mysql or条件可以使用索引而避免全表扫描_MySQL

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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条件都必须是独立索引：

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

| 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