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List:       pgsql-bugs
Subject:    Re: [BUGS] BUG #7509: x NOT IN (select x from z) extremely slow in compare to select x from y except
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2012-08-29 15:20:44
Message-ID: 10470.1346253644 () sss ! pgh ! pa ! us
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stefan@konink.de writes:
> The following is relatively fast:
> bag-2012-aug=# explain select count(*) from (select kvk from kvk_normal
> except select kvk from bag_kvk) as x;

> The 'normal' case basically doesn't finish:
> bag-2012-aug=# explain select count(*) from (select kvk_normal.kvk from
> kvk_normal where kvk_normal.kvk not in (select bag_kvk.kvk from bag_kvk)) as
> x;

NOT IN is difficult to optimize, as well as hard to use, because of its
rather bizarre behavior for nulls.  You might consider using NOT EXISTS
instead.

			regards, tom lane


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