家伙试图总结我的头周围MySQL查询为什么它扫描表中的所有行MySQL查询扫描所有行
我有2个表topic_entry和topic_user
CREATE TABLE `topic_entry` (
`entry_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`topic_id` bigint(20) unsigned NOT NULL,
`entry_created` datetime NOT NULL,
`entry_object` text,
`level` tinyint(4) NOT NULL DEFAULT '3',
PRIMARY KEY (`entry_id`),
KEY `entry_created` (`entry_created`),
KEY `level` (`level`),
KEY `topic_id_2` (`topic_id`,`entry_id`)
) ENGINE=MyISAM;
CREATE TABLE `topic_user` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`topic_id` bigint(20) unsigned NOT NULL,
`user_id` varchar(100) NOT NULL,
`private` enum('y','n') DEFAULT NULL,
`freq` enum('a','d','w') DEFAULT NULL,
`topic_id_freq` varchar(10) DEFAULT NULL,
`casematch` enum('0','1') DEFAULT '0',
`textmatch` enum('0','1') DEFAULT '0',
`topic_name_case` varchar(100) DEFAULT '',
PRIMARY KEY (`id`),
UNIQUE KEY `topic_id_user_id` (`topic_id`,`user_id`),
KEY `user_id` (`user_id`),
KEY `topic_id_freq` (`topic_id_freq`)
) ENGINE=MyISAM;
这是查询我我试图运行
explain
select te.topic_id,te.entry_id
from topic_entry te
WHERE te.topic_id in (select topic_id
from topic_user where user_id ='xyz')
AND te.level=4
AND te.entry_id > 0
ORDER BY te.entry_id DESC
LIMIT 5;
解释输出表明,它扫描所有的行
| 1 | PRIMARY| te | range | PRIMARY,level | PRIMARY | 8 | NULL| **722978** | Using where |
| 2 | DEPENDENT SUBQUERY | topic_user | unique_subquery | topic_id_user_id,user_id | topic_id_user_id | 310 | func,const | 1 | Using index; Using where |
任何指针,以扫描如何避免 – user1590071
它扫描到满足where条件结合起来。您可能需要在user_id和level和entry_id上创建一些索引作为开始,以查看它是否会执行索引扫描而不是顺序扫描。 – 2012-09-12 16:13:14