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Query optimization

From: tmp <skrald(at)amossen(dot)dk>
To: pgsql-performance(at)postgresql(dot)org
Subject: Query optimization
Date: 2008-11-30 18:45:11
Message-ID: ggumvn$1n73$ (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
I am struggeling with the following query which fetches a random subset 
of 200 questions that matches certain tags within certain languages. 
However, the query takes forever to evaluate, even though I have a 
"limit 200" appended. Any ideas on how to optimize it?

QUERY: ================================================

SELECT distinct q.question_id
               FROM question_tags qt, questions q
               WHERE q.question_id = qt.question_id
                     AND q.STATUS = 1
                     AND not q.author_id = 105
                     AND ((qt.language_id = 5 and qt.tag_id in 
(1101,917,734,550,367,183)) or (qt.language_id = 4 and qt.tag_id in 
(856,428)) or (qt.language_id =
  3 and qt.tag_id in (1156,1028,899,771,642,514,385,257,128)) or 
(qt.language_id = 2 and qt.tag_id in 
                      and q.question_id not in (413)
               LIMIT 200

EXPLAIN ANALYZE: =========================================

  Limit  (cost=1.50..1267.27 rows=200 width=4) (actual 
time=278.169..880.934 rows=200 loops=1)
    ->  Unique  (cost=1.50..317614.50 rows=50185 width=4) (actual 
time=278.165..880.843 rows=200 loops=1)
          ->  Merge Join  (cost=1.50..317489.04 rows=50185 width=4) 
(actual time=278.162..880.579 rows=441 loops=1)
                Merge Cond: (qt.question_id = q.question_id)
                ->  Index Scan using question_tags_question_id on 
question_tags qt  (cost=0.00..301256.96 rows=82051 width=4) (actual 
time=24.171..146.811 rows=6067 loops=1)
                      Filter: (((language_id = 5) AND (tag_id = ANY 
('{1101,917,734,550,367,183}'::integer[]))) OR ((language_id = 4) AND 
(tag_id = ANY ('{856,428}'::integer[]))) OR ((language_id = 3) AND 
(tag_id = ANY ('{1156,1028,899,771,642,514,385,257,128}'::integer[]))) 
OR ((language_id = 2) AND (tag_id = ANY 
                ->  Index Scan using questions_pkey on questions q 
(cost=0.00..15464.12 rows=83488 width=4) (actual time=222.956..731.737 
rows=1000 loops=1)
                      Filter: ((q.author_id <> 105) AND (q.question_id 
<> 413) AND (q.status = 1))
  Total runtime: 881.152 ms
(9 rows)


pgsql-performance by date

Next:From: Marc CousinDate: 2008-11-30 19:37:39
Subject: Re: Query optimization
Previous:From: Greg StarkDate: 2008-11-30 13:02:51
Subject: Re: Increasing GROUP BY CHAR columns speed

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