问题
I'm trying to optimize this query:
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;
The size of tables is 38k rows, and 31k and mysql uses "filesort" so it gets pretty slow. I tried to use different indexes, no luck.
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)
What kind of index I need to define to avoid mysql using filesort? Is it possible when order field is not in where clause?
update: Profiling results:
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:
Real query (some more boolean fields, more useless indexes)
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;
Empty set (1.25 sec)
Without 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 | +----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+
SOLUTION
- Query updated to "ORDER BY posts_tags.created_at DESC" (two small changes in app code)
- Index added: index_posts_tags_on_created_at.
That's all!
回答1:
You would need to denormalize a bit, and copy the posts.created_at field into the post_tags table (I called it post_created_at, you could name it how you want):
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;
and then add an index to posts_tags on
(tag_id, post_created_at)
That will allow the query to get all the posts for a tag, in the correct order, without filesort.
回答2:
Try changing KEY index_posts_tags_on_post_id_and_tag_id (post_id,tag_id) to KEY index_posts_tags_tag_id (tag_id) and repost Explain.
What is the distribution of TagIDs withing Posts_Tags?
回答3:
your key index_posts_on_created_at is sorted ascending but you want results sorted descending
来源：https://stackoverflow.com/questions/3015391/mysql-slow-query-inner-join-order-by-causes-filesort