Teach you to locate online MySQL slow query problem hand by hand, package teaching package meeting

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1. 慢查询日志的作用

Slow query log is disabled by default,It is recommended to manually enable,Help us locate online problems.

执行时间超过阈值的SQLWill be written to the slow query log,This can help us to record the execution time is too longSQL语句,Positioning line slowSQL问题,方便我们进行SQL性能调优.

2. 慢查询日志的配置

2.1 查看是否开启了慢查询日志

show variables like 'slow_query_log';
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默认是OFF,不开启,可以手动开启.

2.2 开启慢查询日志

一种方法是可以使用MySQL命令开启：

set global slow_query_log=1;
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另一种方法是修改MySQL配置文件,重新MySQL服务后,开启.

修改配置文件my.cnf,Add the following line of a command

slow_query_log = ON

2.3 设置慢查询日志的阈值

The default threshold for slow query logs is10,单位是秒.

对于线上服务来说,10seconds are too long,我们可以手动修改.

一种是通过MySQL命令修改,比如修改为1秒：

set long_query_time=1;
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另一种方法是修改MySQL配置文件,重新MySQL服务后,开启.

修改配置文件my.cnf,Add the following line of a command

long_query_time = 1

2.4 Modify slow query log location

使用MySQLCommand to view slow query log location：

show variables like '%slow_query_log_file%';
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Want to modify the slow query log location,可以修改MySQL配置文件,重新MySQL服务后,开启.

修改配置文件my.cnf,Add the following line of a command

slow_query_log_file = /usr/local/mysql/data/localhost_slow.log

2.5 Log more slow queriesSQL

By default, management statements are not recorded in the slow query log,管理语句包括ALTER TABLE、 ANALYZE TABLE、 CHECK TABLE、 CREATE INDEX、 DROP INDEX、 OPTIMIZE TABLE和 REPAIR TABLE等.

Management statements are also very important,If you want to be recorded,可以通过MySQL命令修改：

set global log_slow_admin_statements=ON;
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默认情况下,Statements that do not use an index,Is not recorded.

enough shit！Falling into a pit without noticing,Statements that do not use an index are not logged,Why do you want to query the log slowly?？

Want to record without the use of the index of statements,可以通过命令修改：

set global log_queries_not_using_indexes=ON;
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3. 慢查询日志的使用

Manually make a slowSQL,测试一下效果,user表中有100Multimeter data：

select * from user;
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Then look at the contents of the slow query log file：

cat /usr/local/mysql/data/localhost_slow.log
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SQLStatements and execution times are logged.

4. 分析慢查询日志

Sometimes there are many slow query logs,The manual is not very easy for you to see,好在MySQLProvides tools for analyzing slow query logsmysqldumpslow.

常用参数有

-s: 表示按何种方式排序：
　　c: 访问次数
　　l: 锁定时间
　　r: 返回记录
　　t: 查询时间
　　al: 平均锁定时间
　　ar: 平均返回记录数
　　at: 平均查询时间
-t: 返回前面多少条的数据;
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4.1 The query returns the most results10条SQL：

mysqldumpslow -s r -t 10 /usr/local/mysql/data/localhost_slow.log

4.2 The longest query10条SQL：

mysqldumpslow -s t -t 10 /usr/local/mysql/data/localhost_slow.log