Planner selects slow "Bitmap Heap Scan" when "Index Scan" is faster

Hi All

I have a query where the planner makes a wrong cost estimate, it looks
like it underestimates the cost of a "Bitmap Heap Scan" compared to an
"Index Scan".

This it the two plans, I have also pasted them below:
Slow (189ms): http://explain.depesz.com/s/2Wq
Fast (21ms): http://explain.depesz.com/s/ThQ

I have run "VACUUM FULL VERBOSE ANALYZE". I have configured
shared_buffers and effective_cache_size, that didn't solve my problem,
the estimates was kept the same and both queries got faster.

What can I do to fix the cost estimate?

Regards,
Kim Hansen

========

yield=> SELECT version();
version
-------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.3 on x86_64-unknown-linux-gnu, compiled by
gcc-4.4.real (Debian 4.4.5-8) 4.4.5, 64-bit
(1 row)

yield=> explain analyze select "filtered_demands"."pol" as "c0" from
"demands"."filtered_demands" as "filtered_demands" where
("filtered_demands"."pod" = 'VELAG') group by "filtered_demands"."pol"
order by "filtered_demands"."pol" ASC NULLS LAST;

QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=38564.80..38564.80 rows=2 width=6) (actual
time=188.987..189.003 rows=221 loops=1)
Sort Key: pol
Sort Method: quicksort Memory: 35kB
-> HashAggregate (cost=38564.77..38564.79 rows=2 width=6) (actual
time=188.796..188.835 rows=221 loops=1)
-> Bitmap Heap Scan on filtered_demands
(cost=566.23..38503.77 rows=24401 width=6) (actual time=6.501..182.634
rows=18588 loops=1)
Recheck Cond: (pod = 'VELAG'::text)
-> Bitmap Index Scan on filtered_demands_pod_pol_idx
(cost=0.00..560.12 rows=24401 width=0) (actual time=4.917..4.917
rows=18588 loops=1)
Index Cond: (pod = 'VELAG'::text)
Total runtime: 189.065 ms
(9 rows)

yield=> set enable_bitmapscan = false;
SET
yield=> explain analyze select "filtered_demands"."pol" as "c0" from
"demands"."filtered_demands" as "filtered_demands" where
("filtered_demands"."pod" = 'VELAG') group by "filtered_demands"."pol"
order by "filtered_demands"."pol" ASC NULLS LAST;

QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Group (cost=0.00..76534.33 rows=2 width=6) (actual
time=0.028..20.823 rows=221 loops=1)
-> Index Scan using filtered_demands_pod_pol_idx on
filtered_demands (cost=0.00..76473.33 rows=24401 width=6) (actual
time=0.027..17.174 rows=18588 loops=1)
Index Cond: (pod = 'VELAG'::text)
Total runtime: 20.877 ms
(4 rows)

yield=>

--
Kim Rydhof Thor Hansen
Vadgårdsvej 3, 2. tv.
2860 Søborg
Phone: +45 3091 2437