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POGROUP BY query optimization
 

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  1. POGROUP BY query optimization
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    <p>Database is MySQL with MyISAM engine.</p> <p>Table definition:</p> <pre><code>CREATE TABLE IF NOT EXISTS matches ( id int(11) NOT NULL AUTO_INCREMENT, game int(11) NOT NULL, user int(11) NOT NULL, opponent int(11) NOT NULL, tournament int(11) NOT NULL, score int(11) NOT NULL, finish tinyint(4) NOT NULL, PRIMARY KEY ( id ), KEY game ( game ), KEY user ( user ), KEY i_gfu ( game , finish , user ) ) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=3149047 ; </code></pre> <p>I have set an index on <code>(game, finish, user)</code> but this <code>GROUP BY</code> query still needs 0.4 - 0.6 seconds to run:</p> <pre><code>SELECT user AS player , COUNT( id ) AS times FROM matches WHERE finish = 1 AND game = 19 GROUP BY user ORDER BY times DESC </code></pre> <p>The <code>EXPLAIN</code> output:</p> <pre><code>| id | select_type | table | type | possible_keys | key | key_len | | 1 | SIMPLE | matches | ref | game,i_gfu | i_gfu | 5 | | ref | rows | Extra | | const,const | 155855 | Using where; Using temporary; Using filesort | </code></pre> <p>Is there any way I can make it faster? The table has about 800K records.</p> <hr> <p>EDIT: I changed <code>COUNT(id)</code> into <code>COUNT(*)</code> and the time dropped to 0.08 - 0.12 seconds. I think I've tried that before making the index and forgot to change it again after.</p> <p>In the explain output the <strong>Using index</strong> explains the speeding up:</p> <pre><code>| rows | Extra | | 168029 | Using where; Using index; Using temporary; Using filesort | </code></pre> <p>(Side question: is this dropping of a factor of 5 normal?)</p> <p>There are about 2000 users, so the final sorting, even if it uses filesort, it doesn't hurt performance. I tried without <code>ORDER BY</code> and it still takes almost same time.</p>
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    1. COThe reason the count(\*) has a much faster performance than count(id) is MySQL has a specific optimization for the count(\*) case. The count(id) case does a second pass through the data to retrieve the results, where the count(\*) uses existing internal row counters. Use count(\*) whenever possible.
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      1. USThomas Jones-Low
 

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