一、简介
开启慢查询日志，可以让MySQL记录下查询超过指定时间的语句，通过定位分析性能的瓶颈，才能更好的优化数据库系统的性能。
二、参数说明
slow_query_log 慢查询开启状态
slow_query_log_file 慢查询日志存放的位置（这个目录需要MySQL的运行帐号的可写权限，一般设置为MySQL的数据存放目录）
long_query_time 查询超过多少秒才记录
三、设置步骤
1.查看慢查询相关参数
mysql> show variables like 'slow_query_log%';
+---------------------------+----------------------------------+
| Variable_name | Value |
+---------------------------+----------------------------------+
| slow_query_log | OFF |
| slow_query_log_file | /mysql/data/localhost-slow.log |
+---------------------------+----------------------------------+
mysql> show variables like 'long_query_time';
+-----------------+-----------+
| Variable_name | Value |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
2.设置方法
方法一：全局变量设置
将 slow_query_log 全局变量设置为“ON”状态
mysql> set global slow_query_log='ON';
设置慢查询日志存放的位置
mysql> set global slow_query_log_file='/usr/local/mysql/data/slow-query-log.log';
查询超过1秒就记录
mysql> set global long_query_time=1;
方法二：配置文件设置
修改配置文件my.cnf，在[mysqld]下的下方加入
[mysqld]
slow_query_log = ON #开启慢查询
slow_query_log_file = /usr/local/mysql/data/slow-query-log.log #慢查询日志路径
long_query_time = 1 #慢查询时间阈值 单位时间秒
3.重启MySQL服务
service mysql restart
4.查看设置后的参数
mysql> show variables like 'slow_query%';
+---------------------+-------------------------------------------+
| Variable_name | Value |
+---------------------+-------------------------------------------+
| slow_query_log | ON |
| slow_query_log_file | /usr/local/mysql/data/slow-query-log.log |
+---------------------+-------------------------------------------+
mysql> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name | Value |
+-----------------+----------+
| long_query_time | 1.000000 |
+-----------------+----------+
四、测试
1.执行一条慢查询SQL语句
mysql> select sleep(2);
2.查看是否生成慢查询日志
ls /usr/local/mysql/data/slow.log
如果日志存在，MySQL开启慢查询设置成功
五、慢查询日志分析(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@VM_0_12_centos bin]# ./mysqldumpslow -s r -t 20 /usr/local/mysql/mysql-slow.log
[root@VM_0_12_centos bin]# ./mysqldumpslow -s r -t 20 -g 'count' /usr/local/mysql/mysql-slow.log