在我们的应用常有查询较慢的sql语句，开启慢查询日志记录功能，能把这些查询较慢的sql一览无余。
在my.ini配置文件中添加如下配置（这个功能mysql默认是没有开启的）
#开启记录慢查询日志，可以记录查询最慢的sql语句，long_query_time制定慢的时间，单#位为秒，log-queries-not-using-indexes是记录那些没有使用索引查询的sql语句。两个条件满足一个就会记录到慢查询日志中。
log-slow-queries = C:\Program Files\MySQL\MySQL Server 5.5\customLog\log_slow.txt
long-query-time = 1
log-queries-not-using-indexes
利用本人博客 mysql-性能调优1 中的例子，往表中插入一百万条数据，之后，执行查询语句，
mysql> select 10*count(*) from supan; ERROR 2006 (HY000): MySQL server has gone away No connection. Trying to reconnect... Connection id: 1 Current database: test +-------------+ | 10*count(*) | +-------------+ | 9999990 | +-------------+ 1 row in set (2.31 sec)
可见用了2.31秒，这样慢查询日志文件中就有了这个乌龟sql。
如下：
C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld, Version: 5.5.27-log (MySQL Community Server (GPL)). started with:
TCP Port: 3306, Named Pipe: (null)
Time Id Command Argument
C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld, Version: 5.5.27-log (MySQL Community Server (GPL)). started with:
TCP Port: 3306, Named Pipe: (null)
Time Id Command Argument
# Time: 150117 16:39:34
# User@Host: root[root] @ localhost [127.0.0.1]
# Query_time: 1.255072 Lock_time: 0.211012 Rows_sent: 1 Rows_examined: 999999
use test;
SET timestamp=1421483974;
select 10*count(*) from supan;
之后测试log-queries-not-using-indexes指标：
之前是没有在supan表name中建立索引，所以把时间调到很大8，添加log-queries-not-using-indexes 重启mysql，执行查询：
mysql> select * from supan where name = '李四'; ERROR 2006 (HY000): MySQL server has gone away No connection. Trying to reconnect... Connection id: 1 Current database: test Empty set (2.50 sec)
结果日志中存放了这个没有使用索引的记录。可以看一下日志：
C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld, Version: 5.5.27-log (MySQL Community Server (GPL)). started with:
TCP Port: 3306, Named Pipe: (null)
Time Id Command Argument
# Time: 150117 17:11:10
# User@Host: root[root] @ localhost [127.0.0.1]
# Query_time: 1.435082 Lock_time: 0.121007 Rows_sent: 0 Rows_examined: 999999
use test;
SET timestamp=1421485870;
select * from supan where name = '李四';
我们在supan的name字段值添加索引：
CREATE INDEX index_supan_name ON supan (name);
在此执行查询：
mysql> select * from supan where name = '李四'; ERROR 2006 (HY000): MySQL server has gone away No connection. Trying to reconnect... Connection id: 1 Current database: test Empty set, 2 warnings (1.29 sec)
结果日志中没有了这个sql，因为这次查询用到了索引，可以看一下日志：
C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld, Version: 5.5.27-log (MySQL Community Server (GPL)). started with:
TCP Port: 3306, Named Pipe: (null)
Time Id Command Argument
呵呵呵。完毕