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Message-ID: <24b53fa00704091745l560d40ccxec49d7d09e7a97e@mail.gmail.com>
Date: Mon, 9 Apr 2007 17:45:53 -0700
From: "s d" <s.d.sauron@gmail.com>
To: "Jan de Visser" <jdevisser@digitalfairway.com>
Subject: Re: Beginner Question
Cc: pgsql-performance@postgresql.org
In-Reply-To: <200704090751.53347.jdevisser@digitalfairway.com>
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References: <24b53fa00704090209pcdd2c6cof487d6388c1743b8@mail.gmail.com>
	<200704090751.53347.jdevisser@digitalfairway.com>

Hi Jan,
Adding this Index slowed down things by a factor of 4.

Also, the performance is so horrible (example bellow) that i am
certain i am doing something wrong.

Does the following explain gives any ideas ?

Thanks

=#  EXPLAIN ANALYZE select * from word_association where (word1 ='the'
or word2='the') and count > 10;

QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on word_association  (cost=250.86..7256.59 rows=4624
width=22) (actual time=13.461..211.568 rows=6601 loops=1)
   Recheck Cond: (((word1)::text = 'the'::text) OR ((word2)::text =
'the'::text))
   Filter: (count > 10)
   ->  BitmapOr  (cost=250.86..250.86 rows=12243 width=0) (actual
time=9.052..9.052 rows=0 loops=1)
         ->  Bitmap Index Scan on word_association_index1_1
(cost=0.00..153.20 rows=7579 width=0) (actual time=5.786..5.786
rows=7232 loops=1)
               Index Cond: ((word1)::text = 'the'::text)
         ->  Bitmap Index Scan on word_association_index2_1
(cost=0.00..95.34 rows=4664 width=0) (actual time=3.253..3.253
rows=4073 loops=1)
               Index Cond: ((word2)::text = 'the'::text)
  Total runtime: 219.987 ms
(9 rows)


On 4/9/07, Jan de Visser <jdevisser@digitalfairway.com> wrote:
> On Monday 09 April 2007 05:09:53 s d wrote:
> > Hi,
> > I am trying to figure out how to debug a performance problem / use psql
> > explain. The table in question is:
> > # \d word_association;
> >            Table "public.word_association"
> >  Column |          Type          |     Modifiers
> > --------+------------------------+--------------------
> >  word1  | character varying(128) | not null
> >  word2  | character varying(128) | not null
> >  count  | integer                | not null default 0
> > Indexes:
> >     "word1_word2_comb_unique" unique, btree (word1, word2)
> >     "word1_hash_index" hash (word1)
> >     "word2_hash_index" hash (word2)
> >     "word_association_count_index" btree (count)
> >     "word_association_index1_1" btree (word1)
> >     "word_association_index2_1" btree (word2)
> >
> > It has multiple indices since i wanted to see which one the planner choses.
> >
> >
> > # explain select * FROM word_association WHERE (word1 = 'bdss' OR
> > word2 = 'bdss')  AND count >= 10;
> >                                            QUERY PLAN
> > ---------------------------------------------------------------------------
> >--------------------- Bitmap Heap Scan on word_association
> > (cost=11.53..1192.09 rows=155 width=22) Recheck Cond: (((word1)::text =
> > 'bdss'::text) OR ((word2)::text = 'bdss'::text))
> >    Filter: (count >= 10)
> >    ->  BitmapOr  (cost=11.53..11.53 rows=364 width=0)
> >          ->  Bitmap Index Scan on word_association_index1_1
> > (cost=0.00..5.79 rows=190 width=0)
> >                Index Cond: ((word1)::text = 'bdss'::text)
> >          ->  Bitmap Index Scan on word_association_index2_1
> > (cost=0.00..5.67 rows=174 width=0)
> >                Index Cond: ((word2)::text = 'bdss'::text)
> > (8 rows)
> >
> > The questions:
> > 1. i can undestand where the cost=11.53 came from but where did the
> > 1192.09 come form? The values are in milli right ?
> > 2. the query takes  in reality much longer than 1 second.
> >
> > In short, it feels like something is very wrong here (i tried vacuum
> > analyze and it didn't do much diff).
> > any ideas ?
>
> You need an index on (word1, word2, count). In your current setup it will have
> to scan all rows that satisfy word1 and word2 to see if count >= 10.
>
> jan
>
>
> --
> --------------------------------------------------------------
> Jan de Visser jdevisser@digitalfairway.com
>
>   Baruk Khazad! Khazad ai-menu!
> --------------------------------------------------------------
>