在某些情況下，or條件可以避免全表掃描的。

1 .where 語句里面如果帶有or條件, 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 .必須所有的or條件都必須是獨立索引：

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

| 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)

全表掃描了。

3. 用UNION替換OR (適用于索引列)

通常情況下, 用UNION替換WHERE子句中的OR將會起到較好的效果. 對索引列使用OR將造成全表掃描.

注意, 以上規則只針對多個索引列有效. 如果有column沒有被索引, 查詢效率可能會因為你沒有選擇OR而降低.

在下面的例子中, LOC_ID 和REGION上都建有索引.

高效:

selectloc_id?,?loc_desc?,?regionfromlocationwhereloc_id?=?10

union

selectloc_id?,?loc_desc?,?regionfromlocationwhereregion?="melbourne"

低效:

selectloc_id?,?locdesc,?regionfromlocationwhereloc_id?=?10orregion?="melbourne"

如果你堅持要用OR, 那就需要返回記錄最少的索引列寫在最前面.

4. 用in來替換or

這是一條簡單易記的規則，但是實際的執行效果還須檢驗，在oracle8i下，兩者的執行路徑似乎是相同的．

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);

總結

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