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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

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

[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。

[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