Re: bad planner pick... but why?

> FWIW, 8.2 will do better.

i can confirm this is fixed for 8.2b1 - the query runs in 2ms now.

thanks for the great work in planner improvement!

- thomas

----- Original Message -----
From: "Tom Lane" <tgl@sss.pgh.pa.us>
To: <me@alternize.com>
Cc: <pgsql-novice@postgresql.org>
Sent: Monday, October 16, 2006 6:17 PM
Subject: Re: [NOVICE] bad planner pick... but why?


> <me@alternize.com> writes:
>> SELECT mov_id FROM oldtables.movies LEFT JOIN oldtables.content ON
>> movies.mov_id = content.c_m_id
>> WHERE mov_id IN (SELECT DISTINCT rel_movieid FROM infos.rel_persons WHERE
>> rel_personid = 40544)
>
> Try dropping the DISTINCT, which is redundant given the IN.
>
>> query #1 is factor 1000 slower, because the two tables "movies" (~40k
>> entries) and "content" (~30k entries) seem to be joined prior to
>> filtering
>> by the IN (....). any ideas why the planer decides not to first evaluate
>> the
>> IN (...) statement in the first case?
>
> 8.1 doesn't know anything about rearranging join order in the face of
> outer joins.  In the second case, the strict WHERE condition applied to
> the content table allows it to recognize that the outer join can be
> reduced to an inner join, and then it can rearrange the join order.
> (If you thought these queries were equivalent, you're wrong.)
>
> FWIW, 8.2 will do better.
>
> regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
>