Re: Slow query - Mailing list pgsql-performance

I just ran the query you sent me and attached the output of EXPLAIN
ANALYZE as TOMs_plan.txt
It did not speed up the query significantly.

It always seemed to me that UNION is faster than OR, so I tried your
suggestion to use UNION ALL with the original query without
counter-productive LIMIT 1 in EXISTS clause. This reduced the cost of
the plan by 50%, but slowed down the query. Weird ... The plan is shown
in UNION_ALL_plan.txt

AFAIK, the only change I've done since the time when the query took 3
sec. to run was adding more indexes and increasing the size of data by
about 25%. It sounds kind of stupid, but I remember that adding indexes
sometimes slowed down my queries. I will try to drop all the indexes and
add them back again one by one.

Any other ideas?

Thanks.
Oleg




-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Monday, March 24, 2003 1:48 PM
To: Oleg Lebedev
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Slow query


Oleg Lebedev <oleg.lebedev@waterford.org> writes:
> SELECT * FROM media m
> WHERE m.mediatype =3D (SELECT objectid FROM mediatype WHERE
> medianame=3D'Audio')=20 AND EXISTS=20
>         (SELECT * FROM=20
>                 (SELECT objectid AS mediaid=20
>                 FROM media=20
>                 WHERE activity=3D'347667'=20
>                 UNION=20
>                 SELECT ism.media AS mediaid=20
>                 FROM intsetmedia ism, set s=20
>                 WHERE ism.set =3D s.objectid=20
>                 AND s.activity=3D'347667' ) AS a1=20
>         WHERE a1.mediaid =3D m.objectid=20
>         LIMIT 1)=20
> ORDER BY medianame ASC, status DESC=20

Well, one observation is that the LIMIT clause is useless and probably
counterproductive; EXISTS takes only one row from the subselect anyway.
Another is that the UNION is doing it the hard way; UNION implies doing
a duplicate-elimination step, which you don't need here.  UNION ALL
would be a little quicker.  But what I would do is split it into two
EXISTS:

SELECT * FROM media m
WHERE m.mediatype = (SELECT objectid FROM mediatype WHERE
medianame='Audio')
AND (   EXISTS(SELECT 1
               FROM media
               WHERE activity='347667'
               AND objectid = m.objectid)
     OR EXISTS(SELECT 1
               FROM intsetmedia ism, set s
               WHERE ism.set = s.objectid
               AND s.activity='347667'
               AND ism.media = m.objectid))
ORDER BY medianame ASC, status DESC

            regards, tom lane



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