Re: [PERFORM] 512,600ms query becomes 7500ms... but why? Postgres 8.3 query planner quirk?

Bryce Nesbitt Fri, 12 Feb 2010 23:46:02 -0800

Kevin Grittner wrote:

Bryce Nesbitt <bry...@obviously.com> wrote:

I've got a very slow query, which I can make faster by doing
something seemingly trivial.

 
Out of curiosity, what kind of performance do you get with?:
 
EXPLAIN ANALYZE
SELECT contexts.context_key
  FROM contexts
  JOIN articles ON (articles.context_key = contexts.context_key)
  JOIN matview_82034 ON (matview_82034.context_key =
                         contexts.context_key)
  WHERE EXISTS
        (
          SELECT *
            FROM article_words
            JOIN words using (word_key)
            WHERE context_key = contexts.context_key
              AND word = 'insider'
        )
    AND EXISTS
        (
          SELECT *
            FROM article_words
            JOIN words using (word_key)
            WHERE context_key = contexts.context_key
              AND word = 'trading'
        )
    AND EXISTS
        (
          SELECT *
            FROM virtual_ancestors a
            JOIN bp_categories ON (bp_categories.context_key =
                                   a.ancestor_key)
            WHERE a.context_key = contexts.context_key
              AND lower(bp_categories.category) = 'law'
        )
    AND articles.indexed
;

512,600ms query becomes 225,976ms. Twice as fast on pos
Definitely not beating the 7500ms version.
PostgreSQL 8.3.4

EXPLAIN ANALYZE
SELECT contexts.context_key
  FROM contexts
  JOIN articles ON (articles.context_key = contexts.context_key)
  JOIN matview_82034 ON (matview_82034.context_key =
                         contexts.context_key)
  WHERE EXISTS
        (
          SELECT *
            FROM article_words
            JOIN words using (word_key)
            WHERE context_key = contexts.context_key
              AND word = 'insider'
        )
    AND EXISTS
        (
          SELECT *
            FROM article_words
            JOIN words using (word_key)
            WHERE context_key = contexts.context_key
              AND word = 'trading'
        )
    AND EXISTS
        (
          SELECT *
            FROM virtual_ancestors a
            JOIN bp_categories ON (bp_categories.context_key =
                                   a.ancestor_key)
            WHERE a.context_key = contexts.context_key
              AND lower(bp_categories.category) = 'law'
        )
    AND articles.indexed
;




------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=13511.95..13662023.05 rows=19282 width=4) (actual 
time=3070.834..225973.159 rows=622 loops=1)
   ->  Hash Join  (cost=13511.95..356938.38 rows=177937 width=8) (actual 
time=1106.242..7520.756 rows=351337 loops=1)
         Hash Cond: (articles.context_key = matview_82034.context_key)
         ->  Seq Scan on articles  (cost=0.00..334502.90 rows=386266 width=4) 
(actual time=0.030..4031.203 rows=355626 loops=1)
               Filter: indexed
         ->  Hash  (cost=6322.20..6322.20 rows=438220 width=4) (actual 
time=1105.663..1105.663 rows=438220 loops=1)
               ->  Seq Scan on matview_82034  (cost=0.00..6322.20 rows=438220 
width=4) (actual time=7.105..544.072 rows=438220 loops=1)
   ->  Index Scan using contexts_pkey on contexts  (cost=0.00..74.76 rows=1 
width=4) (actual time=0.619..0.619 rows=0 loops=351337)
         Index Cond: (contexts.context_key = articles.context_key)
         Filter: ((subplan) AND (subplan) AND (subplan))
         SubPlan
           ->  Nested Loop  (cost=0.00..30.54 rows=1 width=17) (actual 
time=6.119..6.119 rows=1 loops=983)
                 ->  Index Scan using words_word on words  (cost=0.00..5.50 
rows=1 width=13) (actual time=0.028..0.029 rows=1 loops=983)
                       Index Cond: ((word)::text = 'trading'::text)
                 ->  Index Scan using article_words_cw on article_words  
(cost=0.00..25.02 rows=1 width=8) (actual time=6.082..6.082 rows=1 loops=983)
                       Index Cond: ((public.article_words.context_key = $0) AND 
(public.article_words.word_key = public.words.word_key))
           ->  Nested Loop  (cost=0.00..30.54 rows=1 width=17) (actual 
time=6.196..6.196 rows=0 loops=26494)
                 ->  Index Scan using words_word on words  (cost=0.00..5.50 
rows=1 width=13) (actual time=0.022..0.024 rows=1 loops=26494)
                       Index Cond: ((word)::text = 'insider'::text)
                 ->  Index Scan using article_words_cw on article_words  
(cost=0.00..25.02 rows=1 width=8) (actual time=6.165..6.165 rows=0 loops=26494)
                       Index Cond: ((public.article_words.context_key = $0) AND 
(public.article_words.word_key = public.words.word_key))
           ->  Nested Loop  (cost=0.00..38.38 rows=3 width=29) (actual 
time=0.122..0.122 rows=0 loops=351337)
                 ->  Index Scan using virtual_context_key_idx on 
virtual_ancestors a  (cost=0.00..7.35 rows=5 width=10) (actual 
time=0.074..0.085 rows=5 loops=351337)
                       Index Cond: (context_key = $0)
                 ->  Index Scan using bp_categories_context_key_idx on 
bp_categories  (cost=0.00..6.19 rows=1 width=19) (actual time=0.004..0.004 
rows=0 loops=1740050)
                       Index Cond: (bp_categories.context_key = a.ancestor_key)
                       Filter: (lower(bp_categories.category) = 'law'::text)
 Total runtime: 225976.046 ms
(28 rows)


# select * from version();
 PostgreSQL 8.3.4 on x86_64-pc-linux-gnu, compiled by GCC cc (GCC) 4.1.2 
20061115 (prerelease) (Debian 4.1.1-21)
(1 row)

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Re: [PERFORM] 512,600ms query becomes 7500ms... but why? Postgres 8.3 query planner quirk?

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