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 ;

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。

?

3. 用UNION替换OR (适用于索引列)

? ? ? ?通常情况下, 用UNION替换WHERE子句中的OR将会起到较好的效果. 对索引列使用OR将造成全表扫描.?

? ? ? ?注意, 以上规则只针对多个索引列有效. 如果有column没有被索引, 查询效率可能会因为你没有选择OR而降低.?

?

? ? ? ?在下面的例子中, LOC_ID 和REGION上都建有索引.
? ? ? ?高效:?

?

? ? ?低效:?

?

如果你坚持要用OR, 那就需要返回记录最少的索引列写在最前面.

?

4. 用in来替换or ?

? ? ?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);

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)