mysql關於or的索引問題

摘自：

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

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

問：

不是說，一條sql語句只能用一個索引麼

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

其中 toconuid列 和 tocomuid列 分別爲單列索引

explain後 顯示兩個索引都用了，extra爲 Using union(toconuid,tocomuid); Using where

答：

凡事總有特列。

而MYSQL能夠理解爲把這個語句拆成了兩條語句SELECT * FROM `comment` WHERE `toconuid` = '10'unionSELECT * FROM `comment` WHERE `tocomuid` = '10'

 

在某些狀況下，or條件能夠避免全表掃描的。

1 .where 語句裏面若是帶有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 .必須全部的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 (適用於索引列)

       一般狀況下, 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將形成全表掃描. 

       注意, 以上規則只針對多個索引列有效. 若是有column沒有被索引, 查詢效率可能會由於你沒有選擇OR而下降. 

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

select loc_id , loc_desc , region from location where loc_id = 10
union
select loc_id , loc_desc , region  from location where region = "melbourne"

     低效: 

select loc_id , loc desc , region from location where loc_id = 10 or region = "melbourne"

若是你堅持要用OR, 那就須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，可是實際的執行效果還須檢驗，在oracle8i下，二者的執行路徑彷佛是相同的．　
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);