网站运行很慢的时候，我就特别起知道为什么这么慢，所以我查啊查,数据库绝对是很重要的一部分，里面运行的sql是绝对不能放过的。平时做项目的时候，我也会注意sql语句的书写，写出一些高效的sql来，所以我会经常测试自己写的sql语句。我把我知道的二个方法，总结一下发出来。

一，show profiles 之类的语句来查看

1，查一下profile是不是打开了，默认是不打开的。

mysql> show profiles; 
Empty set (0.02 sec) 
mysql> show variables like "%pro%"; 
+---------------------------+-------+ 
| Variable_name | Value | 
+---------------------------+-------+ 
| profiling | OFF | 
| profiling_history_size | 15 | 
| protocol_version | 10 | 
| slave_compressed_protocol | OFF | 
+---------------------------+-------+ 
4 rows in set (0.00 sec)

我查看一下profiles里面没有东西，所以公司的电脑里面profile是没有打开的，我查看了一下mysql变量，果然是OFF的。

2，开启profile，然后测试

开启profile

mysql> set profiling=1; 
Query OK, 0 rows affected (0.00 sec)

测试如下：

mysql> show tables; 
+----------------+ 
| Tables_in_test | 
+----------------+ 
| aa | 
| bb | 
| comment | 
| string_test | 
| user | 
+----------------+ 
5 rows in set (0.00 sec) 
mysql> select * from aa; 
+----+------+------------+------+ 
| id | name | nname | sex | 
+----+------+------------+------+ 
| 2 | tank | bbbb,4bbbb | NULL | 
| 3 | zhang| 3,c,u | NULL | 
+----+------+------------+------+ 
2 rows in set (0.00 sec) 
mysql> update aa set name='d'; 
Query OK, 2 rows affected (0.00 sec) 
Rows matched: 2 Changed: 2 Warnings: 0 
mysql> delete from bb; 
Query OK, 2 rows affected (0.00 sec) 
mysql> show profiles; 
+----------+------------+------------------------+ 
| Query_ID | Duration | Query | 
+----------+------------+------------------------+ 
| 1 | 0.00054775 | show tables | 
| 2 | 0.00022400 | select * from aa | 
| 3 | 0.00026275 | update aa set name='d' | 
| 4 | 0.00043000 | delete from bb | 
+----------+------------+------------------------+ 
4 rows in set (0.00 sec) 
mysql> show profile; 
+----------------------+-----------+ 
| Status | Duration | 
+----------------------+-----------+ 
| (initialization) | 0.0000247 | 
| checking permissions | 0.0000077 | 
| Opening tables | 0.0000099 | 
| System lock | 0.000004 | 
| Table lock | 0.000005 | 
| init | 0.0003057 | 
| query end | 0.0000062 | 
| freeing items | 0.000057 | 
| closing tables | 0.000008 | 
| logging slow query | 0.0000015 | 
+----------------------+-----------+ 
10 rows in set (0.00 sec) 
mysql> show profile for query 1; 
+----------------------+-----------+ 
| Status | Duration | 
+----------------------+-----------+ 
| (initialization) | 0.000028 | 
| checking permissions | 0.000007 | 
| Opening tables | 0.0000939 | 
| System lock | 0.0000017 | 
| Table lock | 0.0000055 | 
| init | 0.000009 | 
| optimizing | 0.0000027 | 
| statistics | 0.0000085 | 
| preparing | 0.0000065 | 
| executing | 0.000004 | 
| checking permissions | 0.000258 | 
| Sending data | 0.000049 | 
| end | 0.0000037 | 
| query end | 0.0000027 | 
| freeing items | 0.0000307 | 
| closing tables | 0.0000032 | 
| removing tmp table | 0.0000275 | 
| closing tables | 0.0000037 | 
| logging slow query | 0.000002 | 
+----------------------+-----------+ 
19 rows in set (0.00 sec) 
mysql> show profile for query 3; 
+----------------------+-----------+ 
| Status | Duration | 
+----------------------+-----------+ 
| (initialization) | 0.0000475 | 
| checking permissions | 0.0000077 | 
| Opening tables | 0.000026 | 
| System lock | 0.0000042 | 
| Table lock | 0.0000045 | 
| init | 0.0000205 | 
| Updating | 0.0000787 | 
| end | 0.0000567 | 
| query end | 0.000004 | 
| freeing items | 0.0000067 | 
| closing tables | 0.000004 | 
| logging slow query | 0.000002 | 
+----------------------+-----------+ 
12 rows in set (0.00 sec)

二，timestampdiff来查看测试时间

mysql> set @d=now(); 
Query OK, 0 rows affected (0.00 sec) 
mysql> select * from comment; 
+------+-----+------+------------+---------------------+ 
| c_id | mid | name | content | datetime | 
+------+-----+------+------------+---------------------+ 
| 1 | 1 | ?? | 2222222211 | 2010-05-12 00:00:00 | 
| 2 | 1 | ?? | ????(??) | 2010-05-13 00:00:00 | 
| 3 | 2 | tank | ?????? | 0000-00-00 00:00:00 | 
+------+-----+------+------------+---------------------+ 
3 rows in set (0.00 sec) 
mysql> select timestampdiff(second,@d,now()); 
+--------------------------------+ 
| timestampdiff(second,@d,now()) | 
+--------------------------------+ 
| 0 | 
+--------------------------------+ 
1 row in set (0.00 sec) 

这种方法有一点要注意，就是三条sql语句要尽量连一起执行，不然误差太大，根本不准

set @d=now(); 
select * from comment; 
select timestampdiff(second,@d,now()); 

如果是用命令行来执行的话，有一点要注意，就是在select timestampdiff(second,@d,now());后面，一定要多copy一个空行，不然最后一个sql要你自己按回车执行，这样就不准了。

其实我觉得吧，真正要我们关心的是那些查询慢的sql,因为真正影响速度的是他们，关于慢查询的东西，有空写一下。