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POStrange Performance Issues with INNER JOIN vs. LEFT JOIN
 

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  1. POStrange Performance Issues with INNER JOIN vs. LEFT JOIN
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    <p>I was using a query that looked similar to this one:</p> <pre><code>SELECT `episodes`.*, IFNULL(SUM(`views_sum`.`clicks`), 0) as `clicks` FROM `episodes`, `views_sum` WHERE `views_sum`.`index` = "episode" AND `views_sum`.`key` = `episodes`.`id` GROUP BY `episodes`.`id` </code></pre> <p>... which takes ~0.1s to execute. But it's problematic, because some <code>episodes</code> don't have a corresponding <code>views_sum</code> row, so those episodes aren't included in the result.</p> <p>What I want is NULL values when a corresponding <code>views_sum</code> row doesn't exist, so I tried using a LEFT JOIN instead:</p> <pre><code>SELECT `episodes`.*, IFNULL(SUM(`views_sum`.`clicks`), 0) as `clicks` FROM `episodes` LEFT JOIN `views_sum` ON (`views_sum`.`index` = "episode" AND `views_sum`.`key` = `episodes`.`id`) GROUP BY `episodes`.`id` </code></pre> <p>This query produces the same columns, and it also includes the few rows missing from the 1st query.</p> <p><em>BUT</em>, the 2nd query takes 10 times as long! A full second.</p> <p>Why is there such a huge discrepancy between the execution times when the result is so similar? There's nowhere <em>near</em> 10 times as many rows — it's like 60 from the 1st query, and 70 from the 2nd. That's not to mention that the 10 additional rows have no <code>views</code> to sum!</p> <p>Any light shed would be greatly appreciated!</p> <p>(There are indexes on <code>episodes.id</code>, <code>views_sum.index</code>, and <code>views_sum.key</code>.)</p> <p><strong>EDIT:</strong></p> <p>I copy-pasted the SQL from above, and here are the EXPLAINs, in order:</p> <pre><code>id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE views_sum ref index,key index 27 const 6532 Using where; Using temporary; Using filesort 1 SIMPLE episodes eq_ref PRIMARY PRIMARY 4 db102914_itw.views_sum.key 1 Using where id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE episodes ALL NULL NULL NULL NULL 70 Using temporary; Using filesort 1 SIMPLE views_sum ref index,key index 27 const 6532 </code></pre>
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    1. COI corrected your title, because there's no cross join in either example. A cross join produces a cartesian product, but the first query uses ANSI-89 inner join syntax.
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    2. COThanks, OMG. So a CROSS JOIN is an INNER JOIN without any comparison between the two tables?
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