table of Contents
What is the slow log check switch
Check whether the slow log switch is enabled
View slow log variable information
Turn on the slow log variable switch
View the slow log variable information again
How to analyze problematic SQL through slow logs
What is the slow log check switch
The slow log check will store the performance information of the successfully executed SQL statements outside the custom time into the slow log according to the user-defined time, so that the user administrator can optimize the slow log
Check whether the slow log switch is enabled
show variables like 'slow_query_log';You can see that it is OFF by default
View slow log variable information
show variables like '%log%';
Remember this switch is off by default
This is the default storage path for slow log files
Turn on the slow log variable switch
set global log_queries_not_using_indexes=on;
Custom slow log storage time
set global long_query_time=1;
Turn on the slow log switch
set global slow_query_log=on;
View the slow log variable information again
show variables like '%log%'You can see that both the slow log variable and the slow log are turned on
test
Execute a query in mysql
Then copy a session, in another session, do not enter mysql, enter the command, query the slow log file
more /var/lib/mysql/master-slow.logAs you can see, there will be a detailed record of the performance parameters of the SQL you execute
At this time, you will be wondering why I set up only SQL statements that exceed 1S to be saved in the log, and now all the executed SQL statements are stored in the log?
Note: The slow log will record all SQL statements by default. When you set the time for automatic deposit, you must restart the mysql service, otherwise it will not take effect! ! !
systemctl restart mysqld.serviceAfter restarting the service, test again, execute a SQL, and query the log on another port. You will find that if it takes less than 1S, it cannot be recorded in the slow load log.
How to analyze problematic SQL through slow logs
1. Look at the SQL that has many queries and each query takes a long time
2. Look at the SQL with large IO, and see the Rows examine item. The more scan lines, the larger the IO
3. Look at the SQL of the missed index, by analyzing Rows examine and Rows Send, look at the index hit rate
