mysql or条件可以使用索引而避免全表扫描(2)

| Table | Create Table

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

| a     | CREATE TABLE `a` (

  `id` int(1) NOT NULL AUTO_INCREMENT,

  `uid` int(11) NOT NULL,

  `aNum` char(20) DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

explain查看：

mysql> explain select * from a where id=1 or uid =2;

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

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

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

|  1 | SIMPLE      | a     | ALL  | PRIMARY       | NULL | NULL    | NULL |    5 | Using where |

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

1 row in set (0.00 sec)

全表扫描了。

摘自 hguisu的专栏