MySQL or條件可以使用索引而避免全表

在某些情況下，or條件可以避免全表掃描的。

1 .where 語句裡面如果帶有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。