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    <h1>Changing the table</h1> <hr> <p>Based on the advice in this post on <a href="http://mysqldba.blogspot.com/2008/06/how-to-pick-indexes-for-order-by-and.html" rel="nofollow">How to pick indexes for order by and group by queries</a> the table now looks like <pre><code>CREATE TABLE ClusterMatches ( cluster_index INT UNSIGNED, match_index INT UNSIGNED, id INT NOT NULL AUTO_INCREMENT, tfidf FLOAT, PRIMARY KEY (match_index,cluster_index,id,tfidf) ); CREATE TABLE MatchLookup ( match_index INT UNSIGNED NOT NULL PRIMARY KEY, image_match TINYTEXT ); </pre></code></p> <h1>Eliminating Subquery</h1> <p>The query without sorting the results by the SUM(tfidf) looks like <pre><code>SELECT match_index, SUM(tfidf) FROM ClusterMatches WHERE cluster_index in (1,2,3 ... 3000) GROUP BY match_index LIMIT 10;</pre></code></p> <p>Which eliminates using temporary and using filesort </p> <p><pre><code>explain extended SELECT match_index, SUM(tfidf) FROM ClusterMatches WHERE cluster_index in (1,2,3 ... 3000) GROUP BY match_index LIMIT 10; +----+-------------+----------------------+-------+---------------+---------+---------+------+-------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------------------+-------+---------------+---------+---------+------+-------+--------------------------+ | 1 | SIMPLE | ClusterMatches | range | PRIMARY | PRIMARY | 4 | NULL | 14938 | Using where; Using index | +----+-------------+----------------------+-------+---------------+---------+---------+------+-------+--------------------------+</pre></code></p> <h1>Sorting Problem</h1> <p>However if i add the ORDER BY SUM(tfdif) in <code><pre>SELECT match_index, SUM(tfidf) AS total FROM ClusterMatches WHERE cluster_index in (1,2,3 ... 3000) GROUP BY match_index ORDER BY total DESC LIMIT 0,10; +-------------+--------------------+ | match_index | total | +-------------+--------------------+ | 868 | 0.11126546561718 | | 4182 | 0.0238558370620012 | | 2162 | 0.0216601379215717 | | 1406 | 0.0191618576645851 | | 4239 | 0.0168981291353703 | | 1437 | 0.0160425212234259 | | 2599 | 0.0156466849148273 | | 394 | 0.0155945559963584 | | 3116 | 0.0151005545631051 | | 4028 | 0.0149106932803988 | +-------------+--------------------+ 10 rows in set (0.03 sec)</code></pre></p> <p>The result is suitably fast at this scale BUT having the <strong>ORDER BY SUM(tfidf) means it uses temporary and filesort</strong> <code><pre>explain extended SELECT match_index, SUM(tfidf) AS total FROM ClusterMatches WHERE cluster_index IN (1,2,3 ... 3000) GROUP BY match_index ORDER BY total DESC LIMIT 0,10; +----+-------------+----------------------+-------+---------------+---------+---------+------+-------+-----------------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------------------+-------+---------------+---------+---------+------+-------+-----------------------------------------------------------+ | 1 | SIMPLE | ClusterMatches | range | PRIMARY | PRIMARY | 4 | NULL | 65369 | Using where; Using index; Using temporary; Using filesort | +----+-------------+----------------------+-------+---------------+---------+---------+------+-------+-----------------------------------------------------------+</code></pre></p> <h1>Possible Solutions?</h1> <p>Im looking for a solution that doesn't use temporary or filesort, along the lines of <code><pre>SELECT match_index, SUM(tfidf) AS total FROM ClusterMatches WHERE cluster_index IN (1,2,3 ... 3000) GROUP BY cluster_index, match_index HAVING total>0.01 ORDER BY cluster_index;</code></pre> where I dont need to hardcode a threshold for total, any ideas?</p>
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