Question

I am trying to optimize a sql query which is using order by clause. When I use EXPLAIN the query always displays “using filesort”. I am applying this query for a group discussion forum where there are tags attached to posts by users.

Here are the 3 tables I am using: users, user_tag, tags

user_tag is the association mapping table for users and their tags.

CREATE TABLE `usertable` (
 `user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `user_name` varchar(20) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL,
 PRIMARY KEY (`user_name`),
 KEY `user_id` (`user_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `user_tag` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
 `user_id` int(11) unsigned NOT NULL,
 `tag_id` int(11) unsigned NOT NULL,
 `usage_count` int(11) unsigned NOT NULL,
 PRIMARY KEY (`id`),
 KEY `tag_id` (`tag_id`),
 KEY `usage_count` (`usage_count`),
 KEY `user_id` (`user_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

I update the usage_count on server side using programming. Here is the query that’s giving me problem. The query is to find out the tag_id and usage_count for a particular username, sorted by usage count in descending order

select user_tag.tag_id, user_tag.usage_count
  from user_tag inner join usertable on usertable.user_id = user_tag.user_id
 where user_name="abc" order by usage_count DESC;

Here is the explain output:

mysql> explain select
    user_tag.tag_id,
    user_tag.usage_count from user_tag
    inner join usertable on
    user_tag.user_id = usertable.user_id
    where user_name="abc" order by
    user_tag.usage_count desc;

Explain output here

What should I be changing to lose that “Using filesort”

Answer 1

I’m rather rusty with this, but here goes.

The key used to fetch the rows is not the same as the one used in the ORDER BY:

http://dev.mysql.com/doc/refman/5.1/en/order-by-optimization.html

As mentioned by OMG Ponies, an index on user_id, usage_count may resolve the filesort.

KEY `user_id_usage_count` (`user_id`,`usage_count`)