Selecting General Query and Slow Query Log

5.2.1. Selecting General Query and Slow Query Log Output Destinations

As of MySQL 5.1.6, the server provides flexible control over the destination for log output. Log entries can be written to log files to the  general_log and  slow_log tables in the  mysql database. If logging is enabled, 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, if those logs are enabled.)

Note

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 Tables for MySQL Upgrade”.

Currently, 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 log to files and not to tables.

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

• If  --log-output is given with a value, 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).  NONE, if present, takes precedence over any other specifiers.
• If  --log-output is omitted or given without a value, the default is  FILE. (For MySQL 5.1.6 through 5.1.20, the default logging destination is  TABLE.)

The  --general_log option, if given, enables logging to the general query log for the selected log destinations.  --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, use  --general_log_file=file_name. Similarly, the  --slow_query_log option, if given, enables logging to the slow query log for the selected destinations and  --slow_query_log_file=file_name 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.

Examples:

• To write general query log entries to the log table and the log file, use  --log-output=TABLE,FILE to select both log destinations and the  --general_log option to enable the general query log.
• To write general and slow query log entries only to the log tables, use  --log-output=TABLE to select tables as the log destination and the  --general_log and  --slow_query_log options to enable both logs.
• To write slow query log entries only to the log file, use  --log-output=FILE to select files as the log destination and the  --slow_query_log option to enable the slow query log. (In this case, because the default log destination is  FILE, you could omit the  --log-output option.)

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

• The global  log_output system variable indicates the current logging destination. It can be modified at runtime to change the destination.
• The global  general_log and  slow_query_log variables indicate whether the general query log and slow query log are enabled ( ON) or disabled ( OFF). You can set these variables at runtime to control whether the logs are enabled.
• The global  general_log_file and  slow_query_log_file variables indicate the names of the general query log and slow query log files. As of MySQL 5.1.29, you can set these variables at server startup or at runtime to change the names of the log files. Before MySQL 5.1.29, you can set these variables only at runtime, but the  --log and  --log-slow-queries options can be given with a file name argument at startup to change the log file names from their default values.
• The session  sql_log_off variable can be set to  ON or  OFF to disable or enable general query logging for the current connection.

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

• Log entries have a standard format. To display the current structure of the log tables, use these statements:

  SHOW CREATE TABLE mysql.general_log;
  SHOW CREATE TABLE mysql.slow_log;
  

• Log contents are accessible via SQL statements. This enables the use of queries that select only those log entries that satisfy specific criteria. For example, to select log contents associated with a particular client (which can be useful for identifying problematic queries from that client), it is easier to do this using a log table than a log file.
• Logs are accessible remotely through any client that can connect to the server and issue queries (if the client has the appropriate log table privileges). It is not necessary to log in to the server host and directly access the file system.

The log table implementation has the following characteristics:

• In general, the primary purpose of log tables is to provide an interface for users to observe the runtime execution of the server, not to interfere with its runtime execution.
• CREATE TABLE,  ALTER TABLE, and  DROP TABLE are valid operations on a log table. For  ALTER TABLE and  DROP TABLE, the log table cannot be in use and must be disabled, as described later.
• By default, the log tables use the  CSV storage engine that writes data in comma-separated values format. For users who have access to the  .CSV files that contain log table data, the files are easy to import into other programs such as spreadsheets that can process CSV input.
  Beginning with MySQL 5.1.12, the log tables can be altered to use the  MyISAM storage engine. You cannot use  ALTER TABLE to alter a log table that is in use. The log must be disabled first. No engines other than  CSV or  MyISAM are legal for the log tables.
• To disable logging so that you can alter (or drop) a log table, you can use the following strategy. The example uses the general query log; the procedure for the slow query log is similar but uses the  slow_log table and  slow_query_log system variable.

  SET @old_log_state = @@global.general_log;
  SET GLOBAL general_log = 'OFF';
  ALTER TABLE mysql.general_log ENGINE = MyISAM;
  SET GLOBAL general_log = @old_log_state;
  

• TRUNCATE TABLE is a valid operation on a log table. It can be used to expire log entries.
• As of MySQL 5.1.13,  RENAME TABLE is a valid operation on a log table. You can atomically rename a log table (to perform log rotation, for example) using the following strategy:

  USE mysql;
  CREATE TABLE IF NOT EXISTS general_log2 LIKE general_log;
  RENAME TABLE general_log TO general_log_backup, general_log2 TO general_log;
  

• LOCK TABLES cannot be used on a log table.
• INSERT,  DELETE, and  UPDATE cannot be used on a log table. These operations are allowed only internally to the server itself.
• FLUSH TABLES WITH READ LOCK and the state of the global  read_only system variable have no effect on log tables. The server can always write to the log tables.
• Entries written to the log tables are not written to the binary log and thus are not replicated to slave servers.
• To flush the log tables or log files, use  FLUSH TABLES or  FLUSH LOGS, respectively. (From MySQL 5.1.12 to 5.1.20,  FLUSH TABLES ignores log tables and  FLUSH LOGS flushes both the log tables and files.)
• It is not recommended to partition log tables, and doing so is not permitted beginning with MySQL 5.1.20.

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