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Re: PG7.4.5: query not using index on date column

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Dave Steinberg <dave-dated-1101824919(dot)46cd20(at)redterror(dot)net>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: PG7.4.5: query not using index on date column
Date: 2004-11-25 18:44:51
Message-ID: 1247.1101408291@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-sql
Dave Steinberg <dave-dated-1101824919(dot)46cd20(at)redterror(dot)net> writes:
>                ->  Seq Scan on messages  (cost=0.00..21573.04 rows=436426 width=54) (actual time=5.523..6304.657 rows=462931 loops=1)
>                      Filter: ((received_date >= '2004-11-01'::date) AND (received_date <= '2004-11-30'::date))

How many rows in the table altogether?  A rough guess is a few million
based on the estimated seqscan cost.  That would mean that this query
is retrieving about 10% of the table, which is a large enough fraction
that the planner will probably think a seqscan is best.  It may be right.
If you do "set enable_seqscan = off", how does the EXPLAIN ANALYZE
output change?

If it's not right, you may want to try to adjust random_page_cost and/or
effective_cache_size so that the planner's estimated costs are more in
line with reality.  Beware of making such adjustments on the basis of
only one test case, though.

			regards, tom lane

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