:
The query must either not be an administrative statement, or
--log-slow-admin-statementsmust have been specified.The query must have taken at least
long_query_timeseconds, orlog_queries_not_using_indexesmust be enabled and the query used no indexes for row lookups.The query must have examined at least
min_examined_row_limitrows.
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”.
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.
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-erroroption 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>cdshell>mysql-data-directorymv mysql.log mysql.oldshell>mv mysql-slow.log mysql-slow.oldshell>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.
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.
MySQL distributions include several plugins that implement server extensions:
Plugins for authenticating attempts by clients to connect to MySQL Server. Plugins are available for several authentication protocols. See Section 6.3.6, “Pluggable Authentication”.
Semisynchronous replication plugins implement an interface to replication capabilities that permit the master to proceed as long as at least one slave has responded to each transaction. See Section 17.3.8, “Semisynchronous Replication”.
MySQL Enterprise Edition includes a thread pool plugin that manages connection threads to increase server performance by efficiently managing statement execution threads for large numbers of client connections. See Section 5.5.4, “MySQL Enterprise Thread Pool”.
MySQL Enterprise Edition includes an audit plugin for monitoring and logging of connection and query activity. See Section 6.5.2, “MySQL Enterprise Audit”.
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”.
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”.
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
--
option.
plugin_name[=activation_state]
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
--
option.
plugin_name[=activation_state]
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
--
option.
plugin_name[=activation_state]
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'.
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
--
startup option, where plugin_name[=activation_state]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=OFFTells 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
--without a value has the same effect.) If the plugin fails to initialize, the server runs with the plugin disabled.plugin_name--plugin_name=FORCETells 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_PERMANENTLike
FORCE, but in addition prevents the plugin from being unloaded at runtime. If a user attempts to do so withUNINSTALL 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-
option format is a synonym for
plugin_name--.
The
plugin_name=ON--disable-
and
plugin_name--skip-
option formats are synonyms for
plugin_name--.
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
-- or
plugin_name=0--, the
equivalent options are now
plugin_name=1--
and
plugin_name=OFF--,
respectively. You also have the choice of requiring plugins to
start successfully by using
plugin_name=ON--
or
plugin_name=FORCE--.
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_
options likely will need to be omitted at startup. In addition,
because xxxInnoDB is the default
storage engine, it will not start unless you specify another
available storage engine with
--default-storage-engine.
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
NULLin the output fromINFORMATION_SCHEMA.PLUGINSorSHOW PLUGINS.It cannot unload plugins for which the server was started with
--, which prevents plugin unloading at runtime. These can be identified from theplugin_name=FORCE_PLUS_PERMANENTLOAD_OPTIONcolumn of theINFORMATION_SCHEMA.PLUGINStable.
To uninstall a plugin that currently is loaded at server startup with a plugin-loading option, use this procedure.
Remove any options related to the plugin from the
my.cnffile.Restart the server.
Plugins normally are installed using either a plugin-loading option at startup or with
INSTALL PLUGINat runtime, but not both. However, removing options for a plugin from themy.cnffile may not be sufficient to uninstall it if at some pointINSTALL PLUGINhas also been used. If the plugin still appears in the output fromINFORMATION_SCHEMA.PLUGINSorSHOW PLUGINS, useUNINSTALL PLUGINto remove it from themysql.plugintable. Then restart the server again.
There are several ways to determine which plugins are installed in the server:
The
INFORMATION_SCHEMA.PLUGINStable contains a row for each loaded plugin. Any that have aPLUGIN_LIBRARYvalue ofNULLare 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 PLUGINSstatement displays a row for each loaded plugin. Any that have aLibraryvalue ofNULLare 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.plugintable shows which plugins have been registered withINSTALL PLUGIN. The table contains only plugin names and library file names, so it does not provide as much information as thePLUGINStable or theSHOW PLUGINSstatement.
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”
The thread pool feature comprises these components:
A plugin library file contains a plugin for the thread pool code and plugins for several
INFORMATION_SCHEMAtables.For a detailed description of how the thread pool works, see Section 5.5.4.3, “Thread Pool Operation”.
The
INFORMATION_SCHEMAtables are namedTP_THREAD_STATE,TP_THREAD_GROUP_STATE, andTP_THREAD_GROUP_STATS. These tables provide information about thread pool operation. For more information, see Section 21.30, “Thread Pool INFORMATION_SCHEMA Tables”.Several system variables are related to the thread pool. The
thread_handlingsystem variable has a value ofloaded-dynamicallywhen the server successfully loads the thread pool plugin.The other related variables are implemented by the thread pool plugin; they are not available unless it is enabled:
thread_pool_algorithm: The concurrency algorithm to use for scheduling.thread_pool_high_priority_connection: How to schedule statement execution for a session.thread_pool_prio_kickup_timer: How long before the thread pool moves a statement awaiting execution from the low-priority queue to the high-priority queue.thread_pool_max_unused_threads: How many sleeping threads to permit.thread_pool_size: The number of thread groups in the thread pool. This is the most important parameter controlling thread pool performance.thread_pool_stall_limit: The time before an executing statement is considered to be stalled.
If any variable implemented by the plugin is set to an illegal value at startup, plugin initialization fails and the plugin does not load.
For information about setting thread pool parameters, see Section 5.5.4.4, “Thread Pool Tuning”.
The Performance Schema exposes information about the thread pool and may be used to investigate operational performance. For more information, see Chapter 22, MySQL Performance Schema.
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
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.
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_limitsystem 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_connectionsystem 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
autocommitis enabled, are treated as low-priority statements because in this case each statement is a transaction. Thus, given a mix of statements forInnoDBandMyISAMtables, the thread pool prioritizes those forInnoDBover those forMyISAMunlessautocommitis enabled. Withautocommitenabled, 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_timersystem 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.
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 optimalthread_pool_sizesetting 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
InnoDBseems to be usually around 36. For very write-intensive workloads, the optimal setting can sometimes be lower.If the primary storage engine is
MyISAM, thethread_pool_sizesetting 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:
There are 200 statements queued in the low-priority queue.
There are 10 statements queued in the high-priority queue.
thread_pool_prio_kickup_timeris set to 10000 (10 seconds).thread_pool_stall_limitis set to 100 (1 second).
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.
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=
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”.
dir_name
In addition to using different data directories, several other options must have different values for each server instance:
--portcontrols the port number for TCP/IP connections. Alternatively, if the host has multiple network addresses, you can use--bind-addressto cause each server to listen to a different address.--socket={file_name|pipe_name}--socketcontrols 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=nameThis 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.
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=
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
dir_name--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.
Each MySQL Instance on a machine should have its own data
directory. The location is specified using the
--datadir=
option.
dir_name
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.
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
datadirectory 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
dataunder 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.
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.
Copy the data directory to the location where the new data directory should be.
Copy the
my.cnformy.inioption file used by the existing instance. This serves as a basis for the new instance.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”.
Start the new instance, telling it to use the new option file.
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”.
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:
Make sure that each data directory exists, including its own copy of the
mysqldatabase that contains the grant tables.Create two option files. For example, create one file named
C:\my-opts1.cnfthat looks like this:[mysqld] datadir = C:/mydata1 port = 3307
Create a second file named
C:\my-opts2.cnfthat looks like this:[mysqld] datadir = C:/mydata2 port = 3308
Use the
--defaults-fileoption to start each server with its own option file:C:\>
C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnfC:\>C:\mysql\bin\mysqld --defaults-file=C:\my-opts2.cnfEach 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 shutdownC:\>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.
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
mysqld1and the 5.5.53 mysqld using the service namemysqld2. 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 upC:\my.cnflike 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 mysqld1C:\>C:\mysql-5.5.53\bin\mysqld --install mysqld2To start the services, use the services manager, or use NET START with the appropriate service names:
C:\>
NET START mysqld1C:\>NET START mysqld2To stop the services, use the services manager, or use NET STOP with the appropriate service names:
C:\>
NET STOP mysqld1C:\>NET STOP mysqld2Approach 2: Specify options for each server in separate files and use
--defaults-filewhen 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.cnfthat 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.cnfthat 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.cnfC:\>C:\mysql-5.5.53\bin\mysqld --install mysqld2--defaults-file=C:\my-opts2.cnfWhen you install a MySQL server as a service and use a
--defaults-fileoption, 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.
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=
option to mysqld_safe so that the server uses a
different data directory.
dir_name
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.cnfshell>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.sockshell>MYSQL_TCP_PORT=3307shell>export MYSQL_UNIX_PORT MYSQL_TCP_PORTshell>mysql_install_db --user=mysqlshell>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”.
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=to connect using TCP/IP to a remote server, withport_number--host=127.0.0.1--port=to connect using TCP/IP to a local server, or withport_number--host=localhost--socket=to connect to a local server using a Unix socket file or a Windows named pipe.file_nameStart the client with
--protocol=TCPto connect using TCP/IP,--protocol=SOCKETto connect using a Unix socket file,--protocol=PIPEto connect using a named pipe, or--protocol=MEMORYto connect using shared memory. For TCP/IP connections, you may also need to specify--hostand--portoptions. For the other types of connections, you may need to specify a--socketoption to specify a Unix socket file or Windows named-pipe name, or a--shared-memory-base-nameoption to specify the shared-memory name. Shared-memory connections are supported only on Windows.On Unix, set the
MYSQL_UNIX_PORTandMYSQL_TCP_PORTenvironment 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.loginfile 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 useC:\my.cnfon Windows, or the.my.cnffile 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 callingmysql_options(). See Section 23.8.7, “C API Function Descriptions”.If you are using the Perl
DBD::mysqlmodule, 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.
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.
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
For more information on DTrace and writing DTrace scripts, read the DTrace User Guide.
For an introduction to DTrace, see the MySQL Dev Zone article Getting started with DTracing MySQL.
- 5.7.1.1 Connection Probes
- 5.7.1.2 Command Probes
- 5.7.1.3 Query Probes
- 5.7.1.4 Query Parsing Probes
- 5.7.1.5 Query Cache Probes
- 5.7.1.6 Query Execution Probes
- 5.7.1.7 Row-Level Probes
- 5.7.1.8 Read Row Probes
- 5.7.1.9 Index Probes
- 5.7.1.10 Lock Probes
- 5.7.1.11 Filesort Probes
- 5.7.1.12 Statement Probes
- 5.7.1.13 Network Probes
- 5.7.1.14 Keycache Probes
MySQL supports the following static probes, organized into groups of functionality.
Table 5.5 MySQL DTrace Probes
| Group | Probes |
|---|---|
| Connection | connection-start, connection-done |
| Command | command-start, command-done |
| Query | query-start, query-done |
| Query Parsing | query-parse-start,
query-parse-done |
| Query Cache | query-cache-hit, query-cache-miss |
| Query Execution | query-exec-start, query-exec-done |
| Row Level | insert-row-start, insert-row-done |
update-row-start, update-row-done | |
delete-row-start, delete-row-done | |
| Row Reads | read-row-start, read-row-done |
| Index Reads | index-read-row-start,
index-read-row-done |
| Lock | handler-rdlock-start,
handler-rdlock-done |
handler-wrlock-start,
handler-wrlock-done | |
handler-unlock-start,
handler-unlock-done | |
| Filesort | filesort-start, filesort-done |
| Statement | select-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 | |
| Network | net-read-start, net-read-done,
net-write-start,
net-write-done |
| Keycache | keycache-read-start,
keycache-read-block,
keycache-read-done,
keycache-read-hit,
keycache-read-miss,
keycache-write-start,
keycache-write-block,
keycache-write-done |
When extracting the argument data from the probes, each argument
is available as
arg, starting
with Narg0. To identify each argument within
the definitions they are provided with a descriptive name, but
you must access the information using the corresponding
arg parameter.
N
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: Anunsigned longcontaining the connection ID. This is the same as the process ID shown as theIdvalue in the output fromSHOW 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
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.Value Name Description 00 COM_SLEEP Internal thread state 01 COM_QUIT Close connection 02 COM_INIT_DB Select database ( USE ...)03 COM_QUERY Execute a query 04 COM_FIELD_LIST Get a list of fields 05 COM_CREATE_DB Create a database (deprecated) 06 COM_DROP_DB Drop a database (deprecated) 07 COM_REFRESH Refresh connection 08 COM_SHUTDOWN Shutdown server 09 COM_STATISTICS Get statistics 10 COM_PROCESS_INFO Get processes ( SHOW PROCESSLIST)11 COM_CONNECT Initialize connection 12 COM_PROCESS_KILL Kill process 13 COM_DEBUG Get debug information 14 COM_PING Ping 15 COM_TIME Internal thread state 16 COM_DELAYED_INSERT Internal thread state 17 COM_CHANGE_USER Change user 18 COM_BINLOG_DUMP Used by a replication slave or mysqlbinlog to initiate a binary log read 19 COM_TABLE_DUMP Used by a replication slave to get the master table information 20 COM_CONNECT_OUT Used by a replication slave to log a connection to the server 21 COM_REGISTER_SLAVE Used by a replication slave during registration 22 COM_STMT_PREPARE Prepare a statement 23 COM_STMT_EXECUTE Execute a statement 24 COM_STMT_SEND_LONG_DATA Used by a client when requesting extended data 25 COM_STMT_CLOSE Close a prepared statement 26 COM_STMT_RESET Reset a prepared statement 27 COM_SET_OPTION Set a server option 28 COM_STMT_FETCH Fetch a prepared statement user: The user executing the command.host: The client host.
command-done: Triggered when the command execution completes. Thestatusargument 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.
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 theIdvalue in the output fromSHOW 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
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. Thestatusis an integer describing the status of the operation. A0indicates that the query was successfully parsed. A1indicates 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
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
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)
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 theIdvalue in the output fromSHOW 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.Value Description 0 Executed query from sql_parse, top-level query. 1 Executed prepared statement 2 Executed cursor statement 3 Executed 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.
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
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 specifieddatabaseandtable. Thescan_flagis 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. Thestatusreturns 0 on success, or a positive value on failure.
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 specifieddatabaseandtable.index-read-row-done: Triggered when an indexed row read operation within a storage engine completes. Thestatusreturns 0 on success, or a positive value on failure.
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 specifieddatabaseandtable.handler-wrlock-start: Triggered when a write lock is requested on the specifieddatabaseandtable.handler-unlock-start: Triggered when an unlock request is made on the specifieddatabaseandtable.handler-rdlock-done: Triggered when a read lock request completes. Thestatusis 0 if the lock operation succeeded, or>0on failure.handler-wrlock-done: Triggered when a write lock request completes. Thestatusis 0 if the lock operation succeeded, or>0on failure.handler-unlock-done: Triggered when an unlock request completes. Thestatusis 0 if the unlock operation succeeded, or>0on 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
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,databaseandtable, will identify the table being sorted.filesort-done: Triggered when the filesort operation completes. Two arguments are supplied, thestatus(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
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 aSELECTstatement.select-done: Triggered at the end of aSELECTstatement.insert-start: Triggered before aINSERTstatement.insert-done: Triggered at the end of anINSERTstatement.insert-select-start: Triggered before anINSERT ... SELECTstatement.insert-select-done: Triggered at the end of anINSERT ... SELECTstatement.update-start: Triggered before anUPDATEstatement.update-done: Triggered at the end of anUPDATEstatement.multi-update-start: Triggered before anUPDATEstatement involving multiple tables.multi-update-done: Triggered at the end of anUPDATEstatement involving multiple tables.delete-start: Triggered before aDELETEstatement.delete-done: Triggered at the end of aDELETEstatement.multi-delete-start: Triggered before aDELETEstatement involving multiple tables.multi-delete-done: Triggered at the end of aDELETEstatement involving multiple tables.
The arguments for the statement probes are:
query: The query string.status: The status of the query.0for success, and>0for failure.rows: The number of rows affected by the statement. This returns the number rows found forSELECT, the number of rows deleted forDELETE, and the number of rows successfully inserted forINSERT.rowsmatched: The number of rows matched by theWHEREclause of anUPDATEoperation.rowschanged: The number of rows actually changed during anUPDATEoperation.
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
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. Thestatusis anintegerrepresenting the return status for the operation,0for success and1for failure. Thebytesargument 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,0for success and1for 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)
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 specifiedfilepath, reading the specified number ofbytes. Themem_usedandmem_availindicate 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 ofbytes, 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. Themem_usedandmem_availindicate 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 specifiedfilepath, reading the specified number ofbytes. Themem_usedandmem_availindicate 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 ofbytes, to the index file from the keycache.keycache-write-done: Triggered when the keycache write operation has completed. Themem_usedandmem_availindicate memory currently used by the keycache and the amount of memory available within the keycache.