From: tmp <skrald@amossen.dk>
Subject: Query optimization
Date: Sun, 30 Nov 2008 19:45:11 +0100
Organization: Hub.Org Networking Services
Lines: 47
Message-ID: <ggumvn$1n73$1@news.hub.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
User-Agent: Thunderbird 2.0.0.18 (Macintosh/20081105)
To: pgsql-performance@postgresql.org

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 
(1193,1101,1009,917,826,734,642,550,458,367,275,183,91)))
                      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 
('{1193,1101,1009,917,826,734,642,550,458,367,275,183,91}'::integer[]))))
                ->  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)