5.4.1 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
slow_log tables in the
mysql system database. File output, table output, or both can be selected.
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.
log_outputis not specified at startup, the default logging destination is
log_outputis 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.
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.
To write slow query log entries only to the log file, use
--log_output=FILEto select files as the log destination and
--slow_query_logto enable the slow query log. In this case, because the default log destination is
FILE, you could omit the
The system variables associated with log tables and files enable runtime control over logging:
log_outputvariable indicates the current logging destination. It can be modified at runtime to change the destination.
slow_query_logvariables 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.
slow_query_log_filevariables 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
OFF. (This assumes that the general query log itself is enabled.)
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.
By default, the log tables use the
CSVstorage engine that writes data in comma-separated values format. For users who have access to the
.CSVfiles 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
MyISAMstorage engine. You cannot use
ALTER TABLEto alter a log table that is in use. The log must be disabled first. No engines other than
MyISAMare legal for the log tables.
Log Tables and “Too many open files” Errors. If you select
TABLEas a log destination and the log tables use the
CSVstorage 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
.CSVfile, possibly resulting in a “Too many open files” error. To work around this issue, execute
FLUSH TABLESor ensure that the value of
open_files_limitis greater than the value of
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
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 TABLEis a valid operation on a log table. It can be used to expire log entries.
RENAME TABLEis 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 TABLEis a valid operation on a log table.
LOCK TABLEScannot be used on a log table.
Entries written to the log tables are not written to the binary log and thus are not replicated to replicas.
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.