mysql or条件可以使用索引而避免全表

        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：

 

文章来源：http://blog.csdn.net/hguisu/article/details/7106159