[英]Mysql slow query: INNER JOIN + ORDER BY causes filesort
我正在嘗試優化此查詢:
SELECT `posts`.* FROM `posts` INNER JOIN `posts_tags` ON `posts`.id = `posts_tags`.post_id WHERE (((`posts_tags`.tag_id = 1))) ORDER BY posts.created_at DESC;
表的大小是38k行,31k和mysql使用“filesort”因此它變得非常慢。 我試着使用不同的索引,沒有運氣。
CREATE TABLE `posts` ( `id` int(11) NOT NULL auto_increment, `created_at` datetime default NULL, PRIMARY KEY (`id`), KEY `index_posts_on_created_at` (`created_at`), KEY `for_tags` (`trashed`,`published`,`clan_private`,`created_at`) ) ENGINE=InnoDB AUTO_INCREMENT=44390 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci CREATE TABLE `posts_tags` ( `id` int(11) NOT NULL auto_increment, `post_id` int(11) default NULL, `tag_id` int(11) default NULL, `created_at` datetime default NULL, `updated_at` datetime default NULL, PRIMARY KEY (`id`), KEY `index_posts_tags_on_post_id_and_tag_id` (`post_id`,`tag_id`) ) ENGINE=InnoDB AUTO_INCREMENT=63175 DEFAULT CHARSET=utf8
+----+-------------+------------+--------+--------------------------+--------------------------+---------+---------------------+-------+-----------------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+--------------------------+--------------------------+---------+---------------------+-------+-----------------------------------------------------------+ | 1 | SIMPLE | posts_tags | index | index_post_id_and_tag_id | index_post_id_and_tag_id | 10 | NULL | 24159 | Using where; Using index; Using temporary; Using filesort | | 1 | SIMPLE | posts | eq_ref | PRIMARY | PRIMARY | 4 | .posts_tags.post_id | 1 | | +----+-------------+------------+--------+--------------------------+--------------------------+---------+---------------------+-------+-----------------------------------------------------------+ 2 rows in set (0.00 sec)
我需要定義什么樣的索引來避免使用filesort的mysql? 當order字段不在where子句中時是否可能?
更新:分析結果:
mysql> show profile for query 1; +--------------------------------+----------+ | Status | Duration | +--------------------------------+----------+ | starting | 0.000027 | | checking query cache for query | 0.037953 | | Opening tables | 0.000028 | | System lock | 0.010382 | | Table lock | 0.023894 | | init | 0.000057 | | optimizing | 0.010030 | | statistics | 0.000026 | | preparing | 0.000018 | | Creating tmp table | 0.128619 | | executing | 0.000008 | | Copying to tmp table | 1.819463 | | Sorting result | 0.001092 | | Sending data | 0.004239 | | end | 0.000012 | | removing tmp table | 0.000885 | | end | 0.000006 | | end | 0.000005 | | query end | 0.000006 | | storing result in query cache | 0.000005 | | freeing items | 0.000021 | | closing tables | 0.000013 | | logging slow query | 0.000004 | | cleaning up | 0.000006 | +--------------------------------+----------+
UPDATE2:
真正的查詢(一些更多的布爾字段,更多無用的索引)
SELECT `posts`.* FROM `posts` INNER JOIN `posts_tags` ON `posts`.id = `posts_tags`.post_id WHERE ((`posts_tags`.tag_id = 7971)) AND (((posts.trashed = 0) AND (`posts`.`published` = 1 AND `posts`.`clan_private` = 0)) AND ((`posts_tags`.tag_id = 7971))) ORDER BY created_at DESC LIMIT 0, 10;
空集(1.25秒)
沒有ORDER BY - 0.01s。
+----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+ | 1 | SIMPLE | posts_tags | index | index_posts_tags_on_post_id_and_tag_id | index_posts_tags_... | 10 | NULL | 23988 | Using where; Using index | | 1 | SIMPLE | posts | eq_ref | PRIMARY,index_posts_on_trashed_and_crea | PRIMARY | 4 | .posts_tags.post_id | 1 | Using where | +----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+
解
就這樣!
您需要對一些進行非規范化,並將posts.created_at字段復制到post_tags表中(我稱之為post_created_at,您可以根據需要命名):
CREATE TABLE `posts_tags` (
`id` int(11) NOT NULL auto_increment,
`post_id` int(11) default NULL,
`tag_id` int(11) default NULL,
`post_created_at` datetime default NULL,
`created_at` datetime default NULL,
`updated_at` datetime default NULL,
PRIMARY KEY (`id`),
KEY `index_posts_tags_on_post_id_and_tag_id` (`post_id`,`tag_id`)
) ENGINE=InnoDB;
然后在posts_tags上添加索引
(tag_id, post_created_at)
這將允許查詢以正確的順序獲取標記的所有帖子,而不使用filesort。
嘗試將KEY index_posts_tags_on_post_id_and_tag_id
( post_id
, tag_id
)更改為KEY index_posts_tags_tag_id
( tag_id
)並重新發布Explain。
帶有Posts_Tags的TagID的分布是什么?
您的密鑰index_posts_on_created_at
按升序排序,但您希望結果按降序排序
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