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POSQL Nested Query is slow
 

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  1. POSQL Nested Query is slow
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    <p>I have a database where users are represented by hashes. Each user (hash) has associated values (pertaining to the music track that he was listening to). Since, a user can listen to more than one track there are repeat instances of the user and associated data (with data being different as it is now a different track). </p> <p>What I would like to do is select ~10 users at random from this database, and then find their associated data. </p> <p>Currently, the code that I am using is this:</p> <pre><code>SELECT * FROM `tblPlayLogV4` WHERE `titleId` &lt;&gt; 0 AND `hash` IN (SELECT `hash` FROM `tblPlayLogV4` WHERE `titleId` &lt;&gt; 0 AND RAND() &lt;= 0.1 GROUP BY `hash` HAVING COUNT(`hash`) &gt; 500); </code></pre> <p>Why RAND() - because LIMIT is not allowed in inner queries. Idea for RAND() here - <a href="http://www.rndblog.com/how-to-select-random-rows-in-mysql/" rel="nofollow">http://www.rndblog.com/how-to-select-random-rows-in-mysql/</a></p> <p>The above query takes ages to complete. </p> <p>If however, I run the inner query separately, it finishes in 4.53s. I then hard code the result of the inner query in the outer query, and that finishes in about 275 ms. The separated queries are presented below:</p> <pre><code>SELECT `hash` FROM `tblPlayLogV4` WHERE `titleId` &lt;&gt; 0 AND RAND() &lt;= 0.1 GROUP BY `hash` HAVING COUNT(`hash`) &gt; 500); SELECT * FROM `tblPlayLogV4` WHERE `hash` IN ('-29e291921cccd06a5813bca17b7f7c3','-2c08232108dcd93c443d821165c2c79','-58285c1602072da713e51cc6cdc6313','-5bcc2c42482d5805277609a84474aef','-79ecab520d661a1d624de7e7b04f265','-e937c753a96fc9e441f83af97b08489','04d3f1e91e4e92970819190346405d2d','3f9f0cd502de38d47e39367cdfdd6722') AND `titleId`&lt;&gt;0; </code></pre> <p>Can someone please explain to me why this is happening? What is it that I am doing wrong? And if there is a better way for me to formulate my query, do tell me.</p> <p>Number of entries in the database: 6,322,605</p>
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    1. USSam Holder
    1. USrahulthewall
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    1. CODon't know if this will improve performance by so much but you check ``titleId` <> 0` in both inner and outer query. You should be able to remove it from the outer query and still get the same result.
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      1. USjlundqvist
    2. COActually, in the inner query I check for titleID to ensure that only those instances of 'hash' are included in the count where the titleID is not 0. This is to remove hashes where most of the titleIDs are zero. Then in the outer query, I check again because I do not want those records where the titleID is zero.
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      1. USrahulthewall
 

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