SQL语句导致性能问题

前阵子，突然收到服务器的报警信息，于是上服务器找问题，我擦，top看到mysql占的%cpu高得把我吓尿了

从以上的信息看，相信大家已经可以定位到底是那个程序导致服务器CPU负载过高了，但我们要做的是，找出mysql进程下，是什么动作导致服务器出现性能问题

以下做个实验，相信大家看了后也能猜到当时是什么导致高负载的，废话不多说：

表结构如下：

mysql> desc test1;
+---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| id      | int(11)     | NO   | PRI | NULL    | auto_increment |
| role_id | int(11)     | NO   |     | 0       |                |
| referer | varchar(20) | NO   |     |         |                |
+---------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> desc test2;
+--------------+---------+------+-----+---------+----------------+
| Field        | Type    | Null | Key | Default | Extra          |
+--------------+---------+------+-----+---------+----------------+
| id           | int(11) | NO   | PRI | NULL    | auto_increment |
| role_id      | int(11) | NO   | MUL | 0       |                |
| privilege_id | int(11) | NO   |     | 0       |                |
+--------------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

表的索引情况如下：

mysql> show index from test1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| test1 |          0 | PRIMARY  |            1 | id          | A         |         329 |     NULL | NULL   |      | BTREE      |         |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
1 row in set (0.00 sec)

mysql> show index from test2;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| test2 |          0 | PRIMARY  |            1 | id          | A         |       12476 |     NULL | NULL   |      | BTREE      |         |
| test2 |          1 | role_id  |            1 | role_id     | A         |         415 |     NULL | NULL   |      | BTREE      |         |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.00 sec)

当时执行show full processlist后，发现有好几百个连接在执行同一条SQL语句，看见SQL也还好，不复杂，是子查询

mysql> select privilege_id from t2 where role_id in (select role_id from t1 where id=193);

看着以上的SQL语句，写没什么问题啊，但用explain分析一看，我擦

mysql> explain select privilege_id from test2 where role_id in (select role_id from test1 where id=192);
+----+--------------------+-------+-------+---------------+---------+---------+-------+-------+-------------+
| id | select_type        | table | type  | possible_keys | key     | key_len | ref   | rows  | Extra       |
+----+--------------------+-------+-------+---------------+---------+---------+-------+-------+-------------+
|  1 | PRIMARY            | test2 | ALL   | NULL          | NULL    | NULL    | NULL  | 12476 | Using where |
|  2 | DEPENDENT SUBQUERY | test1 | const | PRIMARY       | PRIMARY | 4       | const |     1 |             |
+----+--------------------+-------+-------+---------------+---------+---------+-------+-------+-------------+
2 rows in set (0.00 sec)

当时MySQL版本是：

mysql> select  version();
+-----------+
| version() |
+-----------+
| 5.1.66    |
+-----------+
1 row in set (0.00 sec)

但把SQL语句的子查询修改为以下的写法，执行效率就就像喝了可乐一样爽^0^：

select a.privilege_id from test2 as a inner join test1 as b on a.role_id = b.role_id and b.id=192;

看效果对比：

mysql> select  version();
+-----------+
| version() |
+-----------+
| 5.1.66    |
+-----------+
1 row in set (0.00 sec)

mysql> explain select privilege_id from test2 where role_id in (select role_id from test1 where id=192);
+----+--------------------+-------+-------+---------------+---------+---------+-------+-------+-------------+
| id | select_type        | table | type  | possible_keys | key     | key_len | ref   | rows  | Extra       |
+----+--------------------+-------+-------+---------------+---------+---------+-------+-------+-------------+
|  1 | PRIMARY            | test2 | ALL   | NULL          | NULL    | NULL    | NULL  | 12476 | Using where |
|  2 | DEPENDENT SUBQUERY | test1 | const | PRIMARY       | PRIMARY | 4       | const |     1 |             |
+----+--------------------+-------+-------+---------------+---------+---------+-------+-------+-------------+
2 rows in set (0.00 sec)

mysql> explain select a.privilege_id from test2 as a inner join test1 as b on a.role_id = b.role_id and b.id=192;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | b     | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
|  1 | SIMPLE      | a     | ref   | role_id       | role_id | 4       | const |   32 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
2 rows in set (0.00 sec)

相信大家也能看到修改后跟修改前的差别了吧！

以下用版本为5.5的版本测试下：

以下用版本为5.6的版本测试下：

mysql> select version();
+------------+
| version()  |
+------------+
| 5.6.10-log |
+------------+
1 row in set (0.00 sec)

mysql> explain select privilege_id from test2 where role_id in (select role_id from test1 where id=192);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | test1 | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
|  1 | SIMPLE      | test2 | ref   | role_id       | role_id | 4       | const |   32 | NULL  |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
2 rows in set (0.07 sec)

mysql> explain select a.privilege_id from test2 as a inner join test1 as b on a.role_id = b.role_id and b.id=192;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | b     | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
|  1 | SIMPLE      | a     | ref   | role_id       | role_id | 4       | const |   32 | NULL  |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
2 rows in set (0.04 sec)

看到的两种查询结果是一样高效的，从以上的案例可以看出，写通用且性能高的SQL相当重要，希望大家以后不要踩类似的坑@[email protected]