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:?

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);

來源：mysql關于or的索引問題 - 愛擱淺 - 博客園

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