mysql用or条件性能优化可以使用索引教程

一、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)

 

 

二、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。

 

三、用in来替换or(支持MyISAM和InnoDB)

 

这是一条简单易记的规则，但是实际的执行效果还须检验，在oracle8i下，两者的执行路径似乎是相同的．　

低效: 

select…. from location where loc_id = 10 or loc_id = 20 or loc_id = 30 

 

高效 

select… from location where loc_in  in (10,20,30);

 

四、如果让InnoDB支持索引用union代替，分成多个 sql，走各自的索引。如果可能有重复就用union 不用union all。

 

select count(1) from 

 (

     SELECT * FROM table1 WHERE 1 = 1 AND (3_id = 12345) 

     union 

     SELECT * FROM table1 WHERE 1 = 1 AND (tk_4th_pub_id = 12345) 

     union 

     SELECT * FROM table1  WHERE 1 = 1   AND (tk_pub_id = 12345)

 ) as a where a.refund_status = '1';