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:
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);
转自:http://blog.csdn.net/hguisu/article/details/7106159
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