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POMySQL "IN" queries terribly slow with subquery but fast with explicit values
 

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    <p>I have a MySQL query (Ubu 10.04,Innodb, Core i7, 16Gb RAM, SSD drives, MySQL params optimized):</p> <pre><code>SELECT COUNT(DISTINCT subscriberid) FROM em_link_data WHERE linkid in (SELECT l.id FROM em_link l WHERE l.campaignid = '2900' AND l.link != 'open') </code></pre> <p>The table em_link_data has about 7million rows, em_link has a few thousand. This query will take about <strong>18 seconds</strong> to complete. However, if I substitute the results of the subquery and do this:</p> <pre><code>SELECT COUNT(DISTINCT subscriberid) FROM em_link_data WHERE linkid in (24899,24900,24901,24902); </code></pre> <p>then the query will run in less than 1 millisecond. The subquery alone runs in less than 1ms, the column linkid is indexed.</p> <p>If I rewrite the query as a join, also less than 1ms. Why is a "IN" query so slow with a subquery in it and why so fast with values in it? I can't rewrite the query (bought software) so I was hoping there is some tweak or hint to speedup this query! Any help is appreciated.</p>
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    1. USFrustratedWithFormsDesigner
    1. USFranco
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    1. COshow us EXPLAIN SELECT
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      1. USservermanfail
    2. COWhat does your explain plan say? What indexes do you configured?
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      1. USdiagonalbatman
    3. COMy guess is that `em_link` needs an index containing `campaignid` and `link`.
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      1. USFrustratedWithFormsDesigner
 

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