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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
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mysql> show variables like "%long%";
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+-----------------+-----------+
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| Variable_name | Value |
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+-----------------+-----------+
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| long_query_time | 10.000000 |
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+-----------------+-----------+
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1 row in set (0.00 sec)
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mysql> set global long_query_time=2;
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Query OK, 0 rows affected (0.00 sec)
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mysql> show variables like "%slow%";
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+---------------------+---------------------------------+
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| Variable_name | Value |
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+---------------------+---------------------------------+
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| log_slow_queries | OFF |
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| slow_launch_time | 2 |
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| slow_query_log | OFF |
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| slow_query_log_file | /usr/local/mysql/mysql-slow.log |
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+---------------------+---------------------------------+
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4 rows in set (0.00 sec)
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mysql> set slow_query_log='ON';
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ERROR 1229 (HY000): Variable 'slow_query_log' is a GLOBAL variable and should be set with SET GLOBAL
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mysql> set global slow_query_log='ON';
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Query OK, 0 rows affected (0.28 sec)
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mysql> show variables like "%slow%";
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+---------------------+---------------------------------+
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| Variable_name | Value |
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+---------------------+---------------------------------+
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| log_slow_queries | ON |
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| slow_launch_time | 2 |
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| slow_query_log | ON |
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| slow_query_log_file | /usr/local/mysql/mysql-slow.log |
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+---------------------+---------------------------------+
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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
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long_query_time = 2
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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
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[root@BlackGhost mysql]# cat mysql-slow.log
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/usr/local/mysql/libexec/mysqld, Version: 5.1.26-rc-log (Source distribution). started with:
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Tcp port: 3306 Unix socket: /tmp/mysql.sock
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Time Id Command Argument
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# Time: 100814 13:28:30
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# User@Host: root[root] @ localhost []
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# Query_time: 10.096500 Lock_time: 0.045791 Rows_sent: 1 Rows_examined: 2374192
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SET timestamp=1281763710;
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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);
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# Time: 100814 13:37:02
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# User@Host: root[root] @ localhost []
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# Query_time: 10.394134 Lock_time: 0.000091 Rows_sent: 1 Rows_examined: 2374192
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SET timestamp=1281764222;
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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);
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# Time: 100814 13:37:16
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# User@Host: root[root] @ localhost []
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# Query_time: 4.608920 Lock_time: 0.000078 Rows_sent: 1 Rows_examined: 1260544
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SET timestamp=1281764236;
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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。
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[root@BlackGhost bin]# mysqldumpslow -h
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Option h requires an argument
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ERROR: bad option
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Usage: mysqldumpslow [ OPTS... ] [ LOGS... ]
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Parse and summarize the MySQL slow query log. Options are
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--verbose verbose
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--debug debug
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--help write this text to standard output
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-v verbose
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-d debug
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-s ORDER what to sort by (t, at, l, al, r, ar etc), 'at' is default
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-r reverse the sort order (largest last instead of first)
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-t NUM just show the top n queries
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-a don't abstract all numbers to N and strings to 'S'
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-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
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-g PATTERN grep: only consider stmts that include this string //配置模式
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-h HOSTNAME hostname of db server for *-slow.log filename (can be wildcard), //mysql所以机器名或者IP
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default is '*', i.e. match all
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-i NAME name of server instance (if using mysql.server startup script)
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-l don't subtract lock time from total time
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
(责任编辑：IT) |
