7.4.1 Selecting General Query Log and Slow Query Log Output Destinations

MySQL Server provides flexible control over the destination of output written 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 system database. File output, table output, or both can be selected.

• Log Control at Server Startup

• Log Control at Runtime

• Log Table Benefits and Characteristics

Log Control at Server Startup

The log_output system variable specifies the destination for log output. Setting this variable does not in itself enable the logs; they must be enabled separately.

• If log_output is not specified at startup, the default logging destination is FILE.

• If log_output is specified at startup, its value is a list one or more comma-separated words chosen from 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.

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

Examples:

• To write general query log entries to the log table and the log file, use --log_output=TABLE,FILE to select both log destinations and --general_log 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 --general_log and --slow_query_log 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 --slow_query_log to enable the slow query log. In this case, because the default log destination is FILE, you could omit the log_output setting.

Log Control at Runtime

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

• The log_output variable indicates the current logging destination. It can be modified at runtime to change the destination.

• The 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 general_log_file and slow_query_log_file variables indicate the names of the general query log and slow query log files. You can set these variables at server startup or at runtime to change the names of the log files.

• To disable or enable general query logging for the current session, set the session sql_log_off variable to ON or OFF. (This assumes that the general query log itself is enabled.)

Log Table Benefits and Characteristics

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

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

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

• Log contents are accessible through SQL statements. This enables the use of queries that select only those log entries that satisfy specific criteria. For example, to select log contents associated with a particular client (which can be useful for identifying problematic queries from that client), it is easier to do this using a log table than a log file.

• Logs are accessible remotely through any client that can connect to the server and issue queries (if the client has the appropriate log table privileges). It is not necessary to log in to the server host and directly access the file system.

The log table implementation has the following characteristics:

• In general, the primary purpose of log tables is to provide an interface for users to observe the runtime execution of the server, not to interfere with its runtime execution.

• CREATE TABLE, ALTER TABLE, 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.

  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.

  Log Tables and “Too many open files” Errors.  If you select TABLE as a log destination and the log tables use the CSV storage engine, you may find that disabling and enabling the general query log or slow query log repeatedly at runtime results in a number of open file descriptors for the .CSV file, possibly resulting in a “Too many open files” error. To work around this issue, execute FLUSH TABLES or ensure that the value of open_files_limit is greater than the value of table_open_cache_instances.

• 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.

• 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;
  DROP TABLE IF EXISTS general_log2;
  CREATE TABLE general_log2 LIKE general_log;
  RENAME TABLE general_log TO general_log_backup, general_log2 TO general_log;

• CHECK TABLE is a valid operation on a log table.

• LOCK TABLES cannot be used on a log table.

• INSERT, DELETE, and UPDATE cannot be used on a log table. These operations are permitted only internally to the server itself.

• FLUSH TABLES WITH READ LOCK and the state of the 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 replicas.

• To flush the log tables or log files, use FLUSH TABLES or FLUSH LOGS, respectively.

• Partitioning of log tables is not permitted.

• A mysqldump dump includes statements to recreate those tables so that they are not missing after reloading the dump file. Log table contents are not dumped.