From: Steinar H. Gunderson <sgunderson@bigfoot.com>
To: pgsql-performance@postgresql.org
Subject: Re: why is bitmap index chosen for this query?
Date: Thu, 18 May 2006 21:48:13 +0200
Message-ID: <20060518194813.GA8539@uio.no> (raw)
In-Reply-To: <20060518193818.81332.qmail@web30710.mail.mud.yahoo.com>
References: <16019.1147973659@sss.pgh.pa.us>
<20060518193818.81332.qmail@web30710.mail.mud.yahoo.com>
On Thu, May 18, 2006 at 12:38:18PM -0700, Stephen Byers wrote:
> I repeated explain analyze on the query 5 times and it came up with the same plan.
Yes, but did it end up with the same runtime? That's the interesting part --
the plan will almost always be identical between explain analyze runs given
that you haven't done anything in between them.
> You asked about index order and physical table order. In general the index
> order is indeed close to the same order as the physical table order.
> However, this query is likely an exception. The data is actually from a
> backup server that has filled a hole for some of the time range that I'm
> specifying in my query.
Well, it still isn't all that far-fetched to believe that the data has lots
of correlation (which helps the index scan quite a lot) that the planner
isn't able to pick up. I don't know the details here, so I can't tell you how
the correlation for such a query (WHERE a=foo and b between bar and baz) is
estimated. Something tells me someone else might, though. :-)
/* Steinar */
--
Homepage: http://www.sesse.net/
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To: pgsql-performance@postgresql.org
Cc: sgunderson@bigfoot.com
Subject: Re: why is bitmap index chosen for this query?
In-Reply-To: <20060518194813.GA8539@uio.no>
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