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Re: Slow Query, Optimization Advice Needed
Posted by: Øystein Grøvlen
Date: April 28, 2016 01:25AM

Hi,

It is true that many IN-subqueries perform bad in MySQL 5.5 and earlier. However, I would expect for this particular query that MySQL 5.6 will use the semi-join FirstMatch strategy. That is basically the same strategy as used in earlier MySQL versions. It is pretty efficient in this case since an index can be used to check for matches.

The advantage of semi-join shows when you can either benefit from materializing the inner query, or process the tables in a different order. The former is not possible here since the subquery is dependent on the outer query (it refers to a column of the outer query.) The latter will usually only be an advantage when there is some conditions in the WHERE clause of the inner query that can be applied before the join.

Øystein Grøvlen,
Senior Principal Software Engineer,
MySQL Group, Oracle,
Trondheim, Norway

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