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Re: Understanding tsearch2 performance

From: Oleg Bartunov <oleg(at)sai(dot)msu(dot)su>
To: Ivan Voras <ivoras(at)freebsd(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Understanding tsearch2 performance
Date: 2010-07-14 12:31:28
Message-ID: Pine.LNX.4.64.1007141629000.32129@sn.sai.msu.ru (view raw or flat)
Thread:
Lists: pgsql-performance
Something is not good with statistics,  91 est. vs 8449 actually returned.
Returning 8449 rows could be quite long.

Oleg
On Wed, 14 Jul 2010, Ivan Voras wrote:

> Here's a query and its EXPLAIN ANALYZE output:
>
> cms=> select count(*) from forum;
> count
> -------
> 90675
> (1 row)
>
> cms=> explain analyze select id,title from forum where _fts_ @@
> 'fer'::tsquery;
>                                                      QUERY PLAN
>
> -----------------------------------------------------------------------------------------------------------------------
> Bitmap Heap Scan on forum  (cost=29.21..361.21 rows=91 width=35)
> (actual time=2.946..63.646 rows=8449 loops=1)
>   Recheck Cond: (_fts_ @@ '''fer'''::tsquery)
>   ->  Bitmap Index Scan on forum_fts  (cost=0.00..29.19 rows=91
> width=0) (actual time=2.119..2.119 rows=8449 loops=1)
>         Index Cond: (_fts_ @@ '''fer'''::tsquery)
> Total runtime: 113.641 ms
> (5 rows)
>
> The problem is - tsearch2 seems too slow. I have nothing to compare it
> to but 113 ms for searching through this small table of 90,000 records
> seems too slow. The forum_fts index is of GIN type and the table
> certainly fits into RAM.
>
> When I issue a dumb query without an index, I get a comparable order of
> magnitude performance:
>
> cms=> explain analyze select id,title from forum where content ilike
> '%fer%';
>                                                 QUERY PLAN
>
> ------------------------------------------------------------------------------------------------------------
> Seq Scan on forum  (cost=0.00..7307.44 rows=3395 width=35) (actual
> time=0.030..798.375 rows=10896 loops=1)
>   Filter: (content ~~* '%fer%'::text)
> Total runtime: 864.384 ms
> (3 rows)
>
> cms=> explain analyze select id,title from forum where content like '%fer%';
>                                                QUERY PLAN
>
> -----------------------------------------------------------------------------------------------------------
> Seq Scan on forum  (cost=0.00..7307.44 rows=3395 width=35) (actual
> time=0.024..146.959 rows=7596 loops=1)
>   Filter: (content ~~ '%fer%'::text)
> Total runtime: 191.732 ms
> (3 rows)
>
> Some peculiarities of the setup which might or might not influence this
> performance:
>
> 1) I'm using ICU-patched postgresql because I cannot use my UTF-8 locale
> otherwise - this is why the difference between the dumb queries is large
> (but I don't see how this can influence tsearch2 since it pre-builds the
> tsvector data with lowercase lexemes)
>
> 2) My tsearch2 lexer is somewhat slow (but I don't see how it can
> influence these read-only queries on a pre-built, lexed and indexed data)
>
> Any ideas?
>
>
>

 	Regards,
 		Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83

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