在某些情况下,or条件可以避免全表扫描的。虽然在我在MariaDB10.0.10版本和MySQL5.7.14上都测试不成功,但还是很有必要记录下来。
mysql> show variables like 'version'; +---------------+---------------------+ | Variable_name | Value | +---------------+---------------------+ | version | 10.0.10-MariaDB-log | +---------------+---------------------+ 1 row in set (0.00 sec)
mysql> show variables like 'version'; +---------------+------------+ | Variable_name | Value | +---------------+------------+ | version | 5.7.14-log | +---------------+------------+ 1 row in set, 1 warning (0.00 sec)
一.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)
二.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。
三.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。
四.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);
PS:查看mysql版本的方法
1.在命令行输入“mysql --version",按”Enter“键即可,如下图所示:
2.使用命令行查看mysql版本-mysql变量查看
在命令行输入“mysql",按”Enter“进入mysql命令行模式,输入”show variables like 'version';“即可,如下所示: