Re: *very* inefficient choice made by the planner (regarding

From: Tom Lane
Subject: Re: *very* inefficient choice made by the planner (regarding
Date: ,
Msg-id: 20172.1087568965@sss.pgh.pa.us
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In response to: Re: *very* inefficient choice made by the planner (regarding  (SZUCS Gábor)
List: pgsql-performance


=?iso-8859-1?Q?SZUCS_G=E1bor?= <> writes:
>> It's NULLs inside the subselect that are the issue.
>
> select 1 in (select a from foo)
> select exists ( select 1 from foo where a=1)

> Just a dumb try :)

>   SELECT (exists(select 1 from foo where a isnull) AND NULL)
>        OR exists(select 1 from foo where a=1)

The more general case is where you have a variable (or expression)
on the left of IN.  That could be NULL too, and this still doesn't
give the right result in that case :-(.  With NULL on the left the
correct answer would be FALSE if the subselect has zero rows,
NULL otherwise.

            regards, tom lane


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