From: Tom Lane <tgl@sss.pgh.pa.us>
To: Giorgio Valoti <giorgio_v@mac.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query Plan choice with timestamps
Date: Thu, 07 Aug 2008 11:50:59 -0400
Message-ID: <12707.1218124259@sss.pgh.pa.us> (raw)
In-Reply-To: <321506D5-4400-470C-909E-C7700D49A540@mac.com>
References: <321506D5-4400-470C-909E-C7700D49A540@mac.com>
Giorgio Valoti <giorgio_v@mac.com> writes:
> GroupAggregate (cost=98431.58..119773.92 rows=74226 width=8)
> -> Sort (cost=98431.58..99050.92 rows=247736 width=8)
> Sort Key: (day_trunc(ts))
> -> Seq Scan on blackbox (cost=0.00..72848.36 rows=247736 width=8)
> GroupAggregate (cost=0.00..19109.66 rows=74226 width=8)
> -> Index Scan using test_2_idx on blackbox (cost=0.00..16943.16 rows=247736 width=8)
These numbers seem pretty bogus: there is hardly any scenario in which a
full-table indexscan should be costed as significantly cheaper than a
seqscan. Have you put in silly values for random_page_cost?
If you haven't mucked with the cost parameters, the only way I can think
of to get this result is to have an enormously bloated table that's
mostly empty. Maybe you need to review your vacuuming procedures.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, giorgio_v@mac.com
Subject: Re: Query Plan choice with timestamps
In-Reply-To: <12707.1218124259@sss.pgh.pa.us>
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