欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  数据库

MySQL or条件可以使用索引而避免全表

程序员文章站 2024-02-16 16:58:19
...

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:

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。

MySQL or条件可以使用索引而避免全表