5.4.1 Selecting General Query and Slow Query Log Output Destinations

As of MySQL 5.1.6, MySQL Server provides flexible control over the destination of output to the general query log and the slow query log, if those logs are enabled. Possible destinations for log entries are log files or the general_log and slow_log tables in the mysql database. Either or both destinations can be selected. (Before MySQL 5.1.6, the server uses only log files as the destination for general query log and slow query log entries.)


For new installations of MySQL 5.1.6 or higher, the log tables are created during the installation procedure along with the other system tables. If you upgrade MySQL from a release older than 5.1.6 to MySQL 5.1.6 or higher, you must upgrade the system tables after upgrading to make sure that the log tables exist. See Section 4.4.8, “mysql_upgrade — Check and Upgrade MySQL Tables”.

Logging to tables incurs significantly more server overhead than logging to files. If you enable the general log or slow query log and require highest performance, you should use file logging, not table logging.

Log control at server startup. The --log-output option specifies the destination for log output, if logging is enabled. This option does not in itself enable the logs. Its syntax is --log-output[=value,...]:

The general_log system variable controls logging to the general query log for the selected log destinations. If specified at server startup, general_log takes an optional argument of 1 or 0 to enable or disable the log. To specify a file name other than the default for file logging, set the general_log_file variable. Similarly, the slow_query_log variable controls logging to the slow query log for the selected destinations and setting slow_query_log_file specifies a file name for file logging. If either log is enabled, the server opens the corresponding log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected. Prior to MySQL 5.1.29, the --log and --log-slow-queries options enable the general query log and slow query log. Either option may be given with a file name argument to specify a log file name to override the default.


Log control at runtime. The system variables associated with log tables and files enable runtime control over logging:

The use of tables for log output offers the following benefits:

The log table implementation has the following characteristics: