mysql 关联 filesort_MySQL连接优化 - 绕过filesort
仍然在优化MySQL LEFT JOIN时遇到问题 . 对于下一个(简化的)，该查询需要0.13秒才能完成，而不是0.00 .
我想为这个查询达到0.00左右 .
我已经尝试过全面创建索引和组合索引 . 变化不大 . 基本上只要在EXPLAIN中有FILESORT，就会很慢 . 我不知道该怎么做...在表格之间创建一个索引？它甚至存在吗？
谢谢 .
罪魁祸首：
SELECT
SQL_NO_CACHE p.id
FROM 1_posts p
INNER JOIN 1_topics t
ON (p.cid = t.cid && p.container = t.id)
WHERE
t.cid = 1010699
ORDER BY
p.id DESC
LIMIT 1;
EXPLAIN输出：
+----+-------------+-------+------+-------------------+-------+---------+---------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------+-------+---------+---------------------+------+----------------------------------------------+
| 1 | SIMPLE | t | ref | PRIMARY,cid,cid_2 | cid | 4 | const | 216 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | p | ref | PRIMARY,cid,cid_2 | cid_2 | 8 | const,forumdb.t.id | 12 | |
+----+-------------+-------+------+-------------------+-------+---------+---------------------+------+----------------------------------------------+
现在，相同的简化查询工作正常(使用索引等 . 唯一的区别是括号之间)：
SELECT
SQL_NO_CACHE p.id
FROM
1_posts p
INNER JOIN 1_topics t
ON (p.cid = t.cid)
WHERE
t.cid = 1010699
ORDER BY
p.id DESC
LIMIT 1;
解释：
+----+-------------+-------+-------+-------------------+---------+---------+-------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-------------------+---------+---------+-------+-------+--------------------------+
| 1 | SIMPLE | p | range | PRIMARY,cid,cid_2 | PRIMARY | 4 | NULL | 31720 | Using where; Using index |
| 1 | SIMPLE | t | ref | PRIMARY,cid,cid_2 | cid_2 | 4 | const | 194 | Using index |
+----+-------------+-------+-------+-------------------+---------+---------+-------+-------+--------------------------+
表：
CREATE TABLE `1_posts` (
`cid` int(20) unsigned NOT NULL DEFAULT '0',
`id` int(20) unsigned NOT NULL AUTO_INCREMENT,
`container` int(20) unsigned NOT NULL DEFAULT '0',
`creator` int(20) unsigned NOT NULL DEFAULT '0',
`ref` int(20) unsigned DEFAULT NULL,
`timestamp` int(20) unsigned NOT NULL DEFAULT '0',
`posticon` tinyint(11) DEFAULT NULL,
`last_edited_ts` int(10) unsigned DEFAULT NULL,
`last_edited_by` int(20) unsigned DEFAULT NULL,
`signature` varchar(250) DEFAULT NULL,
`client_ip` int(10) unsigned NOT NULL DEFAULT '0',
`data_format` tinyint(20) unsigned DEFAULT NULL,
`use_bbcode` tinyint(3) unsigned NOT NULL DEFAULT '1',
`use_smileys` tinyint(3) unsigned NOT NULL DEFAULT '1',
`topic_hash` int(10) unsigned NOT NULL DEFAULT '0',
`del_ts` int(10) unsigned NOT NULL DEFAULT '0',
`del_reason` tinyint(4) NOT NULL DEFAULT '0',
PRIMARY KEY (`cid`,`id`),
UNIQUE KEY `cid` (`cid`,`topic_hash`,`container`,`id`,`del_ts`),
KEY `cid_2` (`cid`,`container`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
CREATE TABLE `1_topics` (
`cid` int(10) unsigned NOT NULL DEFAULT '0',
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`container` int(20) NOT NULL DEFAULT '0',
`name` varchar(128) NOT NULL DEFAULT '',
`creator` int(20) unsigned NOT NULL DEFAULT '0',
`last_modified` int(20) unsigned NOT NULL DEFAULT '0',
`views` int(11) NOT NULL DEFAULT '0',
`closed` tinyint(3) unsigned NOT NULL DEFAULT '0',
`sticky` tinyint(3) unsigned NOT NULL DEFAULT '0',
`last_post_id` int(20) unsigned DEFAULT NULL,
`num_posts` int(10) unsigned DEFAULT NULL,
`lp_ts` int(20) unsigned NOT NULL DEFAULT '0',
`posticon` smallint(5) unsigned DEFAULT NULL,
`hidden` tinyint(3) unsigned NOT NULL DEFAULT '0',
`topic_change_ts` int(10) unsigned NOT NULL DEFAULT '0',
`topic_hash` int(10) unsigned NOT NULL DEFAULT '0',
`forum_hash` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`cid`,`id`),
KEY `container` (`container`),
KEY `last_modified` (`last_modified`),
KEY `sticky` (`sticky`),
KEY `topic_hash` (`topic_hash`),
KEY `forum_hash` (`forum_hash`),
KEY `cid` (`cid`,`id`),
KEY `cid_2` (`cid`),
FULLTEXT KEY `name` (`name`)
) ENGINE=MyISAM AUTO_INCREMENT=211963 DEFAULT CHARSET=latin1
这是Gordon指数补充后的EXPLAIN输出：
+----+-------------+-------+------+-------------------------------+-------+---------+---------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------------------+-------+---------+---------------------+------+----------------------------------------------+
| 1 | SIMPLE | t | ref | PRIMARY,cid,cid_2 | cid | 4 | const | 212 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | p | ref | PRIMARY,cid,cid_2,cid_3,cid_4 | cid_3 | 8 | const,forumdb.t.id | 11 | Using index |
+----+-------------+-------+------+-------------------------------+-------+---------+---------------------+------+----------------------------------------------+