之前有用户很不解：SQL语句非常简单，就是select * from test_1 where user_id=1 这种类型，而且user_id上已经建立索引了，怎么还是查询很慢？

test_1的表结构：

CREATE TABLE `test_1` (

  `id` int(11) NOT NULL AUTO_INCREMENT,

  `user_id` varchar(30) NOT NULL,

  `name` varchar(30) DEFAULT NULL,

  PRIMARY KEY (`id`),

  KEY `idx_user_id` (`user_id`)

) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

查看执行计划，可以看出进行了全表扫描，并没有用上user_id的索引。

mysql> explain select * from test_1 where user_id=1;

+----+-------------+--------+------+---------------+------+---------+------+------+-------------+

| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+--------+------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | test_1 | ALL  | idx_user_id   | NULL | NULL    | NULL |    3 | Using where |

+----+-------------+--------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.01 sec)

仔细看下表结构，user_id的字段类型：  `user_id` varchar(30) NOT NULL,

而用户传入的是int，这里会有一个隐式转换的问题。隐式转换会导致全表扫描。

把输入改成字符串类型，执行计划如下，这样就会很快了。

mysql> explain select * from test_1 where user_id=‘1‘;

+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------------+

| id | select_type | table  | type | possible_keys | key         | key_len | ref   | rows | Extra       |

+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------------+

|  1 | SIMPLE      | test_1 | ref  | idx_user_id   | idx_user_id | 92      | const |    1 | Using where |

+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------------+

1 row in set (0.00 sec)

此外，还需要注意的是：

数字类型的0001等价于1

字符串的0001和1不等价

mysql> select * from test_1;

+----+---------+------+

| id | user_id | name |

+----+---------+------+

|  1 | 0001    | kate |

|  2 | 1101    | Jim  |

|  3 | 1       | Jim  |

+----+---------+------+

3 rows in set (0.01 sec)

mysql> select * from test_1 where user_id=1;

+----+---------+------+

| id | user_id | name |

+----+---------+------+

|  1 | 0001    | kate |

|  3 | 1       | Jim  |

+----+---------+------+

2 rows in set (0.00 sec)

mysql> select * from test_1 where user_id=‘1‘;

+----+---------+------+

| id | user_id | name |

+----+---------+------+

|  3 | 1       | Jim  |

+----+---------+------+

1 row in set (0.00 sec)

如果表定义的是int字段，传入的是字符串，则不会发生隐式转换。

看下面的测试：

CREATE TABLE `test_2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL,
`name` varchar(30) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_user_id` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

mysql> explain select * from test_2 where user_id=1;
+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------+
| 1 | SIMPLE | test_2 | ref | idx_user_id | idx_user_id | 4 | const | 2 | |
+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain select * from test_2 where user_id=‘1‘;
+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------+
| 1 | SIMPLE | test_2 | ref | idx_user_id | idx_user_id | 4 | const | 2 | |
+----+-------------+--------+------+---------------+-------------+---------+-------+------+-------+
1 row in set (0.00 sec)

MySQL SQL优化之字符串索引隐式转换