MySQL Slow Log is used to solve the high CPU usage problem of MySQL.

But how can we find out which SQL statement has been executed for a long time? You can refer to the MySQL Slow Log for details.

First, find the MySQL configuration file my. cnf. The configuration for enabling slow queries varies with mysql versions.


For mysql 5.0

[Mysqld]
Long_query_time = 1
Log-slow-queries =/var/log/mysql/slow. log


For mysql 5.1

[Mysqld]
Long_query_time = 1
Slow_query_log = 1
Slow_query_log_file =/var/log/mysql/slow. log


Long_query_time indicates how long the SQL statement will be logged after execution, which is 1 second.
Log-slow-queries and slow_query_log_file


Turn on the above parameters and shut down after a period of operation, saving the trouble of affecting the production environment.

The following is the analysis. The file name here is/var/log/mysql/slow. log.
Mysqldumpslow-help first, mainly used


-S ORDER what to sort by (t, at, l, al, r, ar etc), 'at' is default
-T NUM just show the top n queries
-G PATTERN grep: only consider into ts that include this string

-S is the order, which indicates that the write is not detailed enough, mainly including
C, t, l, r, and ac, at, al, ar are sorted by the number of queries, time, lock time, and number of returned records, respectively, the reverse chronological order of a is added.
-T indicates the top n, that is, the number of data records returned.
-G. You can write a regular expression matching later. It is case insensitive.


Mysqldumpslow-s c-t 20/var/log/mysql/slow. log
Mysqldumpslow-s r-t 20/var/log/mysql/slow. log

The preceding command shows the 20 most frequently accessed SQL statements and the 20 most returned SQL statements in the record set.


Mysqldumpslow-t 10-s t-g "left join"/var/log/mysql/slow. log
Return the first 10 SQL statements containing the left join according to the time.

With this tool, you can find out which SQL statements are performance bottlenecks and optimize them, such as adding indexes and implementing the application.