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
slow_log tables in the
mysql database. Either or both destinations can be selected.
--log-output is given with a value, the value should 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
--log-output is omitted, the default logging destination is
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=FILE to select files as the log destination and
to enable the slow query log. (In this case, because the default log destination is
FILE, you could omit the
Log control at runtime. The system variables associated with log tables and files enable runtime control over logging:
log_output system variable indicates the current logging destination. It
can be modified at runtime to change the destination.
slow_query_log variables indicate whether the general query log and slow
query log are enabled (
ON) or disabled (
You can set these variables at runtime to control whether the logs are enabled.
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 connection, set the
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
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
ALTER TABLE to
alter a log table that is in use. The log must be disabled first. No engines other than
MyISAM are legal for the log
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;
TABLE is a valid operation on a log table. It can be used to expire log entries.
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;
is a valid operation on a log table.
TABLES cannot 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 slave servers.
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.