摘自：

http://www.educity.cn/wenda/590849.html

http://blog.csdn.net/hguisu/article/details/7106159

问：

不是说，一条sql语句只能用一个索引么

但SELECT * FROM `comment` WHERE `toconuid` = ‘10‘ or `tocomuid` = ‘10‘

其中 toconuid列 和 tocomuid列 分别为单列索引

explain后 显示两个索引都用了，extra为 Using union(toconuid,tocomuid); Using where

答：

凡事总有特列。

而MYSQL可以理解为把这个语句拆成了两条语句SELECT * FROM `comment` WHERE `toconuid` = ‘10‘unionSELECT * FROM `comment` WHERE `tocomuid` = ‘10‘

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。

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关于or的索引问题