Re: Understanding tsearch2 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@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83