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To: Giorgio Valoti <giorgio_v@mac.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Query Plan choice with timestamps 
In-reply-to: <321506D5-4400-470C-909E-C7700D49A540@mac.com> 
References: <321506D5-4400-470C-909E-C7700D49A540@mac.com>
Comments: In-reply-to Giorgio Valoti <giorgio_v@mac.com>
	message dated "Thu, 07 Aug 2008 09:50:04 +0200"
Date: Thu, 07 Aug 2008 11:50:59 -0400
Message-ID: <12707.1218124259@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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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