Re: BUG #7509: x NOT IN (select x from z) extremely slow in compare to select x from y except select x from z; - Mailing list pgsql-bugs

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