Choosing between seqscan and bitmap scan

Hi!

There is some strange on current CVS with correct choosing of scans. Although 
bitmap scan is cheaper but postgresql chooses seqscan. Test suite:

CREATE OR REPLACE FUNCTION genvect()
RETURNS tsvector AS
$$    SELECT
        array_to_string(            ARRAY(                SELECT
(random()*random()*random()*1000.0)::int::text               FROM                    generate_series(1, 10 +
(100.0*random())::bigint)           ),            ' '        )::tsvector;
 
$$
LANGUAGE SQL VOLATILE;

SELECT    t::int4 AS id, genvect() AS ts INTO foo
FROM    generate_series(1, 100000) AS t;

CREATE INDEX foo_idx ON foo USING gin (ts);

VACCUM ANALYZE foo;

postgres=# explain  select count(*) from foo where ts @@ '259';                          QUERY PLAN
--------------------------------------------------------------- Aggregate  (cost=5817.27..5817.28 rows=1 width=0)   ->
SeqScan on foo  (cost=0.00..5805.00 rows=4907 width=0)         Filter: (ts @@ '''259'''::tsquery)
 
(3 rows)

Time: 6,370 ms
postgres=# set enable_seqscan = off;
SET
Time: 2,014 ms
postgres=# explain  select count(*) from foo where ts @@ '259';                                   QUERY PLAN
--------------------------------------------------------------------------------- Aggregate  (cost=5767.35..5767.36
rows=1width=0)   ->  Bitmap Heap Scan on foo  (cost=942.46..5755.08 rows=4907 width=0)         Recheck Cond: (ts @@
'''259'''::tsquery)        ->  Bitmap Index Scan on foo_idx  (cost=0.00..941.24 rows=4907 width=0)               Index
Cond:(ts @@ '''259'''::tsquery)
 
(5 rows)

Why does pgsql choose seqscan (5817.28) instead of bitmap one (5767.36)?

Changed options in postgresql.conf:
shared_buffers=128MB
temp_buffers=16MB
work_mem=16MB
maintenance_work_mem=256MB
effective_cache_size=1024MB



-- 
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
  WWW: http://www.sigaev.ru/