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Faster with a sub-query then without

From: Martin Foster <martin(at)ethereal-realms(dot)org>
To: pgsql-performance(at)postgresql(dot)org
Subject: Faster with a sub-query then without
Date: 2004-08-15 03:03:28
Message-ID: 4oATc.14571$S55.11652@clgrps12 (view raw or flat)
Thread:
Lists: pgsql-performance
I thought this could generate some interesting discussion.  Essentially, 
there are three queries below, two using sub-queries to change the way 
the randomized information (works first by author and then by work) and 
the original which simply randomizes out of all works available.

The one not using sub-queries under EXPLAIN ANALYZE proves itself to be 
less efficient and have a far higher cost then those with the penalty of 
a sub-query.   Since this seems to be counter to what I have been told 
in the past, I thought I would bring this forward and get some 
enlightenment.

	Martin Foster
	Creator/Designer Ethereal Realms
	martin(at)ethereal-realms(dot)org

-----------------------------------------------------------------------

SELECT
  g.GalleryID,
  w.WorkID,
  w.WorkName,
  w.WorkImageThumbnail,
  g.GalleryRating,
  g.GalleryPenName
FROM ethereal.Work w, ethereal.Gallery g
WHERE w.GalleryID = g.GalleryID
  AND g.GalleryPrivacy = 'no'
  AND w.WorkImageThumbnail IS NOT NULL
  AND g.PuppeteerLogin = (SELECT PuppeteerLogin
   FROM ethereal.Gallery
   WHERE GalleryType='image'
   GROUP BY PuppeteerLogin
   ORDER BY RANDOM() LIMIT 1)
ORDER BY RANDOM() LIMIT 1

  Limit  (cost=60.70..60.70 rows=1 width=100) (actual time=1.013..1.013 
rows=0 loops=1)
    InitPlan
      ->  Limit  (cost=6.36..6.37 rows=1 width=11) (actual 
time=0.711..0.713 rows=1 loops=1)
            ->  Sort  (cost=6.36..6.45 rows=33 width=11) (actual 
time=0.708..0.708 rows=1 loops=1)
                  Sort Key: random()
                  ->  HashAggregate  (cost=5.45..5.53 rows=33 width=11) 
(actual time=0.420..0.553 rows=46 loops=1)
                        ->  Seq Scan on gallery  (cost=0.00..5.30 
rows=60 width=11) (actual time=0.007..0.227 rows=59 loops=1)
                              Filter: ((gallerytype)::text = 'image'::text)
    ->  Sort  (cost=54.33..54.37 rows=16 width=100) (actual 
time=1.009..1.009 rows=0 loops=1)
          Sort Key: random()
          ->  Nested Loop  (cost=0.00..54.01 rows=16 width=100) (actual 
time=0.981..0.981 rows=0 loops=1)
                ->  Seq Scan on gallery g  (cost=0.00..5.56 rows=2 
width=24) (actual time=0.855..0.888 rows=1 loops=1)
                      Filter: (((galleryprivacy)::text = 'no'::text) AND 
((puppeteerlogin)::text = ($0)::text))
                ->  Index Scan using pkwork on "work" w 
(cost=0.00..24.10 rows=8 width=80) (actual time=0.080..0.080 rows=0 loops=1)
                      Index Cond: (w.galleryid = "outer".galleryid)
                      Filter: (workimagethumbnail IS NOT NULL)
  Total runtime: 1.211 ms

-----------------------------------------------------------------------

SELECT
  g.GalleryID,
  w.WorkID,
  w.WorkName,
  w.WorkImageThumbnail,
  g.GalleryRating,
  g.GalleryPenName
FROM ethereal.Work w, ethereal.Gallery g
WHERE w.GalleryID = g.GalleryID
  AND g.GalleryPrivacy = 'no'
  AND w.WorkImageThumbnail IS NOT NULL
  AND g.GalleryPenName = (SELECT GalleryPenName
   FROM ethereal.Gallery
   WHERE GalleryType='image'
   GROUP BY GalleryPenName
   ORDER BY RANDOM() LIMIT 1)
ORDER BY RANDOM() LIMIT 1

  Limit  (cost=59.92..59.92 rows=1 width=100) (actual time=0.904..0.906 
rows=1 loops=1)
    InitPlan
      ->  Limit  (cost=6.69..6.69 rows=1 width=14) (actual 
time=0.731..0.733 rows=1 loops=1)
            ->  Sort  (cost=6.69..6.79 rows=42 width=14) (actual 
time=0.729..0.729 rows=1 loops=1)
                  Sort Key: random()
                  ->  HashAggregate  (cost=5.45..5.56 rows=42 width=14) 
(actual time=0.431..0.568 rows=48 loops=1)
                        ->  Seq Scan on gallery  (cost=0.00..5.30 
rows=60 width=14) (actual time=0.011..0.233 rows=59 loops=1)
                              Filter: ((gallerytype)::text = 'image'::text)
    ->  Sort  (cost=53.23..53.27 rows=16 width=100) (actual 
time=0.899..0.899 rows=1 loops=1)
          Sort Key: random()
          ->  Nested Loop  (cost=0.00..52.91 rows=16 width=100) (actual 
time=0.808..0.862 rows=6 loops=1)
                ->  Index Scan using idxgallery_pen on gallery g 
(cost=0.00..4.45 rows=2 width=24) (actual time=0.767..0.769 rows=1 loops=1)
                      Index Cond: ((gallerypenname)::text = ($0)::text)
                      Filter: ((galleryprivacy)::text = 'no'::text)
                ->  Index Scan using pkwork on "work" w 
(cost=0.00..24.10 rows=8 width=80) (actual time=0.020..0.042 rows=6 loops=1)
                      Index Cond: (w.galleryid = "outer".galleryid)
                      Filter: (workimagethumbnail IS NOT NULL)
  Total runtime: 1.117 ms

-----------------------------------------------------------------------

SELECT
  g.GalleryID,
  w.WorkID,
  w.WorkName,
  w.WorkImageThumbnail,
  g.GalleryRating,
  g.GalleryPenName
FROM ethereal.Work w, ethereal.Gallery g
WHERE w.GalleryID = g.GalleryID
  AND g.GalleryType = 'image'
  AND g.GalleryPrivacy = 'no'
  AND w.WorkImageThumbnail IS NOT NULL
ORDER BY RANDOM() LIMIT 1

--------
  Limit  (cost=111.73..111.73 rows=1 width=100) (actual 
time=13.021..13.023 rows=1 loops=1)
    ->  Sort  (cost=111.73..113.70 rows=786 width=100) (actual 
time=13.017..13.017 rows=1 loops=1)
          Sort Key: random()
          ->  Hash Join  (cost=5.55..73.93 rows=786 width=100) (actual 
time=1.081..8.320 rows=803 loops=1)
                Hash Cond: ("outer".galleryid = "inner".galleryid)
                ->  Seq Scan on "work" w  (cost=0.00..54.47 rows=817 
width=80) (actual time=0.006..2.207 rows=817 loops=1)
                      Filter: (workimagethumbnail IS NOT NULL)
                ->  Hash  (cost=5.30..5.30 rows=100 width=24) (actual 
time=0.669..0.669 rows=0 loops=1)
                      ->  Seq Scan on gallery g  (cost=0.00..5.30 
rows=100 width=24) (actual time=0.020..0.402 rows=100 loops=1)
                            Filter: ((galleryprivacy)::text = 'no'::text)
  Total runtime: 13.252 ms


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Subject: Re: Faster with a sub-query then without
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Subject: Re: Replication: Slony-I vs. Mammoth Replicator vs. ?

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