在某些情况下，or条件可以避免全表扫描的。

1 .where 语句里面如果带有or条件, myisam表能用到索引，innodb不行。

1)myisam表：

CREATE TABLE IF NOT EXISTS `a` (

`id` int(1) NOT NULL AUTO_INCREMENT,

`uid` int(11) NOT NULL,

`aNum` char(20) DEFAULT NULL,

PRIMARY KEY (`id`),

KEY `uid` (`uid`)

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

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 | index_merge | PRIMARY,uid | PRIMARY,uid | 4,4 | NULL | 2 | Using union(PRIMARY,uid); Using where |

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

1 row in set (0.00 sec)

2)innodb表：

CREATE TABLE IF NOT EXISTS `a` (

`id` int(1) NOT NULL AUTO_INCREMENT,

`uid` int(11) NOT NULL,

`aNum` char(20) DEFAULT NULL,

PRIMARY KEY (`id`),

KEY `uid` (`uid`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=6 ;

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,uid | NULL | NULL | NULL | 5 | Using where |

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

1 row in set (0.00 sec)

2 .必须所有的or条件都必须是索引：

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

| 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