This section discusses MySQL server configuration topics:
Startup options that the server supports
How to set the server SQL mode
Server system variables
Server status variables
When you start the mysqld server, you can specify program options using any of the methods described in the section called “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 the section called “Using Option Files”.
mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. mysql.server reads options from the [mysqld] and [mysql.server] groups. An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.
mysqld accepts many command-line options. For a list, execute mysqld --help. Before MySQL 4.1.1, --help prints the full help message. As of 4.1.1, it prints a brief message; to see the full list, use mysqld --verbose --help.
The following list shows some of the most common server options. Additional options are described elsewhere:
Options that affect security: See the section called “Startup Options for mysqld Concerning Security”.
SSL-related options: See the section called “SSL Command-Line Options”.
Binary log control options: See the section called “The Binary Log”.
Replication-related options: See the section called “Replication Startup Options”.
Options specific to particular storage engines: See MyISAM start, BDB start, InnoDB start.
You can also set the value of a server system variable by using the variable name as an option, as described later in this section.
Display a short help message and exit. Before MySQL 4.1.1, --help displays the full help message. As of 4.1.1, it displays an abbreviated message only. Use both the --verbose and --help options to see the full message.
Use standard SQL syntax instead of MySQL syntax. See the section called “Running MySQL in ANSI Mode”. For more precise control over the server SQL mode, use the --sql-mode option instead.
The path to the MySQL installation directory. All paths are usually resolved relative to this.
Allow 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.
The IP address to bind to.
Write the error log messages to stderr/stdout even if --log-error is specified. On Windows, mysqld will not close the console screen if this option is used.
The directory where character sets are installed. See the section called “The Character Set Used for Data and Sorting”.
Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure as of MySQL 4.0. (MySQL 3.23 is not able to provide a chroot() jail that is 100% closed.) Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.
Use charset as the default server character set. This option is available as of MySQL 4.1.3. See the section called “The Character Set Used for Data and Sorting”.
Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size option to mysqld_safe. See mysqld_safe. Note that on some systems, such as Solaris, you will not get a core file if you are also using the --user option.
Use collation as the default server collation. This option is available as of MySQL 4.1.3. See the section called “The Character Set Used for Data and Sorting”.
The path to the data directory.
If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. The debug_options string often is 'd:t:o,file_name'. See the section called “Creating Trace Files”.
Use charset as the default character set. This option is deprecated in favor of --character-set-server as of MySQL 4.1.3. See the section called “The Character Set Used for Data and Sorting”.
Use collation as the default collation. This option is deprecated in favor of --collation-server as of MySQL 4.1.3. See the section called “The Character Set Used for Data and Sorting”.
This option is a synonym for --default-table-type. It is available as of MySQL 4.1.2.
Set the default table type for tables. See Chapter 15, MySQL Storage Engines and Table Types.
Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone will be the same as the system time zone (given by the value of the system_time_zone system variable. This option is available as of MySQL 4.1.3.
How the DELAYED KEYS option should be used. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAYED KEYS option. ALL delays key writes for all MyISAM tables. Available as of MySQL 4.0.3. See the section called “Tuning Server Parameters”. See the section called “MyISAM Startup Options”.
Note: If you set this variable to ALL, you should not use MyISAM tables from within another program (such as from another MySQL server or with myisamchk) when the table is in use. Doing so will lead to index corruption.
Old form of --delay-key-write=ALL for use prior to MySQL 4.0.3. As of 4.0.3, use --delay-key-write instead.
Read the default keys used by DES_ENCRYPT() and DES_DECRYPT() from this file.
Enable support for named pipes. This option applies only on Windows NT, 2000, and XP systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named pipe connections.
This is a bit mask of different flags you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!
Enable system locking. Note that if you use this option on a system on which lockd does not fully work (as on Linux), you will easily get mysqld to deadlock. This option previously was named --enable-locking.
Note: If you use this option to enable updates to MyISAM tables from many MySQL processes, you have to ensure that these conditions are satisfied:
You should not use the query cache for queries that use tables that are updated by another process.
You should not use --delay-key-write=ALL or DELAY_KEY_WRITE=1 on any shared tables.
The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF --query-cache-size=0.
(This is not done by default because in many setups it's useful to have a mixture of the above options.)
Flush 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 synching to disk. See the section called “What to Do If MySQL Keeps Crashing”.
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
Adds consistency guarantees between the content of InnoDB tables and the binary log. See the section called “The Binary Log”.
Client error messages in given language. lang_name can be given as the language name or as the full pathname to the directory where the language files are installed. See the section called “Setting the Error Message Language”.
Log connections and queries to this file. See the section called “The General Query Log”. If you don't specify a filename, MySQL will use host_name.log as the filename.
The binary log file. Log all queries that change data to this file. Used for backup and replication. See the section called “The Binary Log”. If you don't specify a filename, MySQL will use host_name-bin as the filename.
The index file for binary log filenames. See the section called “The Binary Log”. If you don't specify a filename, MySQL will use host_name-bin.index as the filename.
Log errors and startup messages to this file. See the section called “The Error Log”. If you don't specify a filename, MySQL will use host_name.err as the filename.
Log all ISAM/MyISAM changes to this file (used only when debugging ISAM/MyISAM).
Log some extra information to the log files (update log, binary update log, and slow queries log, whatever log has been activated). For example, username and timestamp are logged for queries. If you are using --log-slow-queries and --log-long-format, queries that are not using indexes also are logged to the slow query log. Note that --log-long-format is deprecated as of MySQL version 4.1, when --log-short-format was introduced (the long log format is the default setting since version 4.1). Also note that starting with MySQL 4.1, the --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log.
If you are using this option with --log-slow-queries, then queries that are not using indexes also are logged to the slow query log. This option is available as of MySQL 4.1. See the section called “The Slow Query Log”.
Log less information to the log files (update log, binary update log, and slow queries log, whatever log has been activated). For example, username and timestamp are not logged for queries. This option was introduced in MySQL 4.1.
Log all queries that have taken more than long_query_time seconds to execute to this file. See the section called “The Slow Query Log”. Note that the default for the amount of information logged has changed in MySQL 4.1. See the --log-long-format and --log-short-format options for details.
Log updates to file# where # is a unique number if not given. See the section called “The Update Log”. The update log is deprecated and is removed in MySQL 5.0.0; you should use the binary log instead (--log-bin). See the section called “The Binary Log”. Starting from version 5.0.0, using --log-update will just turn on the binary log instead (see the section called “Changes in release 5.0.0 (22 Dec 2003: Alpha)”).
Print out warnings such as Aborted connection... to the error log. Enabling this option is recommended, for example, if you use replication (you will get more information about what is happening, such as messages about network failures and reconnections). This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --skip-log-warnings. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1. See the section called “Communication Errors and Aborted Connections”.
This option was named --warnings before MySQL 4.0.
Table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) will have lower priority than selects. This can also be done via {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See the section called “Table Locking Issues”.
Lock the mysqld process in memory. This works on systems such as Solaris that support the mlockall() system call. This might help if you have a problem where the operating system is causing mysqld to swap on disk. Note that use of this option requires that you run the server as root, which is normally not a good idea for security reasons.
Set the MyISAM storage engine recovery mode. The option value is any combination of the values of DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. You can also use a value of "" to disable this option. If this option is used, mysqld will, when it opens a MyISAM table, open check whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with --skip-external-locking.) If this is the case, mysqld will run a check on the table. If the table was corrupted, mysqld will attempt to repair it.
The following options affect how the repair works:
| Option | Description |
| DEFAULT | The same as not giving any option to --myisam-recover. |
| BACKUP | If the data file was changed during recovery, save a backup of the tbl_name.MYD file as tbl_name-datetime.BAK. |
| FORCE | Run recovery even if we will lose more than one row from the .MYD file. |
| QUICK | Don't check the rows in the table if there aren't any delete blocks. |
Before a table is automatically repaired, MySQL will add a note about this in 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 will force a repair of a table even if some rows would be deleted, but it will keep the old data file as a backup so that you can later examine what happened.
This option is available as of MySQL 3.23.25.
When using the NDB storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option.
If the binary includes support for the NDB Cluster storage engine (from version 4.1.3, the MySQL-Max binaries are built with NDB Cluster enabled) the default disabling of support for the NDB Cluster storage engine can be overruled by using this option. Using the NDB Cluster storage engine is necessary for using MySQL Cluster. See Chapter 17, MySQL Cluster.
The --new option can be used to make the server behave as 4.1 in certain respects, easing a 4.0 to 4.1 upgrade:
Hexadecimal strings such as 0xFF are treated as strings by default rather than as numbers. (Works in 4.0.12 and up.)
TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS'. (Works in 4.0.13 and up.) See Chapter 12, Column Types.
This option can be used to help you see how your applications will behave in MySQL 4.1, without actually upgrading to 4.1.
The path to the process ID file used by mysqld_safe.
The port number to use when listening for TCP/IP connections.
Use the 3.20 protocol for compatibility with some very old clients. See the section called “Upgrading from Version 3.20 to 3.21”.
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See the section called “Debugging a MySQL Server”.
To change the number of file descriptors available to mysqld. If this is not set or set to 0, then mysqld will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 then mysqld will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of files. You should try increasing this if mysqld gives you the error "Too many open files."
Skip some optimization stages.
With this option, the SHOW DATABASES statement displays only the names of those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and doesn't do anything (it is enabled by default), because there is now a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See the section called “Privileges Provided by MySQL”.
If this is enabled, a user can't create new users with the GRANT statement, if the user doesn't have the INSERT privilege for the mysql.user table or any column in the table.
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
The name to use for shared-memory connections. This option is available only on Windows. It was added in MySQL 4.1.0.
Disable the BDB storage engine. This saves memory and might speed up some operations. Do not use this option if you require BDB tables.
Turn off the ability to select and insert at the same time on MyISAM tables. (This is to be used only if you think you have found a bug in this feature.)
Ignore the DELAY_KEY_WRITE option for all tables. As of MySQL 4.0.3, you should use --delay-key-write=OFF instead. See the section called “Tuning Server Parameters”.
Don't use system locking. To use isamchk or myisamchk, you must shut down the server. See the section called “MySQL Stability”. In MySQL 3.23, you can use CHECK TABLE and REPAIR TABLE to check and repair MyISAM tables. This option previously was named --skip-locking.
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing a mysqladmin flush-privileges or mysqladmin reload command, or by issuing a FLUSH PRIVILEGES statement.)
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See the section called “How MySQL Uses DNS”.
Disable the InnoDB storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require InnoDB tables.
Disable the ISAM storage engine. As of MySQL 4.1, ISAM is disabled by default, so this option applies only if the server was configured with support for ISAM. This option was added in MySQL 4.1.1.
Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all Host column values in the grant tables must be IP numbers or localhost. See the section called “How MySQL Uses DNS”.
Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster storage engine support, this means that the system will only allocate memory and other resources for this storage engine if it is explicitly enabled.
Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See the section called “How MySQL Uses DNS”.
Don't use new, possibly wrong routines.
This is the old form of --skip-symbolic-links, for use before MySQL 4.0.13.
Enable or disable symbolic link support. This option has different effects on Windows and Unix:
On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by creating a directory.sym file that contains the path to the real directory. See the section called “Using Symbolic Links for Databases on Windows”.
On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY options of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See CREATE TABLE.
This option was added in MySQL 4.0.13.
If MySQL is configured with --with-debug=full, 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 don't need it by using the --skip-safemalloc option.
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.
Don't 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 the section called “Debugging a MySQL Server”.
Disable using thread priorities for faster response time.
On Unix, this option specifies the Unix socket file to use for local connections. The default value is /tmp/mysql.sock. On Windows, the option specifies the pipe name to use for local connections that use a named pipe. The default value is MySQL.
Set the SQL mode for MySQL. See the section called “The Server SQL Mode”. This option was added in 3.23.41.
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to “leak” memory, because it's being allocated to the directory entry cache rather than to the disk cache.
Sets the default transaction isolation level, which can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See SET TRANSACTION.
The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. Starting from MySQL 4.1, 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, NetWare, and OS/2. If the MySQL server is acting as a replication slave, you should not set --tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail.
Run the mysqld server as the user having the name user_name or the numeric user ID user_id. (“User” in this context refers to a system login account, not a MySQL user listed in the grant tables.)
This option is mandatory when starting mysqld as root. The server will change its user ID during its startup sequence, causing it to run as that particular user rather than as root. See the section called “General Security Guidelines”.
Starting from MySQL 3.23.56 and 4.0.12: To avoid a possible security hole where a user adds a --user=root option to some my.cnf file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in /etc/my.cnf and datadir/my.cnf are processed before command-line options, so it is recommended that you put a --user option in /etc/my.cnf and specify a value other than root. The option in /etc/my.cnf will be found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.
Display version information and exit.
As of MySQL 4.0, you can assign a value to a server system variable by using an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.
Note that when setting a variable to a value, MySQL might automatically correct it to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.
It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.
You can find a full description for all variables in the section called “Server System Variables”. The section on tuning server parameters includes information on how to optimize them. See the section called “Tuning Server Parameters”.
You can change the values of most system variables for a running server with the SET statement. See SET OPTION.
If you want to restrict the maximum value that a startup option can be set to with SET, you can define this by using the --maximum-var_name command-line option.
The MySQL server can operate in different SQL modes, and (as of MySQL 4.1) can apply these modes differentially for different clients. This allows an application to tailor server operation to its own requirements.
Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.
You can set the default SQL mode by starting mysqld with the --sql-mode="modes" option. The value also can be empty (--sql-mode="") if you want to reset it.
Beginning with MySQL 4.1, you can also change the SQL mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement. 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. Any client can change its session sql_mode value.
modes is a list of different modes separated by comma (‘,’) characters. You can retrieve the current mode by issuing a SELECT @@sql_mode statement. The default value is empty (no modes set).
The most important sql_mode values are probably:
Change syntax and behavior to be more conformant to standard SQL. (New in MySQL 4.1.1)
If a value could not be inserted as given into a transactional table, abort the statement. For a non-transactional table, abort the statement if the value occurs in a single-row statement or the first row of a multiple-row statement. More detail is given later in this section. (New in MySQL 5.0.2)
Make MySQL behave like a “traditional” SQL database system. A simple description of this mode is “give an error instead a warning” when inserting an incorrect value in a column. Note: If you are using non-transactional storage engines, the INSERT/UPDATE will abort as soon as the error is noticed, which may not be what you want. (New in MySQL 5.0.2)
When this manual refers to strict mode, it means a mode where at least one of STRICT_TRANS_TABLES or STRICT_ALL_TABLES is enabled.
The following list describes all the supported modes:
Don't do full checking of dates. Check only that the month is in the range from 1 to 12 and the day is in the range from 1 to 31. This is very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation). This mode applies to DATE and DATETIME columns. It does not apply TIMESTAMP columns, which always require a valid date.
This mode is new in MySQL 5.0.2. Before 5.0.2, this was the default MySQL date-handling mode. As of 5.0.2, enabling strict mode causes the server to require that month and day values be legal, not just in the range from 1 to 12 and 1 to 31. For example, '2004-04-31' is legal with strict mode disabled, but illegal with strict mode enabled. To allow such dates in strict mode, enable ALLOW_INVALID_DATES as well.
Treat ‘"’ as an identifier quote character (like the ‘`’ quote character) and not as a string quote character. You can still use ‘`’ to quote identifers in ANSI mode. With ANSI_QUOTES enabled, you cannot use double quotes to quote a literal string, because it will be interpreted as an identifier. (New in MySQL 4.0.0)
Produce an error in strict mode (otherwise a warning) when we encounter a division by zero (or MOD(X,0)) during an INSERT/ UPDATE. If this mode is not given, MySQL instead returns NULL for divisions by zero. If used with IGNORE, MySQL generates a warning for divisions by zero, but the result of the operation is NULL. (New in MySQL 5.0.2)
Allow spaces between a function name and the ‘(’ character. This forces all function names to be treated as reserved words. As a result, if you want to access any database, table, or column name that is a reserved word, you must quote it. For example, because there is a USER() function, the name of the user table in the mysql database and the User column in that table become reserved, so you must quote them:
SELECT "User" FROM mysql."user";
(New in MySQL 4.0.0)
NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number. (New in MySQL 4.1.1)
This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (This is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with different contents than the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. As of MySQL 4.1.1, mysqldump automatically includes a statement in the dump output to enable NO_AUTO_VALUE_ON_ZERO.
When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers. (New in MySQL 4.0.15)
Don't print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
Don't print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
Don't print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
Don't allow '0000-00-00' as a valid date. You can still insert zero dates with the IGNORE option. (New in MySQL 5.0.2)
Don't accept dates where the month or day part is 0. If used with the IGNORE option, we insert a '0000-00-00' date for any such date. (New in MySQL 5.0.2)
In subtraction operations, don't mark the result as UNSIGNED if one of the operands is unsigned. Note that this makes UNSIGNED BIGINT not 100% usable in all contexts. See the section called “Cast Functions and Operators”. (New in MySQL 4.0.2)
Don't allow queries that in the GROUP BY part refer to a not selected column. (New in MySQL 4.0.0)
Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR. (New in MySQL 4.0.0)
Treat REAL as a synonym for FLOAT rather than as a synonym for DOUBLE. (New in MySQL 4.0.0)
Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail follows. (New in MySQL 5.0.2)
Enable strict mode for transactional storage engines, and when possible for non-transactional storage engines. Additional detail follows. (New in MySQL 5.0.2)
When strict mode is enabled, MySQL returns an error if a value is invalid or missing (no value given for a column that does not have an explicit DEFAULT value in its definition). For transactional tables, this occurs when either of STRICT_ALL_TABLES or STRICT_TRANS_TABLES is enabled. The statement is aborted and rolled back. For non-transactional tables, MySQL handles invalid or missing values as follows:
If the statement inserts or modifies a single row, an error occurs if a value is invalid or missing. The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows, the effect depends on which strict option is enabled:
For STRICT_ALL_TABLES, if a value is invalid or missing for the first row, MySQL returns an error and no rows are changed. If a value is invalid or missing for the second or later row, MySQL returns an error and ignores the rest of the rows. However, in this case, the earlier rows will already have been inserted or updated. This means that you might get a partial update, which might not be what you want. To avoid this, it's best to use single-row statements.
For STRICT_TRANS_TABLES, if a value is invalid or missing for the first row, MySQL returns an error and no rows are changed. For the second or later row, if a value is invalid, MySQL insert the closest valid value. If a value is missing, MySQL inserts the implicit default value for the column data type. In this case, MySQL generates a warning rather than an error and continues processing the statement.
Strict mode disallows invalid date values such as '2004-04-31'. It does not disallow dates with zero parts such as 2004-04-00' or “zero” dates. To disallow these, enable the NO_ZERO_IN_DATE and NO_ZERO_DATE SQL modes.
If you are not using strict mode (that is, neither STRICT_TRANS_TABLES nor STRICT_ALL_TABLES is enabled), MySQL produces a warning for all values that are adjusted when inserted. See SHOW WARNINGS.
The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available as of MySQL 4.1.1, except TRADITIONAL (5.0.2).
Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, ONLY_FULL_GROUP_BY. See the section called “Running MySQL in ANSI Mode”.
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.
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.
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.
The server maintains many system variables that indicate how it is configured. All of them have default values. They can be set at server startup using options on the command line or in option files. Most of them can be set at runtime using the SET statement.
Beginning with MySQL 4.0.3, the mysqld server maintains two kinds of variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections.
When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified in option files or on the command line. After the server starts, those global variables that are dynamic can be changed by connecting to the server and issuing a SET GLOBAL var_name statement. To change a global variable, you must have the SUPER privilege.
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 those session variables that are dynamic, the client can change them by issuing a SET SESSION var_name statement. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.
A change to a global variable is visible to any client that accesses that global variable. However, it affects the corresponding session variable that is initialized from the global variable only for clients that connect after the change. It does not affect the session variable for any client that is already connected (not even that of the client that issues the SET GLOBAL statement).
When setting a variable using a startup option, variable values can be given with a suffix of K, M, or G to indicate kilobytes, megabytes, or gigabytes, respectively. For example, the following command starts the server with a key buffer size of 16 megabytes:
mysqld --key_buffer_size=16M
Before MySQL 4.0, use this syntax instead:
mysqld --set-variable=key_buffer_size=16M
The lettercase of suffix letters does not matter; 16M and 16m are equivalent.
At runtime, use the SET statement to set system variables. In this context, suffix letters cannot be used, but the value can take the form of an expression:
mysql> SET sort_buffer_size = 10 * 1024 * 1024;
To specify explicitly whether to set the global or session variable, use the GLOBAL or SESSION options:
mysql> SET GLOBAL sort_buffer_size = 10 * 1024 * 1024; mysql> SET SESSION sort_buffer_size = 10 * 1024 * 1024;
Without either option, the statement sets the session variable.
The variables that can be set at runtime are listed in the section called “Dynamic System Variables”.
If you want to restrict the maximum value to which a system variable can be set with the SET statement, you can specify this maximum by using an option of the form --maximum-var_name at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M. This feature is available as of MySQL 4.0.2.
You can view system variables and their values by using the SHOW VARIABLES statement. See the section called “System Variables” for more information.
mysql> SHOW VARIABLES; +---------------------------------+------------------------------+ | Variable_name | Value | +---------------------------------+------------------------------| | back_log | 50 | | basedir | /usr/local/mysql | | bdb_cache_size | 8388572 | | bdb_home | /usr/local/mysql | | bdb_log_buffer_size | 32768 | | bdb_logdir | | | bdb_max_lock | 10000 | | bdb_shared_data | OFF | | bdb_tmpdir | /tmp/ | | bdb_version | Sleepycat Software: ... | | binlog_cache_size | 32768 | | bulk_insert_buffer_size | 8388608 | | character_set | latin1 | | character_sets | latin1 big5 czech euc_kr | | concurrent_insert | ON | | connect_timeout | 5 | | convert_character_set | | | datadir | /usr/local/mysql/data/ | | default_week_format | 0 | | delay_key_write | ON | | delayed_insert_limit | 100 | | delayed_insert_timeout | 300 | | delayed_queue_size | 1000 | | flush | OFF | | flush_time | 0 | | ft_boolean_syntax | + -><()~*:""&| | | ft_max_word_len | 84 | | ft_min_word_len | 4 | | ft_query_expansion_limit | 20 | | ft_stopword_file | (built-in) | | have_bdb | YES | | have_innodb | YES | | have_isam | YES | | have_openssl | YES | | have_query_cache | YES | | have_raid | NO | | have_symlink | DISABLED | | init_file | | | innodb_additional_mem_pool_size | 1048576 | | innodb_buffer_pool_size | 8388608 | | innodb_data_file_path | ibdata1:10M:autoextend | | innodb_data_home_dir | | | innodb_fast_shutdown | ON | | innodb_file_io_threads | 4 | | innodb_flush_log_at_trx_commit | 1 | | innodb_flush_method | | | innodb_force_recovery | 0 | | innodb_lock_wait_timeout | 50 | | innodb_log_arch_dir | | | innodb_log_archive | OFF | | innodb_log_buffer_size | 1048576 | | innodb_log_file_size | 5242880 | | innodb_log_files_in_group | 2 | | innodb_log_group_home_dir | ./ | | innodb_mirrored_log_groups | 1 | | innodb_thread_concurrency | 8 | | interactive_timeout | 28800 | | join_buffer_size | 131072 | | key_buffer_size | 16773120 | | key_cache_age_threshold | 300 | | key_cache_block_size | 1024 | | key_cache_division_limit | 100 | | language | /usr/local/mysql/share/... | | large_files_support | ON | | local_infile | ON | | locked_in_memory | OFF | | log | OFF | | log_bin | OFF | | log_slave_updates | OFF | | log_slow_queries | OFF | | log_update | OFF | | log_warnings | 1 | | long_query_time | 10 | | low_priority_updates | OFF | | lower_case_table_names | 0 | | max_allowed_packet | 1047552 | | max_binlog_cache_size | 4294967295 | | max_binlog_size | 1073741824 | | max_connect_errors | 10 | | max_connections | 100 | | max_delayed_threads | 20 | | max_error_count | 64 | | max_heap_table_size | 16777216 | | max_join_size | 4294967295 | | max_relay_log_size | 0 | | max_sort_length | 1024 | | max_tmp_tables | 32 | | max_user_connections | 0 | | max_write_lock_count | 4294967295 | | myisam_max_extra_sort_file_size | 268435456 | | myisam_max_sort_file_size | 2147483647 | | myisam_recover_options | force | | myisam_repair_threads | 1 | | myisam_sort_buffer_size | 8388608 | | net_buffer_length | 16384 | | net_read_timeout | 30 | | net_retry_count | 10 | | net_write_timeout | 60 | | open_files_limit | 1024 | | pid_file | /usr/local/mysql/name.pid | | port | 3306 | | protocol_version | 10 | | query_cache_limit | 1048576 | | query_cache_size | 0 | | query_cache_type | ON | | read_buffer_size | 131072 | | read_rnd_buffer_size | 262144 | | rpl_recovery_rank | 0 | | server_id | 0 | | skip_external_locking | ON | | skip_networking | OFF | | skip_show_database | OFF | | slave_net_timeout | 3600 | | slow_launch_time | 2 | | socket | /tmp/mysql.sock | | sort_buffer_size | 2097116 | | sql_mode | | | table_cache | 64 | | table_type | MYISAM | | thread_cache_size | 3 | | thread_stack | 131072 | | timezone | EEST | | tmp_table_size | 33554432 | | tmpdir | /tmp/:/mnt/hd2/tmp/ | | tx_isolation | READ-COMMITTED | | version | 4.0.4-beta | | wait_timeout | 28800 | +---------------------------------+------------------------------+
Most system variables are described here. Variables with no version indicated have been present since at least MySQL 3.22. InnoDB system variables are listed at InnoDB start.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
Information on tuning these variables can be found in the section called “Tuning Server Parameters”.
This is ON if mysqld was started with --ansi. See the section called “Running MySQL in ANSI Mode”. This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for sql_mode.
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.
In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than your operating system limit will be ineffective.
The MySQL installation base directory. This variable can be set with the --basedir option.
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should start mysqld with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.14.
The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should set this to 0 or start mysqld with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.31.
The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.
The maximum number of locks you can have active on a BDB table (10,000 by default). You should increase this if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:
bdb: Lock table is out of available locks Got error 12 from ...
This variable was added in MySQL 3.23.29.
This is ON if you are using --bdb-shared-data. This variable was added in MySQL 3.23.29.
The value of the --bdb-tmpdir option. This variable was added in MySQL 3.23.14.
See the description for version_bdb.
The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has binary log enabled (--log-bin option). If you often use big, multiple-statement transactions, you can increase this to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. This variable was added in MySQL 3.23.29. See the section called “The Binary Log”.
MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. Note: This cache is used only when adding data to a non-empty table. The default value is 8MB. This variable was added in MySQL 4.0.3. This variable previously was named myisam_bulk_insert_tree_size.
The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.
The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.
The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.
The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.
The server default character set. This variable was added in MySQL 4.1.1.
The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.
The supported character sets. This variable was added in MySQL 3.23.15 and removed in MySQL 4.1.1. (Use SHOW CHARACTER SET for a list of character sets.)
The directory where character sets are installed. This variable was added in MySQL 4.1.2.
The collation of the connection character set. This variable was added in MySQL 4.1.1.
The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.
The server default collation. This variable was added in MySQL 4.1.1.
If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new. This variable was added in MySQL 3.23.7.
The number of seconds the mysqld server waits for a connect packet before responding with Bad handshake.
The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.
The MySQL data directory. This variable can be set with the --datadir option.
The default mode value to use for the WEEK() function. This variable is available as of MySQL 4.0.14.
This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.
| Option | Description |
| OFF | DELAYED_KEY_WRITE is ignored. |
| ON | MySQL honors the DELAY_KEY_WRITE option for CREATE TABLE. This is the default value. |
| ALL | All new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled. |
If DELAY_KEY_WRITE is enabled, this means that the key buffer for tables with this option are not flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP,FORCE). See the section called “mysqld Command-Line Options” and the section called “MyISAM Startup Options”.
Note that --external-locking doesn't offer any protection against index corruption for tables that use delayed key writes.
This variable was added in MySQL 3.23.8.
After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.
How long an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement will wait until there is room in the queue again.
The number of days for automatic binary log removal. The default is 0, which means “no automatic removal.” This variable was added in MySQL 4.1.0.
This is ON if you have started mysqld with the --flush option. This variable was added in MySQL 3.22.9.
If this is set to a non-zero value, all tables will be closed every flush_time seconds to free up resources and sync unflushed data to disk. We recommend this option only on Windows 9x or Me, or on systems with minimal resources available. This variable was added in MySQL 3.22.18.
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. This variable was added in MySQL 4.0.1. See the section called “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 non-alphanumeric character.
Either the first or second character must be a space.
No duplicates are allowed 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.
The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.
The file from which to read the list of stopwords for full-text searches. 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 myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. This variable was added in MySQL 4.0.10.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
The maximum allowed result length for the GROUP_CONCAT() function. This variable was added in MySQL 4.1.0.
YES if mysqld supports ARCHIVE tables. This variable was added in MySQL 4.1.3.
YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.
Whether the zlib compression library is available to the server. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used. This variable was added in MySQL 4.1.1.
Whether the crypt() system call is available to the server. If not, the CRYPT() function cannot be used. This variable was added in MySQL 4.0.10.
Whether the server supports spatial data types. This variable was added in MySQL 4.1.3.
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.
YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.
YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used. This variable was added in MySQL 4.1.2.
YES if mysqld supports SSL (encryption) of the client/server protocol. This variable was added in MySQL 3.23.43.
YES if mysqld supports the query cache. This variable was added in MySQL 4.0.2.
YES if mysqld supports the RAID option. This variable was added in MySQL 3.23.30.
Whether RTREE indexes are available. (These are used for spatial indexed in MyISAM tables.) This variable was added in MySQL 4.1.3.
Whether symbolic link support is enabled. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options. This variable was added in MySQL 4.0.0.
A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global server variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect:
SET GLOBAL init_connect='SET AUTOCOMMIT=0';
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld] init_connect='SET AUTOCOMMIT=0'
Note that the content of init_connect is not executed for users having the SUPER privilege; this is in case that content has been wrongly set (contains a wrong query, for example with a syntax error), thus making all connections fail. Not executing it for SUPER users enables those to open a connection and fix init_connect. This variable was added in MySQL 4.1.2.
The name of the file specified with the --init-file option when you start the server. This is 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. This variable was added in MySQL 3.23.2.
This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.
The InnoDB system variables are listed at InnoDB start.
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
The size of the buffer that is used for full joins (joins that do not use indexes). Normally the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.
Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.
The maximum allowable setting for key_buffer_size is 4GB. 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.
Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache.
For even more speed when writing many rows at the same time, use LOCK TABLES. See LOCK TABLES.
You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. See SHOW.
The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.
The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_used status variable and the buffer block size. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is:
(Key_blocks_used * key_cache_block_size) / key_buffer_size
Before MySQL 4.1.1, key cache blocks are 1024 bytes, so the fraction of the key buffer in use is:
(Key_blocks_used * 1024) / key_buffer_size
See MyISAM key cache.
This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See MyISAM key cache.
The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See MyISAM key cache.
The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See MyISAM key cache.
The language used for error messages.
Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.
The type of license the server has. This variable was added in MySQL 4.0.19.
Whether LOCAL is supported for LOAD DATA INFILE statements. This variable was added in MySQL 4.0.3.
Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.
Whether logging of all queries to the general query log is enabled. See the section called “The General Query Log”.
Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See the section called “The Binary Log”.
The location of the error log. This variable was added in MySQL 4.0.10.
Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this to have any effect. This variable was added in MySQL 3.23.17. See the section called “Replication Startup Options”.
Whether slow queries should be logged. “Slow” is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See the section called “The Slow Query Log”.
Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See the section called “The Update Log”.
Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1.
If a query takes longer than this many seconds, the Slow_queries status variable is incremented. If you are using the --log-slow-queries option, 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. See the section called “The Slow Query Log”.
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named sql_low_priority_updates. It was added in MySQL 3.22.5.
This variable indicates whether the filesystem where the data directory is located has case insensitive filenames. ON means filenames are case insensitive, OFF means they are case sensitive. This variable was added in MySQL 4.0.19.
If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. See the section called “Identifier Case Sensitivity”.
You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.
The maximum size of one packet or any generated/intermediate string.
The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch big (possibly wrong) packets.
You must increase this value if you are using big BLOB columns or long strings. It should be as big as the biggest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter.
If a multiple-statement transaction requires more than this amount of memory, you will get the error Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage. This variable was added in MySQL 3.23.29.
If a write to the binary log exceeds the given value, rotate the binary logs. You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33.
Note if you are using transactions: A transaction is written in one chunk to the binary log, hence it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs bigger than max_binlog_size.
If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.
If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the FLUSH HOSTS statement.
The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that mysqld requires. See the section called “How MySQL Opens and Closes Tables” for comments on file descriptor limits. Also see the section called “Too many connections”.
Don't start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, this disables DELAYED entirely. This variable was added in MySQL 3.23.0.
The maximum number of error, warning, and note messages to be stored for display by SHOW ERRORS or SHOW WARNINGS. This variable was added in MySQL 4.1.0.
This variable sets the maximum size to which MEMORY (HEAP) tables are allowed to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or TRUNCATE TABLE, or altered with ALTER TABLE. This variable was added in MySQL 3.23.0.
This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.
Don't allow SELECT statements that probably will need to examine more than max_join_size row combinations or are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows.
Setting this variable to a value other than DEFAULT resets the SQL_BIG_SELECTS value to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored.
If a query result already is in the query cache, no result size check is performed, because the result has already been computed and it does not burden the server to send it to the client.
This variable previously was named sql_max_join_size.
The cutoff on the size of index values that determines which filesort algorithm to use. See ORDER BY optimization. This variable was added in MySQL 4.1.1
If a write by a replication slave to its relay log exceeds the given value, rotate the relay log. This variable enables you to put different size constraints on relay logs and binary logs. However, setting the variable to 0 makes MySQL use max_binlog_size for both binary logs and relay logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. This variable was added in MySQL 4.0.14. See the section called “Replication Implementation Details”.
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer will assume that no more than this number of key seeks will be required when searching for matching rows in a table by scanning a key, regardless of the actual cardinality of the key (see the section called “SHOW INDEX Syntax”). By setting this to a low value (100?), you can force MySQL to prefer keys instead of table scans.
This variable was added in MySQL 4.0.14.
The number of bytes to use when sorting BLOB or TEXT values. Only the first max_sort_length bytes of each value are used; the rest are ignored.
The maximum number of temporary tables a client can keep open at the same time. (This option doesn't yet do anything.)
The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means “no limit.” This variable was added in MySQL 3.23.34.
After this many write locks, allow some read locks to run in between. This variable was added in MySQL 3.23.7.
The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 8. The default value is 4. This variable was added in MySQL 4.1.2. See myisam_data_pointer_size.
If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
The maximum size of the temporary file MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be bigger than this value, the index will be created using the key cache instead, which is slower. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
The value of the --myisam-recover option. This variable was added in MySQL 3.23.36.
If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1. Note: Multi-threaded repair is still alpha quality code. This variable was added in MySQL 4.0.13.
The buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. This variable was added in MySQL 3.23.16.
On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.
The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout. This variable was added in MySQL 3.23.20.
If a read 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. This variable was added in MySQL 3.23.7.
The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout. This variable was added in MySQL 3.23.20.
This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave mysqld as a startup option. The value is 0 on systems where MySQL can't change the number of open files. This variable was added in MySQL 3.23.20.
Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans. This variable was added in MySQL 5.0.1.
The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the original algorithm used before MySQL 5.0.1 that performs an exhaustive search. This variable was added in MySQL 5.0.1.
The pathname of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.
The port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
The allocation size of memory blocks that are allocated for objects created during query parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit. This variable was added in MySQL 4.0.16.
Don't cache results that are bigger than this. The default value is 1MB. This variable was added in MySQL 4.0.1.
The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in the section called “Query Cache Configuration”. This variable is present from MySQL 4.1.
The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. Note that this amount of memory will be allocated even if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.
Set query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache.
| Option | Description |
| 0 or OFF | Don't cache or retrieve results. Note that this will not deallocate the query cache buffer. To do that, you should set query_cache_size to 0. |
| 1 or ON | Cache all query results except for those that begin with SELECT SQL_NO_CACHE. |
| 2 or DEMAND | Cache results only for queries that begin with SELECT SQL_CACHE. |
This variable was added in MySQL 4.0.3.
Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing queries for the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect. This variable was added in MySQL 4.0.19.
The size of the persistent buffer used for query parsing and execution. This buffer is not freed between queries. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations. This variable was added in MySQL 4.0.16.
The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you might want to increase this value. This variable was added in MySQL 4.0.3. Previously, it was named record_buffer.
When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except from slave threads or from users with the SUPER privilege. This can be useful to ensure that a slave server accepts no updates from clients. This variable was added in MySQL 4.0.14.
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (enabled). This variable was added in MySQL 4.1.1.
When reading rows in sorted order after a sort, the rows are read through this buffer to avoid disk seeks. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries. This variable was added in MySQL 4.0.3. Previously, it was named record_rnd_buffer.
Don't show databases for which the user has no database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also skip_show_database.
This variable was removed in MySQL 4.0.5. Instead, use the SHOW DATABASES privilege to control access by MySQL accounts to database names.
If the MySQL server has been started with the --secure-auth option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is ON, otherwise it is OFF.
You should enable this option if you want to prevent all usage of passwords in old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1.
Server startup will fail with an error if this option is enabled and the privilege tables are in pre-4.1 format.
When used as a client-side option, the client refuses to connect to a server if the server requires a password in old format for the client account.
The value of the --server-id option. It is used for master and slave replication servers. This variable was added in MySQL 3.23.26.
Whether or not the server allows shared-memory connections. Currently, only Windows servers support this. This variable was added in MySQL 4.1.1.
Whether or not the server allows shared-memory connections. Currently, only Windows servers support this. This variable was added in MySQL 4.1.0.
This is OFF if mysqld uses external locking. This variable was added in MySQL 4.0.3. Previously, it was named skip_locking.
This is ON if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable was added in MySQL 3.22.23.
This prevents people from using the SHOW DATABASES statement if they don't have the SHOW DATABASES privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.
Whether to use compression of the slave/master protocol if both the slave and the master support it. This variable was added in MySQL 4.0.3.
The number of seconds to wait for more data from a master/slave connection before aborting the read. This variable was added in MySQL 3.23.40.
If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.
On Unix, this is the Unix socket file used for local client connections. On Windows, this is the name of the named pipe used for local client connections.
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See the section called “Where MySQL Stores Temporary Files”.
The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See the section called “The Server SQL Mode”.
The number of events from the master that a slave server should skip. It was added in MySQL 3.23.33.
This variable is a synonym for table_type. It was added in MySQL 4.1.2.
If positive, the MySQL server will synchronize its binary log to disk (fdatasync()) after every sync_binlog'th write to this binary log. Note that there is one write to the binary log per statement if in autocommit mode, and otherwise one write per transaction. The default value is 0 which does no sync'ing to disk. A value of 1 is the safest choice, because in case of crash you will lose at most one statement/transaction from the binary log; but it is also the slowest choice (unless the disk has a battery-backed cache, which makes sync'ing very fast). This variable was added in MySQL 4.1.3.
This was added as a command-line option in MySQL 4.0.18, and is also a settable global variable since MySQL 4.1.3. If set to 1, when a non-temporary table is created it will synchronize its .frm file to disk (fdatasync()); this is slower but safer in case of crash. Default is 1.
The server system time zone. When the server starts, it attempts to determine the time zone of the host machine. For example, if the TZ environment variable is set, its value is used to set system_time_zone. This variable was added in MySQL 4.1.3.
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See the section called “Server Status Variables”. If the value of Opened_tables is large and you don't do FLUSH TABLES a lot (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable.
For more information about the table cache, see the section called “How MySQL Opens and Closes Tables”.
The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See the section called “mysqld Command-Line Options”.
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't already thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables (see the section called “Server Status Variables” for details) you can see how efficient the thread cache is. This variable was added in MySQL 3.23.16.
On Solaris, mysqld calls thr_setconcurrency() with this value. This function allows applications to give the threads system a hint about the desired number of threads that should be run at the same time. This variable was added in MySQL 3.23.7.
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See the section called “The MySQL Benchmark Suite”.
The current time zone. The initial value of this is 'SYSTEM' (use the value of system_time_zone), but can be specified explicitly at server startup time with the --default-time-zone option. This variable was added in MySQL 4.1.3.
The time zone for the server. This is set from the TZ environment variable when mysqld is started. The time zone also can be set by giving a --timezone argument to mysqld_safe. This variable was added in MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been replaced by the system_time_zone variable. See the section called “Time Zone Problems”.
If an in-memory temporary table exceeds this size, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.
The directory used for temporary files and temporary tables. Starting from MySQL 4.1, 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, NetWare, and OS/2.
This feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail.
This variable was added in MySQL 3.22.4.
The allocation size of memory blocks that are allocated for storing queries that are part of a transaction to be stored in the binary log when doing a commit. This variable was added in MySQL 4.0.16.
The size of the persistent buffer for transaction_alloc_blocks that is not freed between queries. By making this big enough to fit all queries in a common transaction, you can avoid a lot of malloc() calls. This variable was added in MySQL 4.0.16.
The default transaction isolation level. This variable was added in MySQL 4.0.3.
This variable controls whether updates can be made using a view that does not contain a primary key in the underlying table, if the update contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL.
The variable can have two values:
1 or YES: Issue a warning only (not an error message). This is the default value.
0 or NO: Prohibit the update.
This variable was added in MySQL 5.0.2.
The version number for the server.
The BDB storage engine version. This variable was added in MySQL 3.23.31 with the name bdb_version and renamed to version_bdb in MySQL 4.1.1.
The configure script has a --with-comment option that allows a comment to be specified when building MySQL. This variable contains the value of that comment. This variable was added in MySQL 4.0.17.
The type of machine MySQL was built on. This variable was added in MySQL 4.1.1.
The type of operating system MySQL was built on. This variable was added in MySQL 4.0.19.
The number of seconds the server waits for activity on a non-interactive connection before closing it.
On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.
Beginning with MySQL 4.0.3, many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also select their values using SELECT. See the section called “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.
| Variable Name | Value Type | Type |
| autocommit | boolean | SESSION |
| big_tables | boolean | SESSION |
| binlog_cache_size | numeric | GLOBAL |
| bulk_insert_buffer_size | numeric | GLOBAL | SESSION |
| character_set_client | string | GLOBAL | SESSION |
| character_set_connection | string | GLOBAL | SESSION |
| character_set_results | string | GLOBAL | SESSION |
| character_set_server | string | GLOBAL | SESSION |
| collation_connection | string | GLOBAL | SESSION |
| collation_server | string | GLOBAL | SESSION |
| concurrent_insert | boolean | GLOBAL |
| connect_timeout | numeric | GLOBAL |
| convert_character_set | string | GLOBAL | SESSION |
| default_week_format | numeric | GLOBAL | SESSION |
| delay_key_write | OFF | ON | ALL | GLOBAL |
| delayed_insert_limit | numeric | GLOBAL |
| delayed_insert_timeout | numeric | GLOBAL |
| delayed_queue_size | numeric | GLOBAL |
| error_count | numeric | SESSION |
| expire_logs_days | numeric | GLOBAL |
| flush | boolean | GLOBAL |
| flush_time | numeric | GLOBAL |
| foreign_key_checks | boolean | SESSION |
| ft_boolean_syntax | numeric | GLOBAL |
| group_concat_max_len | numeric | GLOBAL | SESSION |
| identity | numeric | SESSION |
| insert_id | boolean | SESSION |
| interactive_timeout | numeric | GLOBAL | SESSION |
| join_buffer_size | numeric | GLOBAL | SESSION |
| key_buffer_size | numeric | GLOBAL |
| last_insert_id | numeric | SESSION |
| local_infile | boolean | GLOBAL |
| log_warnings | numeric | GLOBAL |
| long_query_time | numeric | GLOBAL | SESSION |
| low_priority_updates | boolean | GLOBAL | SESSION |
| max_allowed_packet | numeric | GLOBAL | SESSION |
| max_binlog_cache_size | numeric | GLOBAL |
| max_binlog_size | numeric | GLOBAL |
| max_connect_errors | numeric | GLOBAL |
| max_connections | numeric | GLOBAL |
| max_delayed_threads | numeric | GLOBAL |
| max_error_count | numeric | GLOBAL | SESSION |
| max_heap_table_size | numeric | GLOBAL | SESSION |
| max_insert_delayed_threads | numeric | GLOBAL |
| max_join_size | numeric | GLOBAL | SESSION |
| max_relay_log_size | numeric | GLOBAL |
| max_seeks_for_key | numeric | GLOBAL | SESSION |
| max_sort_length | numeric | GLOBAL | SESSION |
| max_tmp_tables | numeric | GLOBAL | SESSION |
| max_user_connections | numeric | GLOBAL |
| max_write_lock_count | numeric | GLOBAL |
| myisam_data_pointer_size | numeric | GLOBAL |
| myisam_max_extra_sort_file_size | numeric | GLOBAL | SESSION |
| myisam_max_sort_file_size | numeric | GLOBAL | SESSION |
| myisam_repair_threads | numeric | GLOBAL | SESSION |
| myisam_sort_buffer_size | numeric | GLOBAL | SESSION |
| net_buffer_length | numeric | GLOBAL | SESSION |
| net_read_timeout | numeric | GLOBAL | SESSION |
| net_retry_count | numeric | GLOBAL | SESSION |
| net_write_timeout | numeric | GLOBAL | SESSION |
| old_passwords | numeric | GLOBAL | SESSION |
| optimizer_prune_level | numeric | GLOBAL | SESSION |
| optimizer_search_depth | numeric | GLOBAL | SESSION |
| preload_buffer_size | numeric | GLOBAL | SESSION |
| query_alloc_block_size | numeric | GLOBAL | SESSION |
| query_cache_limit | numeric | GLOBAL |
| query_cache_size | numeric | GLOBAL |
| query_cache_type | enumeration | GLOBAL | SESSION |
| query_cache_wlock_invalidate | boolean | GLOBAL | SESSION |
| query_prealloc_size | numeric | GLOBAL | SESSION |
| range_alloc_block_size | numeric | GLOBAL | SESSION |
| read_buffer_size | numeric | GLOBAL | SESSION |
| read_only | numeric | GLOBAL |
| read_rnd_buffer_size | numeric | GLOBAL | SESSION |
| rpl_recovery_rank | numeric | GLOBAL |
| safe_show_database | boolean | GLOBAL |
| secure_auth | boolean | GLOBAL |
| server_id | numeric | GLOBAL |
| slave_compressed_protocol | boolean | GLOBAL |
| slave_net_timeout | numeric | GLOBAL |
| slow_launch_time | numeric | GLOBAL |
| sort_buffer_size | numeric | GLOBAL | SESSION |
| sql_auto_is_null | boolean | SESSION |
| sql_big_selects | boolean | SESSION |
| sql_big_tables | boolean | SESSION |
| sql_buffer_result | boolean | SESSION |
| sql_log_bin | boolean | SESSION |
| sql_log_off | boolean | SESSION |
| sql_log_update | boolean | SESSION |
| sql_low_priority_updates | boolean | GLOBAL | SESSION |
| sql_max_join_size | numeric | GLOBAL | SESSION |
| sql_mode | enumeration | GLOBAL | SESSION |
| sql_quote_show_create | boolean | SESSION |
| sql_safe_updates | boolean | SESSION |
| sql_select_limit | numeric | SESSION |
| sql_slave_skip_counter | numeric | GLOBAL |
| updatable_views_with_limit | enumeration | GLOBAL | SESSION |
| sql_warnings | boolean | SESSION |
| sync_binlog | numeric | GLOBAL |
| sync_frm | boolean | GLOBAL |
| storage_engine | enumeration | GLOBAL | SESSION |
| table_cache | numeric | GLOBAL |
| table_type | enumeration | GLOBAL | SESSION |
| thread_cache_size | numeric | GLOBAL |
| time_zone | string | GLOBAL | SESSION |
| timestamp | boolean | SESSION |
| tmp_table_size | enumeration | GLOBAL | SESSION |
| transaction_alloc_block_size | numeric | GLOBAL | SESSION |
| transaction_prealloc_size | numeric | GLOBAL | SESSION |
| tx_isolation | enumeration | GLOBAL | SESSION |
| unique_checks | boolean | SESSION |
| wait_timeout | numeric | GLOBAL | SESSION |
| warning_count | numeric | SESSION |
Variables that are marked as “string” take a string value. Variables that are marked as “numeric” take a numeric value. Variables that are marked as “boolean” can be set to 0, 1, ON or OFF. 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 enumeration-valued system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1.
The server maintains many status variables that provide information about its operations. You can view these variables and their values by using the SHOW STATUS statement:
mysql> SHOW 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 | 60 | | Created_tmp_tables | 8340 | | Delayed_errors | 0 | | Delayed_insert_threads | 0 | | Delayed_writes | 0 | | Flush_commands | 1 | | Handler_delete | 462604 | | Handler_read_first | 105881 | | Handler_read_key | 27820558 | | Handler_read_next | 390681754 | | Handler_read_prev | 6022500 | | Handler_read_rnd | 30546748 | | Handler_read_rnd_next | 246216530 | | Handler_update | 16945404 | | Handler_write | 60356676 | | Key_blocks_used | 14955 | | Key_read_requests | 96854827 | | Key_reads | 162040 | | Key_write_requests | 7589728 | | Key_writes | 3813196 | | Max_used_connections | 0 | | Not_flushed_delayed_rows | 0 | | Not_flushed_key_blocks | 0 | | Open_files | 2 | | Open_streams | 0 | | Open_tables | 1 | | Opened_tables | 44600 | | Qcache_free_blocks | 36 | | Qcache_free_memory | 138488 | | Qcache_hits | 79570 | | Qcache_inserts | 27087 | | Qcache_lowmem_prunes | 3114 | | Qcache_not_cached | 22989 | | Qcache_queries_in_cache | 415 | | Qcache_total_blocks | 912 | | Questions | 2026873 | | Select_full_join | 0 | | Select_full_range_join | 0 | | Select_range | 99646 | | Select_range_check | 0 | | Select_scan | 30802 | | Slave_open_temp_tables | 0 | | Slave_running | OFF | | Slow_launch_threads | 0 | | Slow_queries | 0 | | Sort_merge_passes | 30 | | Sort_range | 500 | | Sort_rows | 30296250 | | Sort_scan | 4650 | | Table_locks_immediate | 1920382 | | Table_locks_waited | 0 | | Threads_cached | 0 | | Threads_connected | 1 | | Threads_created | 30022 | | Threads_running | 1 | | Uptime | 80380 | +--------------------------+------------+
Many status variables are reset to 0 by the FLUSH STATUS statement.
The status variables have the following meanings. The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. The Qcache_xxx query cache variables were added beginning with MySQL 4.0.1. Otherwise, variables with no version indicated have been present since at least MySQL 3.22.
The number of connections that were aborted because the client died without closing the connection properly. See the section called “Communication Errors and Aborted Connections”.
The number of tries to connect to the MySQL server that failed. See the section called “Communication Errors and Aborted Connections”.
The number of transactions that used the temporary binary log cache but that exceeded the value of binlog_cache_size and used a temporary file to store statements from the transaction. This variable was added in MySQL 4.1.2.
The number of transactions that used the temporary binary log cache. This variable was added in MySQL 4.1.2.
The number of bytes received from all clients. This variable was added in MySQL 3.23.7.
The number of bytes sent to all clients. This variable was added in MySQL 3.23.7.
The number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_insert count DELETE and INSERT statements.
The number of connection attempts (successful or not) to the MySQL server.
The number of temporary tables on disk created automatically by the server while executing statements. This variable was added in MySQL 3.23.24.
How many temporary files mysqld has created. This variable was added in MySQL 3.23.28.
The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is big, you may want to increase the tmp_table_size value to cause temporary tables to be memory-based instead of disk-based.
The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
The number of INSERT DELAYED handler threads in use.
The number of INSERT DELAYED rows written.
The number of executed FLUSH statements.
The number of internal COMMIT statements. This variable was added in MySQL 4.0.2.
The MySQL server can ask the NDB Cluster storage engine if it knows about a table with a given name. This is called discovery. Handler_discover indicates the number of time tables have been discovered. This variable was added in MySQL 4.1.2.
The number of times a row was deleted from a table.
The number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming that col1 is indexed.
The number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed.
The number of requests to read the next row in key order. This will be 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. This variable was added in MySQL 3.23.6.
The number of requests to read a row based on a fixed position. This will be 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 whole tables or you have joins that don't use keys properly.
The number of requests to read the next row in the data file. This will be 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 internal ROLLBACK statements. This variable was added in MySQL 4.0.2.
The number of requests to update a row in a table.
The number of requests to insert a row in a table.
The number of key blocks in the key cache that have changed but haven't yet been flushed to disk. This variable was added in MySQL 4.1.1. It used to be known as Not_flushed_key_blocks.
This variable was added in MySQL 4.1.2. the section called “Server System Variables”.
The number of used blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of key_buffer_size in the section called “Server System Variables”.
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_reads is big, then your key_buffer_size value is probably too small. The cache miss rate can be calculated as Key_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. Useful for comparing the cost of different query plans for the same query. The default value of −1 means that no query has been compiled yet. This variable was added in MySQL 5.0.1.
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 DELAY queues.
The old name for Key_blocks_not_flushed before MySQL 4.1.1.
The number of files that are open.
The number of streams that are open (used mainly for logging).
The number of tables that are open.
The number of tables that have been opened. If Opened_tables is big, your table_cache value is probably too small.
The number of free memory blocks in query cache.
The amount of free memory for query cache.
The number of cache hits.
The number of queries added to the cache.
The number of queries that were deleted from the cache because of low memory.
The number of non-cached queries (not cachable, or due to query_cache_type).
The number of queries registered in the cache.
The total number of blocks in the query cache.
The number of queries that have been sent to the server.
The status of failsafe replication (not yet implemented).
The number of joins that do not use indexes. If this value is not 0, you should carefully check the indexes of your tables. This variable was added in MySQL 3.23.25.
The number of joins that used a range search on a reference table. This variable was added in MySQL 3.23.25.
The number of joins that used ranges on the first table. (It's normally not critical even if this is big.) This variable was added in MySQL 3.23.25.
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.) This variable was added in MySQL 3.23.25.
The number of joins that did a full scan of the first table. This variable was added in MySQL 3.23.25.
The number of temporary tables currently open by the slave SQL thread. This variable was added in MySQL 3.23.29.
This is ON if this server is a slave that is connected to a master. This variable was added in MySQL 3.23.16.
The number of threads that have taken more than slow_launch_time seconds to create. This variable was added in MySQL 3.23.15.
The number of queries that have taken more than long_query_time seconds. See the section called “The Slow Query Log”.
The number of merge passes the sort algorithm has had to do. If this value is large, you should consider increasing the value of the sort_buffer_size system variable. This variable was added in MySQL 3.23.28.
The number of sorts that were done with ranges. This variable was added in MySQL 3.23.25.
The number of sorted rows. This variable was added in MySQL 3.23.25.
The number of sorts that were done by scanning the table. This variable was added in MySQL 3.23.25.
Variables used for SSL connections. These variables were added in MySQL 4.0.0.
The number of times that a table lock was acquired immediately. This variable was added as of MySQL 3.23.33.
The number of times that a table lock could not be acquired 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. This variable was added as of MySQL 3.23.33.
The number of threads in the thread cache. This variable was added in MySQL 3.23.17.
The number of currently open connections.
The number of threads created to handle connections. If Threads_created is big, you may want to increase the thread_cache_size value. The cache hit rate can be calculated as Threads_created/Connections. This variable was added in MySQL 3.23.31.
The number of threads that are not sleeping.
The number of seconds the server has been up.