Optimize MySQL count query with JOIN

Question

I have a query that takes about 20 seconds, I would like to understand if there is a way to optimize it. Table 1:

CREATE TABLE IF NOT EXISTS `sessions` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=9845765 ;

And table 2:

CREATE TABLE IF NOT EXISTS `access` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `session_id` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
KEY `session_id ` (`session_id `)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=9467799 ;

Now, what I am trying to do is to count all the access connected to all sessions about one user, so my query is:

SELECT COUNT(*)
FROM access
INNER JOIN sessions ON access.session_id=session.id
WHERE session.user_id='6';

It takes almost 20 seconds...and for user_id 6 there are about 3 millions sessions stored. There is anything I can do to optimize that query?

Answer 1

Change this line from the session table:

KEY `user_id` (`user_id`)

To this:

KEY `user_id` (`user_id`, `id`)

What this will do for you is allow you to complete the query from the index, without going back to the raw table. As it is, you need to do an index scan on the session table for your user_id , and for each item go back to the table to find the id for the join to the access table. By including the id in the index, you can skip going back to the table.

Sadly, this will make your inserts slower into that table, and it seems like this may be a bid deal, given just one user has 3 millions sessions. Sql Server and Oracle would address this by allowing you to include the id column in your index, without actually indexing on it, saving a little work at insert time, and also by allowing you specify a lower fill factor for the index, reducing the need to re-build or re-order the indexes at insert, but MySql doesn't support these.