authentication_windows_log_level
This variable is available only if the authentication_windows Windows authentication plugin is enabled and debugging code is enabled. See Section 6.3.6.3, "The Windows Native Authentication Plugin".
This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.
This variable was added in MySQL 5.5.16.
authentication_windows_use_principal_name
This variable is available only if the authentication_windows Windows authentication plugin is enabled. See Section 6.3.6.3, "The Windows Native Authentication Plugin".
A client that authenticates using the InitSecurityContext() function should provide a string identifying the service to which it connects (targetName). MySQL uses the principal name (UPN) of the account under which the server is running. The UPN has the form user_id@computer_name and need not be registered anywhere to be used. This UPN is sent by the server at the beginning of authentication handshake.
This variable controls whether the server sends the UPN in the initial challenge. By default, the variable is enabled. For security reasons, it can be disabled to avoid sending the server's account name to a client in clear text. If the variable is disabled, the server always sends a 0x00 byte in the first challenge, the client does not specify targetName, and as a result, NTLM authentication is used.
If the server fails to obtain its UPN (which will happen primarily in environments that do not support Kerberos authentication), the UPN is not sent by the server and NTLM authentication is used.
This variable was added in MySQL 5.5.16.
autocommit
The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. If autocommit is 0 and you change it to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 13.3.1, "START TRANSACTION, COMMIT, and ROLLBACK Syntax".
By default, client connections begin with autocommit set to 1. To cause clients to begin with a default of 0, set the global autocommit value by starting the server with the --autocommit=0 option. To set the variable using an option file, include these lines:
[mysqld]autocommit=0
Before MySQL 5.5.8, the global autocommit value cannot be set at startup. As a workaround, set the init_connect system variable:
SET GLOBAL init_connect='SET autocommit=0';
The init_connect variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld]init_connect='SET autocommit=0'
The content of init_connect is not executed for users that have the SUPER privilege (unlike the effect of setting the global autocommit value at startup).
automatic_sp_privileges
When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (The ALTER ROUTINE privilege is required to drop the routine.) The server also automatically drops those privileges from the creator when the routine is dropped. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.
The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.
See also Section 19.2.2, "Stored Routines and MySQL Privileges".
back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.
In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. back_log cannot be set higher than your operating system limit.
basedir
The MySQL installation base directory. This variable can be set with the --basedir option. Relative path names for other variables usually are resolved relative to the base directory.
big_tables
If set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error The table tbl_name is full does not occur for SELECT operations that require a large temporary table. The default value for a new connection is 0 (use in-memory temporary tables). Normally, you should never need to set this variable, because in-memory tables are automatically converted to disk-based tables as required.
Note
This variable was formerly named sql_big_tables.
bulk_insert_buffer_size
MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE when adding data to nonempty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.
character_set_client
The character set for statements that arrive from the client. The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 10.1.4, "Connection Character Sets and Collations".) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests:
The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.
The client requests a character set not known to the server. For example, a Japanese-enabled client requests sjis when connecting to a server not configured with sjis support.
mysqld was started with the --skip-character-set-client-handshake option, which causes it to ignore client character set configuration. This reproduces MySQL 4.0 behavior and is useful should you wish to upgrade the server without upgrading all the clients.
ucs2, utf16, and utf32 cannot be used as a client character set, which means that they also do not work for SET NAMES or SET CHARACTER SET.
character_set_connection
The character set used for literals that do not have a character set introducer and for number-to-string conversion.
character_set_database
The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server.
character_set_filesystem
The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, which means that no conversion occurs. For systems on which multi-byte file names are permitted, a different value may be more appropriate. For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8'.
character_set_results
The character set used for returning query results such as result sets or error messages to the client.
character_set_server
The server's default character set.
character_set_system
The character set used by the server for storing identifiers. The value is always utf8.
character_sets_dir
The directory where character sets are installed.
collation_connection
The collation of the connection character set.
collation_database
The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server.
collation_server
The server's default collation.
completion_type
The transaction completion type. This variable can take the values shown in the following table. As of MySQL 5.5.3, the variable can be assigned using either the name values or corresponding integer values. Before 5.5.3, only the integer values can be used.
completion_type affects transactions that begin with START TRANSACTION or BEGIN and end with COMMIT or ROLLBACK. It does not apply to implicit commits resulting from execution of the statements listed in Section 13.3.3, "Statements That Cause an Implicit Commit". It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.
concurrent_insert
If AUTO (the default), MySQL permits INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file. If you start mysqld with --skip-new, this variable is set to NEVER.
This variable can take the values shown in the following table. As of MySQL 5.5.3, the variable can be assigned using either the name values or corresponding integer values. Before 5.5.3, only the integer values can be used.
See also Section 8.10.3, "Concurrent Inserts".
connect_timeout
The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 10 seconds.
Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.
datadir
The MySQL data directory. This variable can be set with the --datadir option.
date_format
This variable is unused.
datetime_format
This variable is unused.
debug
This variable indicates the current debugging settings. It is available only for servers built with debugging support. The initial value comes from the value of instances of the --debug option given at server startup. The global and session values may be set at runtime; the SUPER privilege is required, even for the session value.
Assigning a value that begins with + or - cause the value to added to or subtracted from the current value:
mysql> SET debug = 'T';mysql> SELECT @@debug;+---------+| @@debug |+---------+| T |+---------+mysql> SET debug = '+P';mysql> SELECT @@debug;+---------+| @@debug |+---------+| P:T |+---------+mysql> SET debug = '-P';mysql> SELECT @@debug;+---------+| @@debug |+---------+| T |+---------+
debug_sync
This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 option (see Section 2.10.4, "MySQL Source-Configuration Options"). If Debug Sync is not compiled in, this system variable is not available.
The global variable value is read only and indicates whether the facility is enabled. By default, Debug Sync is disabled and the value of debug_sync is OFF. If the server is started with --debug-sync-timeout=N, where N is a timeout value greater than 0, Debug Sync is enabled and the value of debug_sync is ON - current signal followed by the signal name. Also, N becomes the default timeout for individual synchronization points.
The session value can be read by any user and will have the same value as the global variable. The session value can be set by users that have the SUPER privilege to control synchronization points.
For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.
default_storage_engine
The default storage engine. To set the storage engine at server startup, use the --default-storage-engine option. See Section 5.1.3, "Server Command Options".
If you disable the default storage engine at server startup, you must set the default engine to a different engine or the server will not start.
This variable was added in MySQL 5.5.3 to be used in preference to storage_engine, which is now deprecated.
default_week_format
The default mode value to use for the WEEK() function. See Section 12.7, "Date and Time Functions".
delay_key_write
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.
If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover-options option (for example, --myisam-recover-options=BACKUP,FORCE). See Section 5.1.3, "Server Command Options", and Section 14.5.1, "MyISAM Startup Options".
Warning
If you enable external locking with --external-locking, there is no protection against index corruption for tables that use delayed key writes.
delayed_insert_limit
After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it permits them to execute before continuing to insert delayed rows.
delayed_insert_timeout
How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
delayed_queue_size
This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement waits until there is room in the queue again.
div_precision_increment
This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.
mysql> SELECT 1/7;+--------+| 1/7 |+--------+| 0.1429 |+--------+mysql> SET div_precision_increment = 12;mysql> SELECT 1/7;+----------------+| 1/7 |+----------------+| 0.142857142857 |+----------------+
engine_condition_pushdown
The engine condition pushdown optimization enables processing for certain comparisons to be "pushed down" to the storage engine level for more efficient execution. For more information, see Section 8.13.3, "Engine Condition Pushdown Optimization".
Engine condition pushdown is used only by the NDBCLUSTER storage engine. Enabling this optimization on a MySQL Server acting as a MySQL Cluster SQL node causes WHERE conditions on unindexed columns to be evaluated on the cluster's data nodes and only the rows that match to be sent back to the SQL node that issued the query. This greatly reduces the amount of cluster data that must be sent over the network, increasing the efficiency with which results are returned.
The engine_condition_pushdown variable controls whether engine condition pushdown is enabled. By default, this variable is ON (1). Setting it to OFF (0) disables pushdown.
This variable is deprecated as of MySQL 5.5.3 and is removed in MySQL 5.6. Use the engine_condition_pushdown flag of the optimizer_switch variable instead. See Section 8.8.5.2, "Controlling Switchable Optimizations".
error_count
The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.18, "SHOW ERRORS Syntax".
event_scheduler
This variable indicates the status of the Event Scheduler; possible values are ON, OFF, and DISABLED, with the default being OFF. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in the Overview section of the Events chapter.
expire_logs_days
The number of days for automatic binary log file removal. The default is 0, which means "no automatic removal." Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.2, "MySQL Server Logs".
To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 13.4.1.1, "PURGE BINARY LOGS Syntax".
external_user
The external user name used during the authentication process, as set by the plugin used to authenticate the client. With native (built-in) MySQL authentication, or if the plugin does not set the value, this variable is NULL. See Section 6.3.7, "Proxy Users".
This variable was added in MySQL 5.5.7.
flush
If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section C.5.4.2, "What to Do If MySQL Keeps Crashing". This variable is set to ON if you start mysqld with the --flush option.
flush_time
If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on systems with minimal resources.
foreign_key_checks
If set to 1 (the default), foreign key constraints for InnoDB tables are checked. If set to 0, they are ignored. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. See Section 14.3.5.4, "FOREIGN KEY Constraints".
Setting foreign_key_checks to 0 also affects data definition statements: DROP SCHEMA drops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.
ft_boolean_syntax
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 12.9.2, "Boolean Full-Text Searches".
The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:
Operator function is determined by position within the string.
The replacement value must be 14 characters.
Each character must be an ASCII nonalphanumeric character.
Either the first or second character must be a space.
No duplicates are permitted except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.
Positions 10, 13, and 14 (which by default are set to ":", "&", and "|") are reserved for future extensions.
ft_max_word_len
The maximum length of the word to be included in a FULLTEXT index.
Note
FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_min_word_len
The minimum length of the word to be included in a FULLTEXT index.
Note
FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_query_expansion_limit
The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION.
ft_stopword_file
The file from which to read the list of stopwords for full-text searches. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the storage/myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. See also Section 12.9.4, "Full-Text Stopwords".
Note
FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.
general_log
Whether the general query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The default value depends on whether the --general_log option is given. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.
general_log_file
The name of the general query log file. The default value is host_name.log, but the initial value can be changed with the --general_log_file option.
group_concat_max_len
The maximum permitted result length in bytes for the GROUP_CONCAT() function. The default is 1024.
have_compress
YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used.
have_crypt
YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used.
have_csv
YES if mysqld supports CSV tables, NO if not.
This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.
have_dynamic_loading
YES if mysqld supports dynamic loading of plugins, NO if not.
have_geometry
YES if the server supports spatial data types, NO if not.
have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.
This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.
have_openssl
This variable is an alias for have_ssl.
have_partitioning
YES if mysqld supports partitioning.
This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.
have_profiling
YES if statement profiling capability is present, NO if not. If present, the profiling system variable controls whether this capability is enabled or disabled. See Section 13.7.5.32, "SHOW PROFILES Syntax".
have_query_cache
YES if mysqld supports the query cache, NO if not.
have_rtree_keys
YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)
have_ssl
YES if mysqld supports SSL connections, NO if not. DISABLED indicates that the server was compiled with SSL support, but but was not started with the appropriate --ssl-xxx options. For more information, see Section 6.3.8.2, "Configuring MySQL for SSL".
have_symlink
YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options, and on Windows for support of data directory symlinks. If the server is started with the --skip-symbolic-links option, the value is DISABLED.
hostname
The server sets this variable to the server host name at startup.
identity
This variable is a synonym for the last_insert_id variable. It exists for compatibility with other database systems. You can read its value with SELECT @@identity, and set it using SET identity.
init_connect
A string to be executed by the server for each client that connects. The string consists of one or more SQL statements, separated by semicolon characters. For example, each client session begins by default with autocommit mode enabled. For older servers (before MySQL 5.5.8), there is no global autocommit system variable to specify that autocommit should be disabled by default, but as a workaround init_connect can be used to achieve the same effect:
SET GLOBAL init_connect='SET autocommit=0';
The init_connect variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld]init_connect='SET autocommit=0'
The content of init_connect is not executed for users that have the SUPER privilege. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.
init_file
The name of the file specified with the --init-file option when you start the server. This should be a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. No statement terminator such as ;, \g, or \G should be given at the end of each statement.
Note that the --init-file option is unavailable if MySQL was configured with the DISABLE_GRANT_OPTIONS compiler flag. See Section 2.10.4, "MySQL Source-Configuration Options".
innodb_xxx
InnoDB system variables are listed in Section 14.3.4, "InnoDB Startup Options and System Variables". These variables control many aspects of storage, memory use, and I/O patterns for InnoDB tables, and are especially important now that InnoDB is the default storage engine.
insert_id
The value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.
interactive_timeout
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
join_buffer_size
The minimum size of the buffer that is used for plain index scans, range index scans, and joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary. There is no gain from setting the buffer larger than required to hold each matching row, and all joins allocate at least the minimum size, so use caution in setting this variable to a large value globally. It is better to keep the global setting small and change to a larger setting only in sessions that are doing large joins. Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.
The maximum permissible setting for join_buffer_size is 4GB. Values larger than 4GB are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).
keep_files_on_create
If a MyISAM table is created with no DATA DIRECTORY option, the .MYD file is created in the database directory. By default, if MyISAM finds an existing .MYD file in this case, it overwrites it. The same applies to .MYI files for tables created with no INDEX DIRECTORY option. To suppress this behavior, set the keep_files_on_create variable to ON (1), in which case MyISAM will not overwrite existing files and returns an error instead. The default value is OFF (0).
If a MyISAM table is created with a DATA DIRECTORY or INDEX DIRECTORY option and an existing .MYD or .MYI file is found, MyISAM always returns an error. It will not overwrite a file in the specified directory.
key_buffer_size
Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.
The maximum permissible setting for key_buffer_size is 4GB on 32-bit platforms. Values larger than 4GB are permitted for 64-bit platforms. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. The value of this variable indicates the amount of memory requested. Internally, the server allocates as much memory as possible up to this amount, but the actual allocation might be less.
You can increase the value to get better index handling for all reads and multiple writes; on a system whose primary function is to run MySQL using the MyISAM storage engine, 25% of the machine's total memory is an acceptable value for this variable. However, you should be aware that, if you make the value too large (for example, more than 50% of the machine's total memory), your system might start to page and become extremely slow. This is because MySQL relies on the operating system to perform file system caching for data reads, so you must leave some room for the file system cache. You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.
For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 8.2.2.1, "Speed of INSERT Statements".
You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 13.7.5, "SHOW Syntax".) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.
The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size, which is available from the key_cache_block_size system variable:
1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)
This value is an approximation because some space in the key buffer is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).
It is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 8.9.2, "The MyISAM Key Cache".
key_cache_age_threshold
This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See Section 8.9.2, "The MyISAM Key Cache".
key_cache_block_size
The size in bytes of blocks in the key cache. The default value is 1024. See Section 8.9.2, "The MyISAM Key Cache".
key_cache_division_limit
The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. See Section 8.9.2, "The MyISAM Key Cache".
language
The directory where error messages are located. See Section 10.2, "Setting the Error Message Language".
language is removed as of MySQL 5.5.0. Similar information is available from the lc_messages_dir and lc_messages variables.
large_files_support
Whether mysqld was compiled with options for large file support.
large_pages
Whether large page support is enabled (via the --large-pages option). See Section 8.11.4.2, "Enabling Large Page Support".
large_page_size
If large page support is enabled, this shows the size of memory pages. Currently, large memory pages are supported only on Linux; on other platforms, the value of this variable is always 0. See Section 8.11.4.2, "Enabling Large Page Support".
last_insert_id
The value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.
lc_messages
The locale to use for error messages. The server converts the value to a language name and combines it with the value of the lc_messages_dir to produce the location for the error message file. See Section 10.2, "Setting the Error Message Language".
lc_messages_dir
The directory where error messages are located. The value is used together with the value of lc_messages to produce the location for the error message file. See Section 10.2, "Setting the Error Message Language".
lc_time_names
This variable specifies the locale that controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting. For further information, see Section 10.7, "MySQL Server Locale Support".
license
The type of license the server has.
local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. If this variable is disabled, clients cannot use LOCAL in LOAD DATA statements. See Section 6.1.6, "Security Issues with LOAD DATA LOCAL".
lock_wait_timeout
This variable specifies the timeout in seconds for attempts to acquire metadata locks. The permissible values range from 1 to 31536000 (1 year). The default is 31536000.
This timeout applies to all statements that use metadata locks. These include DML and DDL operations on tables, views, stored procedures, and stored functions, as well as LOCK TABLES, FLUSH TABLES WITH READ LOCK, and HANDLER statements.
This timeout does not apply to implicit accesses to system tables in the mysql database, such as grant tables modified by GRANT or REVOKE statements or table logging statements. The timeout does apply to system tables accessed directly, such as with SELECT or UPDATE.
The timeout value applies separately for each metadata lock attempt. A given statement can require more than one lock, so it is possible for the statement to block for longer than the lock_wait_timeout value before reporting a timeout error. When lock timeout occurs, ER_LOCK_WAIT_TIMEOUT is reported.
lock_wait_timeout does not apply to delayed inserts, which always execute with a timeout of 1 year. This is done to avoid unnecessary timeouts because a session that issues a delayed insert receives no notification of delayed insert timeouts.
This variable was added in MySQL 5.5.3.
locked_in_memory
Whether mysqld was locked in memory with --memlock.
log
Whether logging of all statements to the general query log is enabled. See Section 5.2.3, "The General Query Log".
This variable is deprecated and is removed in MySQL 5.6. Use general_log instead.
log_bin
Whether the binary log is enabled. If the --log-bin option is used, then the value of this variable is ON; otherwise it is OFF. This variable reports only on the status of binary logging (enabled or disabled); it does not actually report the value to which --log-bin is set.
See Section 5.2.4, "The Binary Log".
log_bin_trust_function_creators
This variable applies when binary logging is enabled. It controls whether stored function creators can be trusted not to create stored functions that will cause unsafe events to be written to the binary log. If set to 0 (the default), users are not permitted to create or alter stored functions unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege. A setting of 0 also enforces the restriction that a function must be declared with the DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. This variable also applies to trigger creation. See Section 19.7, "Binary Logging of Stored Programs".
log_error
The location of the error log.
log_output
The destination for general query log and slow query log output. The value can be a comma-separated list of one or more of the words TABLE (log to tables), FILE (log to files), or NONE (do not log to tables or files). The default value is FILE. NONE, if present, takes precedence over any other specifiers. If the value is NONE log entries are not written even if the logs are enabled. If the logs are not enabled, no logging occurs even if the value of log_output is not NONE. For more information, see Section 5.2.1, "Selecting General Query and Slow Query Log Output Destinations".
log_queries_not_using_indexes
Whether queries that do not use indexes are logged to the slow query log. See Section 5.2.5, "The Slow Query Log".
log_slow_queries
Whether slow queries should be logged. "Slow" is determined by the value of the long_query_time variable. See Section 5.2.5, "The Slow Query Log".
This variable is deprecated and is removed in MySQL 5.6. Use slow_query_log instead.
log_warnings
Whether to produce additional warning messages to the error log. It is enabled (1) by default and can be disabled by setting it to 0. Aborted connections and access-denied errors for new connection attempts are logged if the value is greater than 1. The server logs messages about statements that are unsafe for statement-based logging only if the value is greater than 0.
long_query_time
If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If the slow query log is enabled, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds. For logging to a file, times are written including the microseconds part. For logging to tables, only integer times are written; the microseconds part is ignored. See Section 5.2.5, "The Slow Query Log".
low_priority_updates
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This affects only storage engines that use only table-level locking (such as MyISAM, MEMORY, and MERGE). This variable previously was named sql_low_priority_updates.
lower_case_file_system
This variable describes the case sensitivity of file names on the file system where the data directory is located. OFF means file names are case sensitive, ON means they are not case sensitive. This variable is read only because it reflects a file system attribute and setting it would have no effect on the file system.
lower_case_table_names
If set to 0, table names are stored as specified and comparisons are case sensitive. If set to 1, table names are stored in lowercase on disk and comparisons are not case sensitive. If set to 2, table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. For additional information, see Section 9.2.2, "Identifier Case Sensitivity".
You should not set this variable to 0 if you are running MySQL on a system that has case-insensitive file names (such as Windows or Mac OS X). If you set this variable to 0 on such a system and access MyISAM tablenames using different lettercases, index corruption may result. On Windows the default value is 1. On Mac OS X, the default value is 2.
If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.
The setting of this variable has no effect on replication filtering options. This is a known issue which is fixed in MySQL 5.6. See Section 16.2.3, "How Servers Evaluate Replication Filtering Rules", for more information.
You should not use different settings for lower_case_table_names on replication masters and slaves. In particular, you should not do this when the slave uses a case-sensitive file system, as this can cause replication to fail. This is a known issue which is fixed in MySQL 5.6. For more information, see Section 16.4.1.35, "Replication and Variables".
max_allowed_packet
The maximum size of one packet or any generated/intermediate string.
The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch large (possibly incorrect) packets.
You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 1GB. The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple.
When you change the message buffer size by changing the value of the max_allowed_packet variable, you should also change the buffer size on the client side if your client program permits it. On the client side, max_allowed_packet has a default of 1GB. Some programs such as mysql and mysqldump enable you to change the client-side value by setting max_allowed_packet on the command line or in an option file.
The session value of this variable is read only.
max_connect_errors
If more than this many successive connection requests from a host are interrupted without a successful connection, the server blocks that host from further connections. You can unblock blocked hosts by flushing the host cache. To do so, issue a FLUSH HOSTS statement or execute a mysqladmin flush-hosts command. If a connection is established successfully within fewer than max_connect_errors attempts after a previous connection was interrupted, the error count for the host is cleared to zero. However, once a host is blocked, flushing the host cache is the only way to unblock it.
max_connections
The maximum permitted number of simultaneous client connections. By default, this is 151. See Section C.5.2.7, "Too many connections", for more information.
Increasing this value increases the number of file descriptors that mysqld requires. See Section 8.4.3.1, "How MySQL Opens and Closes Tables", for comments on file descriptor limits.
max_delayed_threads
Do not start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row is inserted as if the DELAYED attribute was not specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, this disables DELAYED entirely.
For the SESSION value of this variable, the only valid values are 0 or the GLOBAL value.
max_error_count
The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS and SHOW WARNINGS statements.
max_heap_table_size
This variable sets the maximum size to which user-created MEMORY tables are permitted to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or altered with ALTER TABLE or TRUNCATE TABLE. A server restart also sets the maximum size of existing MEMORY tables to the global max_heap_table_size value.
This variable is also used in conjunction with tmp_table_size to limit the size of internal in-memory tables. See Section 8.4.3.3, "How MySQL Uses Internal Temporary Tables".
max_heap_table_size is not replicated. See Section 16.4.1.20, "Replication and MEMORY Tables", and Section 16.4.1.35, "Replication and Variables", for more information.
max_insert_delayed_threads
This variable is a synonym for max_delayed_threads.
max_join_size
Do not permit SELECT statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than max_join_size disk seeks. By setting this value, you can catch 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 value of sql_big_selects to 0. If you set the sql_big_selects value again, the max_join_size variable is ignored.
If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.
This variable previously was named sql_max_join_size.
max_length_for_sort_data
The cutoff on the size of index values that determines which filesort algorithm to use. See Section 8.13.9, "ORDER BY Optimization".
max_long_data_size
The maximum size of parameter values that can be sent with the mysql_stmt_send_long_data() C API function. If not set at server startup, the default is the value of the max_allowed_packet system variable. This variable is deprecated. In MySQL 5.6, it is removed and the maximum parameter size is controlled by max_allowed_packet.
max_prepared_stmt_count
This variable limits the total number of prepared statements in the server. (The sum of the number of prepared statements across all sessions.) It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. The default value is 16,382. The permissible range of values is from 0 to 1 million. Setting the value to 0 disables prepared statements.
max_relay_log_size
If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 16.2.1, "Replication Implementation Details".
max_seeks_for_key
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 13.7.5.23, "SHOW INDEX Syntax"). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.
max_sort_length
The number of bytes to use when sorting data values. Only the first max_sort_length bytes of each value are used; the rest are ignored.
max_sp_recursion_depth
The number of times that any given stored procedure may be called recursively. The default value for this option is 0, which completely disables recursion in stored procedures. The maximum value is 255.
Stored procedure recursion increases the demand on thread stack space. If you increase the value of max_sp_recursion_depth, it may be necessary to increase thread stack size by increasing the value of thread_stack at server startup.
max_tmp_tables
This variable is unused.
max_user_connections
The maximum number of simultaneous connections permitted to any given MySQL user account. A value of 0 (the default) means "no limit."
This variable has a global value that can be set at server startup or runtime. It also has a read-only session value that indicates the effective simultaneous-connection limit that applies to the account associated with the current session. The session value is initialized as follows:
If the user account has a nonzero MAX_USER_CONNECTIONS resource limit, the session max_user_connections value is set to that limit.
Otherwise, the session max_user_connections value is set to the global value.
Account resource limits are specified using the GRANT statement. See Section 6.3.4, "Setting Account Resource Limits", and Section 13.7.1.3, "GRANT Syntax".
max_write_lock_count
After this many write locks, permit some pending read lock requests to be processed in between.
metadata_locks_cache_size
The size of the metadata locks cache. The server uses this cache to avoid creation and destruction of synchronization objects. This is particularly helpful on systems where such operations are expensive, such as Windows XP. This variable was added in MySQL 5.5.19.
min_examined_row_limit
Queries that examine fewer than this number of rows are not logged to the slow query log.
myisam_data_pointer_size
The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 6. See Section C.5.2.12, "The table is full".
myisam_max_sort_file_size
The maximum size of the temporary file that MySQL is permitted to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. The value is given in bytes.
The default value is 2GB. If MyISAM index files exceed this size and disk space is available, increasing the value may help performance. The space must be available in the file system containing the directory where the original index file is located.
myisam_mmap_size
The maximum amount of memory to use for memory mapping compressed MyISAM files. If many compressed MyISAM tables are used, the value can be decreased to reduce the likelihood of memory-swapping problems. This variable was added in MySQL 5.5.1.
myisam_recover_options
The value of the --myisam-recover-options option. See Section 5.1.3, "Server Command Options".
myisam_repair_threads
If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1.
Note
Multi-threaded repair is still beta-quality code.
myisam_sort_buffer_size
The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE.
The maximum permissible setting for myisam_sort_buffer_size is 4GB. Values larger than 4GB are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).
myisam_stats_method
How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has three possible values, nulls_equal, nulls_unequal, and nulls_ignored. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1. For nulls_ignored, NULL values are ignored.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 8.3.7, "InnoDB and MyISAM Index Statistics Collection".
myisam_use_mmap
Use memory mapping for reading and writing MyISAM tables.
named_pipe
(Windows only.) Indicates whether the server supports connections over named pipes.
net_buffer_length
Each client thread is associated with a connection buffer and result buffer. Both begin with a size given by net_buffer_length but are dynamically enlarged up to max_allowed_packet bytes as needed. The result buffer shrinks to net_buffer_length after each SQL statement.
This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of statements sent by clients. If statements exceed this length, the connection buffer is automatically enlarged. The maximum value to which net_buffer_length can be set is 1MB.
The session value of this variable is read only.
net_read_timeout
The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout.
net_retry_count
If a read or write on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads.
net_write_timeout
The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout.
new
This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. In MySQL 5.5, its value is always OFF.
old
old is a compatibility variable. It is disabled by default, but can be enabled at startup to revert the server to behaviors present in older versions.
Currently, when old is enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with no FOR clause apply only to how indexes are used for row retrieval and not to resolution of ORDER BY or GROUP BY clauses. (See Section 13.2.9.3, "Index Hint Syntax".) Take care about enabling this in a replication setup. With statement-based binary logging, having different modes for the master and slaves might lead to replication errors.
old_alter_table
When this variable is enabled, the server does not use the optimized method of processing an ALTER TABLE operation. It reverts to using a temporary table, copying over the data, and then renaming the temporary table to the original, as used by MySQL 5.0 and earlier. For more information on the operation of ALTER TABLE, see Section 13.1.7, "ALTER TABLE Syntax".
old_passwords
This variable determines the type of password hashing performed by the PASSWORD() function and statements such as CREATE USER and SET PASSWORD. The following table shows the permitted values of old_passwords, as well as the type of password hashing each value produces, and the plugins that use each hashing type.
If old_passwords=1, PASSWORD('str') returns the same value as OLD_PASSWORD('str').
For information about authentication plugins and hashing formats, see Section 6.3.6, "Pluggable Authentication", and Section 6.1.2.4, "Password Hashing in MySQL".
one_shot
This is not a variable, but it can be used when setting some variables. It is described in Section 13.7.4, "SET Syntax".
open_files_limit
The number of files that the operating system permits mysqld to open. This is the real value permitted by the system and might be different from the value you gave using the --open-files-limit option to mysqld or mysqld_safe. The value is 0 on systems where MySQL cannot change the number of open files.
optimizer_prune_level
Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans.
optimizer_search_depth
The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to 63, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. The value of 63 is deprecated and will be treated as invalid in a future MySQL release.
optimizer_switch
The optimizer_switch system variable enables control over optimizer behavior. The value of this variable is a set of flags, each of which has a value of on or off to indicate whether the corresponding optimizer behavior is enabled or disabled. This variable has global and session values and can be changed at runtime. The global default can be set at server startup.
To see the current set of optimizer flags, select the variable value:
mysql> SELECT @@optimizer_switch\G*************************** 1. row ***************************@@optimizer_switch: index_merge=on,index_merge_union=on, index_merge_sort_union=on, index_merge_intersection=on, engine_condition_pushdown=on
For more information about the syntax of this variable and the optimizer behaviors that it controls, see Section 8.8.5.2, "Controlling Switchable Optimizations".
performance_schema_xxx
Performance Schema system variables are listed in Section 21.8, "Performance Schema System Variables".
pid_file
The path name of the process ID (PID) file. This variable can be set with the --pid-file option.
plugin_dir
The path name of the plugin directory.
If the plugin directory is writable by the server, it may be possible for a user to write executable code to a file in the directory using SELECT ... INTO DUMPFILE. This can be prevented by making plugin_dir read only to the server or by setting --secure-file-priv to a directory where SELECT writes can be made safely.
port
The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
preload_buffer_size
The size of the buffer that is allocated when preloading indexes.
profiling
If set to 0 or OFF (the default), statement profiling is disabled. If set to 1 or ON, statement profiling is enabled and the SHOW PROFILE and SHOW PROFILES statements provide access to profiling information. See Section 13.7.5.32, "SHOW PROFILES Syntax".
profiling_history_size
The number of statements for which to maintain profiling information if profiling is enabled. The default value is 15. The maximum value is 100. Setting the value to 0 effectively disables profiling. See Section 13.7.5.32, "SHOW PROFILES Syntax".
protocol_version
The version of the client/server protocol used by the MySQL server.
proxy_user
If the current client is a proxy for another user, this variable is the proxy user account name. Otherwise, this variable is NULL. See Section 6.3.7, "Proxy Users".
This variable was added in MySQL 5.5.7.
pseudo_thread_id
This variable is for internal server use.
query_alloc_block_size
The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this parameter.
query_cache_limit
Do not cache results that are larger than this number of bytes. The default value is 1MB.
query_cache_min_res_unit
The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 8.9.3.3, "Query Cache Configuration".
query_cache_size
The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. To reduce overhead significantly, you should also start the server with query_cache_type=0 if you will not be using the query cache. The permissible values are multiples of 1024; other values are rounded down to the nearest multiple. Note that query_cache_size bytes of memory are allocated even if query_cache_type is set to 0. See Section 8.9.3.3, "Query Cache Configuration", for more information.
The query cache needs a minimum size of about 40KB to allocate its structures. (The exact size depends on system architecture.) If you set the value of query_cache_size too small, a warning will occur, as described in Section 8.9.3.3, "Query Cache Configuration".
query_cache_type
Set the query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache. Possible values are shown in the following table.
This variable defaults to ON.
If the server is started with query_cache_type set to 0, it does not acquire the query cache mutex at all, which means that the query cache cannot be enabled at runtime and there is reduced overhead in query execution.
query_cache_wlock_invalidate
Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing statements that read from the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect.
query_prealloc_size
The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.
rand_seed1
The rand_seed1 and rand_seed2 variables exist as session variables only, and can be set but not read. The variables-but not their values-are shown in the output of SHOW VARIABLES.
The purpose of these variables is to support replication of the RAND() function. For statements that invoke RAND(), the master passes two values to the slave, where they are used to seed the random number generator. The slave uses these values to set the session variables rand_seed1 and rand_seed2 so that RAND() on the slave generates the same value as on the master.
rand_seed2
See the description for rand_seed1.
range_alloc_block_size
The size of blocks that are allocated when doing range optimization.
read_buffer_size
Each thread that does a sequential scan for a MyISAM table allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value, which defaults to 131072. The value of this variable should be a multiple of 4KB. If it is set to a value that is not a multiple of 4KB, its value will be rounded down to the nearest multiple of 4KB.
The maximum permissible setting for read_buffer_size is 2GB.
For more information about memory use during different operations, see Section 8.11.4.1, "How MySQL Uses Memory".
read_only
This variable is off by default. When it is enabled, the server permits no updates except from users that have the SUPER privilege or (on a slave server) from updates performed by slave threads. In replication setups, it can be useful to enable read_only on slave servers to ensure that slaves accept updates only from the master server and not from clients.
read_only does not apply to TEMPORARY tables, nor does it prevent the server from inserting rows into the log tables (see Section 5.2.1, "Selecting General Query and Slow Query Log Output Destinations"). This variable does not prevent the use of ANALYZE TABLE or OPTIMIZE TABLE statements because its purpose is to prevent changes to table structure or contents. Analysis and optimization do not qualify as such changes. This means, for example, that consistency checks on read-only slaves can be performed with mysqlcheck --all-databases --analyze.
read_only exists only as a GLOBAL variable, so changes to its value require the SUPER privilege. Changes to read_only on a master server are not replicated to slave servers. The value can be set on a slave server independent of the setting on the master.
Important
In MySQL 5.5, enabling read_only prevents the use of the SET PASSWORD statement by any user not having the SUPER privilege. This is not necessarily the case for all MySQL release series. When replicating from one MySQL release series to another (for example, from a MySQL 5.0 master to a MySQL 5.1 or later slave), you should check the documentation for the versions running on both master and slave to determine whether the behavior of read_only in this regard is or is not the same, and, if it is different, whether this has an impact on your applications.
The following conditions apply:
If you attempt to enable read_only while you have any explicit locks (acquired with LOCK TABLES) or have a pending transaction, an error occurs.
If you attempt to enable read_only while other clients hold explicit table locks or have pending transactions, the attempt blocks until the locks are released and the transactions end. While the attempt to enable read_only is pending, requests by other clients for table locks or to begin transactions also block until read_only has been set.
read_only can be enabled while you hold a global read lock (acquired with FLUSH TABLES WITH READ LOCK) because that does not involve table locks.
As of MySQL 5.5.3, attempts to set read_only block for active transactions that hold metadata locks until those transactions end.
read_rnd_buffer_size
When reading rows from a MyISAM table in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. See Section 8.13.9, "ORDER BY Optimization". Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries.
The maximum permissible setting for read_rnd_buffer_size is 2GB.
For more information about memory use during different operations, see Section 8.11.4.1, "How MySQL Uses Memory".
relay_log_purge
Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (ON).
relay_log_space_limit
The maximum amount of space to use for all relay logs.
report_host
The value of the --report-host option.
report_password
The value of the --report-password option. Not the same as the password used for the MySQL replication user account.
report_port
The value of the --report-port option.
report_user
The value of the --report-user option. Not the same as the name for the MySQL replication user account.
rpl_semi_sync_master_enabled
Controls whether semisynchronous replication is enabled on the master. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.
This variable is available only if the master-side semisynchronous replication plugin is installed.
rpl_semi_sync_master_timeout
A value in milliseconds that controls how long the master waits on a commit for acknowledgment from a slave before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).
This variable is available only if the master-side semisynchronous replication plugin is installed.
rpl_semi_sync_master_trace_level
The semisynchronous replication debug trace level on the master. Currently, four levels are defined:
1 = general level (for example, time function failures)
16 = detail level (more verbose information)
32 = net wait level (more information about network waits)
64 = function level (information about function entry and exit)
This variable is available only if the master-side semisynchronous replication plugin is installed.
rpl_semi_sync_master_wait_no_slave
With semisynchronous replication, for each transaction, the master waits until timeout for acknowledgment of receipt from some semisynchronous slave. If no response occurs during this period, the master reverts to normal replication. This variable controls whether the master waits for the timeout to expire before reverting to normal replication even if the slave count drops to zero during the timeout period.
If the value is ON (the default), it is permissible for the slave count to drop to zero during the timeout period (for example, if slaves disconnect). The master still waits for the timeout, so as long as some slave reconnects and acknowledges the transaction within the timeout interval, semisynchronous replication continues.
If the value is OFF, the master reverts to normal replication if the slave count drops to zero during the timeout period.
This variable is available only if the master-side semisynchronous replication plugin is installed.
rpl_semi_sync_slave_enabled
Controls whether semisynchronous replication is enabled on the slave. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.
This variable is available only if the slave-side semisynchronous replication plugin is installed.
rpl_semi_sync_slave_trace_level
The semisynchronous replication debug trace level on the slave. See rpl_semi_sync_master_trace_level for the permissible values.
This variable is available only if the slave-side semisynchronous replication plugin is installed.
secure_auth
If this variable is enabled, the server blocks connections by clients that attempt to use accounts that have passwords stored in the old (pre-4.1) format.
Enable this variable to prevent all use of passwords employing the old format (and hence insecure communication over the network).
Server startup fails with an error if this variable is enabled and the privilege tables are in pre-4.1 format. See Section C.5.2.4, "Client does not support authentication protocol".
secure_file_priv
By default, this variable is empty. If set to the name of a directory, it limits the effect of the LOAD_FILE() function and the LOAD DATA and SELECT ... INTO OUTFILE statements to work only with files in that directory.
server_id
The server ID, used in replication to give each master and slave a unique identity. This variable is set by the --server-id option. For each server participating in replication, you should pick a positive integer in the range from 1 to 232 � 1 to act as that server's ID.
shared_memory
(Windows only.) Whether the server permits shared-memory connections.
shared_memory_base_name
(Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. The default name is MYSQL. The name is case sensitive.
skip_external_locking
This is OFF if mysqld uses external locking, ON if external locking is disabled. This affects only MyISAM table access.
skip_name_resolve
This variable is set from the value of the --skip-name-resolve option. If it is ON, mysqld resolves host names when checking client connections. If OFF, mysqld uses only IP numbers and all Host column values in the grant tables must be IP addresses or localhost. See Section 8.11.5.2, "DNS Lookup Optimization and the Host Cache".
This variable was added in MySQL 5.5.5.
skip_networking
This is ON if the server permits only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe or shared memory. This variable can be set to ON with the --skip-networking option.
skip_show_database
This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES privilege. This can improve security if you have concerns about users being able to see databases belonging to other users. Its effect depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is permitted only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is permitted to all users, but displays the names of only those databases for which the user has the SHOW DATABASES or other privilege.
slow_launch_time
If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable.
slow_query_log
Whether the slow query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The default value depends on whether the --slow_query_log option is given. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.
"Slow" is determined by the value of the long_query_time variable. See Section 5.2.5, "The Slow Query Log".
slow_query_log_file
The name of the slow query log file. The default value is host_name-slow.log, but the initial value can be changed with the --slow_query_log_file option.
socket
On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)
On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is MySQL (not case sensitive).
sort_buffer_size
Each session that needs to do a sort allocates a buffer of this size. sort_buffer_size is not specific to any storage engine and applies in a general manner for optimization. See Section 8.13.9, "ORDER BY Optimization", for example.
If you see many Sort_merge_passes per second in SHOW GLOBAL STATUS output, you can consider increasing the sort_buffer_size value to speed up ORDER BY or GROUP BY operations that cannot be improved with query optimization or improved indexing. The entire buffer is allocated even if it is not all needed, so setting it larger than required globally will slow down most queries that sort. It is best to increase it as a session setting, and only for the sessions that need a larger size. On Linux, there are thresholds of 256KB and 2MB where larger values may significantly slow down memory allocation, so you should consider staying below one of those values. Experiment to find the best value for your workload. See Section C.5.4.4, "Where MySQL Stores Temporary Files".
The maximum permissible setting for sort_buffer_size is 4GB. Values larger than 4GB are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).
sql_auto_is_null
If this variable is set to 1, then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:
SELECT * FROM tbl_name WHERE auto_col IS NULL
If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 12.14, "Information Functions". If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.
The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison is used by some ODBC programs, such as Access. See Section 22.1.7.1.1, "Obtaining Auto-Increment Values". This behavior can be disabled by setting sql_auto_is_null to 0.
The default value of sql_auto_is_null is 0 as of MySQL 5.5.3, and 1 for earlier versions.
sql_big_selects
If set to 0, MySQL aborts SELECT statements that are likely to take a very long time to execute (that is, statements for which the optimizer estimates that the number of examined rows exceeds the value of max_join_size). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is 1, which permits all SELECT statements.
If you set the max_join_size system variable to a value other than DEFAULT, sql_big_selects is set to 0.
sql_buffer_result
If set to 1, sql_buffer_result forces results from SELECT statements to be put into temporary tables. This helps MySQL free the table locks early and can be beneficial in cases where it takes a long time to send results to the client. The default value is 0.
sql_log_bin
This variable controls whether logging to the binary log is done. The default value is 1 (do logging). To change logging for the current session, change the session value of this variable. The session user must have the SUPER privilege to set this variable.
Beginning with MySQL 5.5.5, it is no longer possible to set @@session.sql_log_bin within a transaction or subquery. (Bug #53437)
sql_log_off
This variable controls whether logging to the general query log is done. The default value is 0 (do logging). To change logging for the current session, change the session value of this variable. The session user must have the SUPER privilege to set this option. The default value is 0.
sql_log_update
This variable is deprecated, and is mapped to sql_log_bin. It was removed in MySQL 5.5.3.
sql_mode
The current server SQL mode, which can be set dynamically. See Section 5.1.7, "Server SQL Modes".
sql_notes
If set to 1 (the default), warnings of Note level increment warning_count and the server records them. If set to 0, Note warnings do not increment warning_count and the server does not record them. mysqldump includes output to set this variable to 0 so that reloading the dump file does not produce warnings for events that do not affect the integrity of the reload operation.
sql_quote_show_create
If set to 1 (the default), the server quotes identifiers for SHOW CREATE TABLE and SHOW CREATE DATABASE statements. If set to 0, quoting is disabled. This option is enabled by default so that replication works for identifiers that require quoting. See Section 13.7.5.12, "SHOW CREATE TABLE Syntax", and Section 13.7.5.8, "SHOW CREATE DATABASE Syntax".
sql_safe_updates
If set to 1, MySQL aborts UPDATE or DELETE statements that do not use a key in the WHERE clause or a LIMIT clause. This makes it possible to catch UPDATE or DELETE statements where keys are not used properly and that would probably change or delete a large number of rows. The default value is 0.
sql_select_limit
The maximum number of rows to return from SELECT statements. The default value for a new connection is the maximum number of rows that the server permits per table. Typical default values are (232)�1 or (264)�1. If you have changed the limit, the default value can be restored by assigning a value of DEFAULT.
If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of sql_select_limit.
sql_select_limit does not apply to SELECT statements executed within stored routines. It also does not apply to SELECT statements that do not produce a result set to be returned to the client. These include SELECT statements in subqueries, CREATE TABLE ... SELECT, and INSERT INTO ... SELECT.
sql_warnings
This variable controls whether single-row INSERT statements produce an information string if warnings occur. The default is 0. Set the value to 1 to produce an information string.
ssl_ca
The path to a file with a list of trusted SSL CAs.
ssl_capath
The path to a directory that contains trusted SSL CA certificates in PEM format.
ssl_cert
The name of the SSL certificate file to use for establishing a secure connection.
ssl_cipher
A list of permissible ciphers to use for SSL encryption.
ssl_key
The name of the SSL key file to use for establishing a secure connection.
storage_engine
The default storage engine (table type). To set the storage engine at server startup, use the --default-storage-engine option. See Section 5.1.3, "Server Command Options".
This variable is deprecated as of MySQL 5.5.3. Use default_storage_engine instead.
stored_program_cache
Sets a soft upper limit for the number of cached stored routines per connection. The value of this variable is specified in terms of the number of stored routines held in each of the two caches maintained by the MySQL Server for, respectively, stored procedures and stored functions.
Whenever a stored routine is executed this cache size is checked before the first or top-level statement in the routine is parsed; if the number of routines of the same type (stored procedures or stored functions according to which is being executed) exceeds the limit specified by this variable, the corresponding cache is flushed and memory previously allocated for cached objects is freed. This allows the cache to be flushed safely, even when there are dependencies between stored routines.
sync_frm
If this variable is set to 1, when any nontemporary table is created its .frm file is synchronized to disk (using fdatasync()). This is slower but safer in case of a crash. The default is 1.
system_time_zone
The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script.
The system_time_zone variable differs from time_zone. Although they might have the same value, the latter variable is used to initialize the time zone for each client that connects. See Section 10.6, "MySQL Server Time Zone Support".
table_definition_cache
The number of table definitions (from .frm files) that can be stored in the definition cache. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache. The minimum and default values are both 400.
table_lock_wait_timeout
This variable is unused. It was removed in 5.5.3.
table_open_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See Section 5.1.6, "Server Status Variables". If the value of Opened_tables is large and you do not use FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_open_cache variable. For more information about the table cache, see Section 8.4.3.1, "How MySQL Opens and Closes Tables".
table_type
This variable was removed in MySQL 5.5.3. Use storage_engine instead.
thread_cache_size
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. Normally, this does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.1.6, "Server Status Variables".
thread_concurrency
This variable is specific to Solaris systems, for which mysqld invokes the thr_setconcurrency() with the variable value. This function enables applications to give the threads system a hint about the desired number of threads that should be run at the same time.
thread_handling
The thread-handling model used by the server for connection threads. The permissible user-settable values are no-threads (the server uses a single thread) and one-thread-per-connection (the server uses one thread to handle each client connection; this is the default). no-threads is useful for debugging under Linux; see MySQL Internals: Porting to Other Systems.
If the thread pool plugin is enabled, the server sets the thread_handling value to dynamically-loaded. See Section 8.11.6.1, "Thread Pool Components and Installation".
thread_pool_algorithm
This variable controls which algorithm the thread pool plugin uses:
A value of 0 (the default) uses a conservative low-concurrency algorithm which is most well tested and is known to produce very good results.
A value of 1 increases the concurrency and uses a more aggressive algorithm which at times has been known to perform 5�10% better on optimal thread counts, but has degrading performance as the number of connections increases. Its use should be considered as experimental and not supported.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 8.11.6, "The Thread Pool Plugin"
thread_pool_high_priority_connection
This variable affects queuing of new statements prior to execution. If the value is 0 (false, the default), statement queuing uses both the low-priority and high-priority queues. If the value is 1 (true), queued statements always go to the high-priority queue.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 8.11.6, "The Thread Pool Plugin"
thread_pool_max_unused_threads
The maximum permitted number of unused threads in the thread pool. This variable makes it possible to limit the amount of memory used by sleeping threads.
A value of 0 (the default) means no limit on the number of sleeping threads. A value of N where N is greater than 0 means 1 consumer thread and N�1 reserve threads. In this case, if a thread is ready to sleep but the number of sleeping threads is already at the maximum, the thread exits rather than going to sleep.
A sleeping thread is either sleeping as a consumer thread or a reserve thread. The thread pool permits one thread to be the consumer thread when sleeping. If a thread goes to sleep and there is no existing consumer thread, it will sleep as a consumer thread. When a thread must be woken up, a consumer thread is selected if there is one. A reserve thread is selected only when there is no consumer thread to wake up.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 8.11.6, "The Thread Pool Plugin"
thread_pool_prio_kickup_timer
This variable affects statements waiting for execution in the low-priority queue. The value is the number of milliseconds before a waiting statement is moved to the high-priority queue. The default is 1000 (1 second). The range of values is 0 to 232 � 2.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 8.11.6, "The Thread Pool Plugin"
thread_pool_size
The number of thread groups in the thread pool. This is the most important parameter controlling thread pool performance. It affects how many statements can execute simultaneously. The default value is 16, with a range from 1 to 64 of permissible values. If a value outside this range is specified, the thread pool plugin does not load and the server writes a message to the error log.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 8.11.6, "The Thread Pool Plugin"
thread_pool_stall_limit
This variable affects executing statements. The value is the amount of time a statement has to finish after starting to execute before it becomes defined as stalled, at which point the thread pool permits the thread group to begin executing another statement. The value is measured in 10 millisecond units, so a value of 6 (the default) means 60ms. The range of values is 4 to 600 (40ms to 6s). Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.
This variable was added in MySQL 5.5.16. It is available only if the thread pool plugin is enabled. See Section 8.11.6, "The Thread Pool Plugin"
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. See Section 8.12.2, "The MySQL Benchmark Suite". The default of 192KB (256KB for 64-bit systems) is large enough for normal operation. If the thread stack size is too small, it limits the complexity of the SQL statements that the server can handle, the recursion depth of stored procedures, and other memory-consuming actions.
time_format
This variable is unused.
time_zone
The current time zone. This variable is used to initialize the time zone for each client that connects. By default, the initial value of this is 'SYSTEM' (which means, "use the value of system_time_zone"). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 10.6, "MySQL Server Time Zone Support".
timed_mutexes
This variable controls whether InnoDB mutexes are timed. If this variable is set to 0 or OFF (the default), mutex timing is disabled. If the variable is set to 1 or ON, mutex timing is enabled. With timing enabled, the os_wait_times value in the output from SHOW ENGINE INNODB MUTEX indicates the amount of time (in ms) spent in operating system waits. Otherwise, the value is 0.
timestamp = {timestamp_value | DEFAULT}
Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp, not a MySQL timestamp.
SET timestamp affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE(). The server can be started with the --sysdate-is-now option to cause SYSDATE() to be an alias for NOW(), in which case SET timestamp affects both functions.
tmp_table_size
The maximum size of internal in-memory temporary tables. (The actual limit is determined as the minimum of tmp_table_size and max_heap_table_size.) If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size (and max_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of memory. This variable does not apply to user-created MEMORY tables.
You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.
See also Section 8.4.3.3, "How MySQL Uses Internal Temporary Tables".
tmpdir
The directory used for temporary files and temporary tables. This variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (":") on Unix and semicolon characters (";") on Windows.
The multiple-directory feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based file system or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails. You can set the slave's temporary directory using the slave_load_tmpdir variable. In that case, the slave will not use the general tmpdir value and you can set tmpdir to a nonpermanent location.
transaction_alloc_block_size
The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size.
transaction_prealloc_size
There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is transaction_prealloc_size. For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by transaction_alloc_block_size bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size bytes.
By making transaction_prealloc_size sufficiently large to contain all statements within a single transaction, you can avoid many malloc() calls.
tx_isolation
The default transaction isolation level. Defaults to REPEATABLE-READ.
This variable can be set directly, or indirectly using the SET TRANSACTION statement. See Section 13.3.6, "SET TRANSACTION Syntax". If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotation marks, with the space replaced by a dash. For example:
SET tx_isolation = 'READ-COMMITTED';
Any unique prefix of a valid value may be used to set the value of this variable.
The default transaction isolation level can also be set at startup using the --transaction-isolation server option.
unique_checks
If set to 1 (the default), uniqueness checks for secondary indexes in InnoDB tables are performed. If set to 0, storage engines are permitted to assume that duplicate keys are not present in input data. If you know for certain that your data does not contain uniqueness violations, you can set this to 0 to speed up large table imports to InnoDB.
Note that setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still permitted to check for them and issue duplicate-key errors if it detects them.
updatable_views_with_limit
This variable controls whether updates to a view can be made when the view does not contain all columns of the primary key defined in the underlying table, if the update statement contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL.
The variable can have two values:
version
The version number for the server. The value might also include a suffix indicating server build or configuration information. -log indicates that one or more of the general log, slow query log, or binary log are enabled. -debug indicates that the server was built with debugging support enabled.
version_comment
The CMake configuration program has a COMPILATION_COMMENT option that permits a comment to be specified when building MySQL. This variable contains the value of that comment. See Section 2.10.4, "MySQL Source-Configuration Options".
version_compile_machine
The type of machine or architecture on which MySQL was built.
version_compile_os
The type of operating system on which MySQL was built.
wait_timeout
The number of seconds the server waits for activity on a noninteractive connection before closing it.
On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.
warning_count
The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.41, "SHOW WARNINGS Syntax".