linux下开启mysql慢查询，分析查询语句

原文地址：http://zhumeng8337797.blog.163.com/blog/static/10076891420111017012021/

一，为什么要开启这个查询呢？

数据库是很容易产生瓶颈的地方，现在Nosql大家讨论这么热，估计都被数据库搞郁闷了。mysql中最影响速度的就是那些查询非常慢的语句，这些慢的语句，可能是写的不够合理或者是大数据下多表的联合查询等等，所以我们要找出这些语句，分析原因，加以优化。这也是发这篇博文的原因

二，开启mysql的慢查询

方法1，用命令开启慢查询

查看复制打印?

mysql> show variables like "%long%"; //查看一下默认为慢查询的时间10秒

+-----------------+-----------+

| Variable_name | Value |

+-----------------+-----------+

| long_query_time | 10.000000 |

+-----------------+-----------+

1 row in set (0.00 sec)

mysql> set global long_query_time=2; //设置成2秒，加上global,下次进mysql已然生效

Query OK, 0 rows affected (0.00 sec)

mysql> show variables like "%slow%"; //查看一下慢查询是不是已经开启

+---------------------+---------------------------------+

| Variable_name | Value |

+---------------------+---------------------------------+

| log_slow_queries | OFF |

| slow_launch_time | 2 |

| slow_query_log | OFF |

| slow_query_log_file | /usr/local/mysql/mysql-slow.log |

+---------------------+---------------------------------+

4 rows in set (0.00 sec)

mysql> set slow_query_log='ON'; //加上global，不然会报错的。

ERROR 1229 (HY000): Variable 'slow_query_log' is a GLOBAL variable and should be set with SET GLOBAL

mysql> set global slow_query_log='ON'; //启用慢查询

Query OK, 0 rows affected (0.28 sec)

mysql> show variables like "%slow%"; //查看是否已经开启

+---------------------+---------------------------------+

| Variable_name | Value |

+---------------------+---------------------------------+

| log_slow_queries | ON |

| slow_launch_time | 2 |

| slow_query_log | ON |

| slow_query_log_file | /usr/local/mysql/mysql-slow.log |

+---------------------+---------------------------------+

4 rows in set (0.00 sec)

方法2,修改mysql的配置文件my.cnf

在[mysqld]里面加上以下内容

long_query_time = 2

log-slow-queries = /usr/local/mysql/mysql-slow.log

重起一下

/usr/local/mysql/libexec/mysqld restart

三，分析工具

分析工具干什么事的呢，其实就是把mysql-slow.log里面记录下来的数据，分析一下显示出来。其实自己写一个shell脚本也是可以把要的信息取出来的。我们来看一下mysql-slow.log里面到底是什么东西

查看复制打印?

[root@BlackGhost mysql]# cat mysql-slow.log //查看命令

/usr/local/mysql/libexec/mysqld, Version: 5.1.26-rc-log (Source distribution). started with:

Tcp port: 3306 Unix socket: /tmp/mysql.sock

Time Id Command Argument

# Time: 100814 13:28:30

# User@Host: root[root] @ localhost []

# Query_time: 10.096500 Lock_time: 0.045791 Rows_sent: 1 Rows_examined: 2374192

SET timestamp=1281763710;

select count(distinct ad_code) as x from ad_visit_history where ad_code in (select ad_code from ad_list where media_id=15);

# Time: 100814 13:37:02

# User@Host: root[root] @ localhost []

# Query_time: 10.394134 Lock_time: 0.000091 Rows_sent: 1 Rows_examined: 2374192

SET timestamp=1281764222;

select count(distinct ad_code) as x from ad_visit_history where ad_code in (select ad_code from ad_list where media_id=15);

# Time: 100814 13:37:16

# User@Host: root[root] @ localhost []

# Query_time: 4.608920 Lock_time: 0.000078 Rows_sent: 1 Rows_examined: 1260544

SET timestamp=1281764236;

select count(*) as cou from ad_visit_history where ad_code in (select ad_code from ad_list where id=41) order by id desc;

看到了，就是记录一下sql语句的执行情况，包括执行时间，锁定时间等，所以要不要分析工具看个人情况，分析工具很多，在这儿只说一下mysql自带的慢查询分析工具mysqldumpslow的使用方法。

查看复制打印?

[root@BlackGhost bin]# mysqldumpslow -h

Option h requires an argument

ERROR: bad option

Usage: mysqldumpslow [ OPTS... ] [ LOGS... ]

Parse and summarize the MySQL slow query log. Options are

--verbose verbose

--debug debug

--help write this text to standard output

-v verbose

-d debug //查错

-s ORDER what to sort by (t, at, l, al, r, ar etc), 'at' is default //排序方式query次数，时间，lock的时间和返回的记录数来排序

-r reverse the sort order (largest last instead of first) //倒排序

-t NUM just show the top n queries //显示前N多个

-a don't abstract all numbers to N and strings to 'S'

-n NUM abstract numbers with at least n digits within names //抽象的数字，至 少有n位内的名称

-g PATTERN grep: only consider stmts that include this string //配置模式

-h HOSTNAME hostname of db server for *-slow.log filename (can be wildcard), //mysql所以机器名或者IP

default is '*', i.e. match all

-i NAME name of server instance (if using mysql.server startup script)

-l don't subtract lock time from total time //总时间中不减去锁定时间

例子：

[root@BlackGhost
bin]# ./mysqldumpslow -s r -t 20 /usr/local/mysql/mysql-slow.log

[root@BlackGhost
bin]# ./mysqldumpslow -s r -t 20 -g 'count' /usr/local/mysql/mysql-slow.log