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么
但SELECT * FROM `comment` WHERE `toconuid` = '10' or `tocomuid` = '10'
其中 toconuid列 和 tocomuid列 分別為單列索引
explain后 顯示兩個索引都用了，extra為 Using union(toconuid,tocomuid); Using where
答：
凡事總有特列。
而MYSQL可以理解為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 (適用於索引列)
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:
- 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"
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);