Index usage

Hi. I'm having some trouble on the use of indexes.

The querys below are exactly the same but refer to different months.
One case uses indexes, the other doesn't.

Is there anything I can do? Increasing index mem size?

Query 2 hash 9105 entries matching the given conditions
Query 2 hash 9248 entries matching the given conditions

QUERY 1:

explain select ra_datacolh::date, count(distinct ra_utente) from requisicaoanalise where (ra_datacolh::date >= '2003-4-01'::date and ra_datacolh::date < '2003-5-1'::date) and isactive=0 group by ra_datacolh;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Aggregate (cost=0.00..12300.68 rows=313 width=8)
-> Group (cost=0.00..12292.86 rows=3130 width=8)
-> Index Scan using requisicaoanalise_datacolh on requisicaoanalise (cost=0.00..12285.03 rows=3130 width=8)
Index Cond: ((ra_datacolh >= '2003-04-01'::date) AND (ra_datacolh < '2003-05-01'::date))
Filter: (isactive = 0)

QUERY 2:

netlab=> explain select ra_datacolh::date, count(distinct ra_utente) from requisicaoanalise where (ra_datacolh::date >= '2003-6-01'::date and ra_datacolh::date < '2003-7-1'::date) and isactive=0 group by ra_datacolh;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=32452.98..32632.06 rows=2388 width=8)
-> Group (cost=32452.98..32572.36 rows=23877 width=8)
-> Sort (cost=32452.98..32512.67 rows=23877 width=8)
Sort Key: ra_datacolh
-> Seq Scan on requisicaoanalise (cost=0.00..30716.71 rows=23877 width=8)
Filter: ((ra_datacolh >= '2003-06-01'::date) AND (ra_datacolh < '2003-07-01'::date) AND (isactive = 0))

--
Pedro Miguel G. Alves pmalves(at)think(dot)pt
THINK - Tecnologias de Informação www.think.pt
Tel: +351 21 413 46 00 Av. José Gomes Ferreira
Fax: +351 21 413 46 09 nº 13 1495-139 ALGÉS