On Fri, Feb 4, 2011 at 7:08 AM, Ivan Voras <ivoras@freebsd.org> wrote:
> -> BitmapAnd (cost=1282.94..1282.94
> rows=1430 width=0) (actual time=5.508..5.508 rows=0 loops=1)
> -> Bitmap Index Scan on
> news_index_layout_id_state (cost=0.00..150.14 rows=2587 width=0) (actual
> time=0.909..0.909 rows=3464 loops=1)
> Index Cond: ((layout_id = 8980)
> AND (state = 2))
> -> BitmapOr (cost=1132.20..1132.20
> rows=20127 width=0) (actual time=4.136..4.136 rows=0 loops=1)
> -> Bitmap Index Scan on
> news_visible_from (cost=0.00..1122.09 rows=19976 width=0) (actual
> time=3.367..3.367 rows=19932 loops=1)
> Index Cond: (visible_from
> IS NULL)
> -> Bitmap Index Scan on
> news_visible_to (cost=0.00..9.40 rows=151 width=0) (actual
> time=0.766..0.766 rows=43 loops=1)
> Index Cond: (1296806570 <=
> visible_to)
I think this part of the query is the problem. Since the planner
doesn't support cross-column statistics, it can't spot the correlation
between these different search conditions, resulting in a badly broken
selectivity estimate.
Sometimes you can work around this by adding a single column, computed
with a trigger, that contains enough information to test the whole
WHERE-clause condition using a single indexable test against the
column value. Or sometimes you can get around it by partitioning the
data into multiple tables, say with the visible_from IS NULL rows in a
different table from the rest.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
