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POMySQL dependent subqueries when should be independent
 

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  1. POMySQL dependent subqueries when should be independent
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    <p>when I explain this query:</p> <pre><code>SELECT DISTINCT(u.mail) FROM ep_point_transactions pt JOIN ep_transaction_status ts ON ts.tid = pt.tid JOIN users u ON u.uid = pt.uid WHERE ts.anulada IS NULL AND pt.anulada IS NULL AND ts.source = 'rake' AND (pt.educapuntos * pt.multiplicador) &gt;= 7500 AND ts.timestamp &lt; '2010-11-30 23:00:00' AND pt.uid NOT IN ( SELECT distinct(pt2.uid) FROM ep_point_transactions pt2 JOIN ep_transaction_status ts2 ON ts2.tid = pt2.tid WHERE ts2.anulada IS NULL AND pt2.anulada IS NULL AND ts2.source = 'rake' AND ts2.timestamp &gt; '2010-11-30 23:00:00' ); </code></pre> <p>I'm obtaining this output:</p> <pre><code>+----+--------------------+-------+--------+----------------+---------+---------+-------------------------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+-------+--------+----------------+---------+---------+-------------------------+------+------------------------------+ | 1 | PRIMARY | ts | ref | PRIMARY,source | source | 194 | const | 2997 | Using where; Using temporary | | 1 | PRIMARY | pt | ref | PRIMARY | PRIMARY | 4 | educapoker.ts.tid | 30 | Using where | | 1 | PRIMARY | u | eq_ref | PRIMARY | PRIMARY | 4 | educapoker.pt.uid | 1 | | | 2 | DEPENDENT SUBQUERY | ts2 | ref | PRIMARY,source | source | 194 | const | 2997 | Using where; Using temporary | | 2 | DEPENDENT SUBQUERY | pt2 | eq_ref | PRIMARY | PRIMARY | 8 | educapoker.ts2.tid,func | 1 | Using where | +----+--------------------+-------+--------+----------------+---------+---------+-------------------------+------+------------------------------+ </code></pre> <p>My doubt here is why MySQL is considering the subquery dependent if I can execute it isolately.</p> <p>As far as I know, in a dependent subquery, MySQL executes the subquery once for each parent query row, so it can be really inefficient.</p> <p>Can anyone bring me some light here, please? Thanks.</p>
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    1. UScarles
    1. UScarles
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    1. This table or related slice is empty.
    1. COWhat happens if you move the subquery to your where clause?
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      1. POMySQL dependent subqueries when should be independent
      1. USBrynJ
    2. COMySQL performs better filtering in FROM clause. The problem here is that the optimizer is executing the subquery once per each result (thousands), because it's is considering the subquery as being dependent of the main one. This is making the server enter in a never-ending execution.
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      1. POMySQL dependent subqueries when should be independent
      1. UScarles
 

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