Date: Tue, 25 Jul 2006 01:08:55 -0400
From: mark@mark.mielke.cc
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Bruce Momjian <bruce@momjian.us>, Jie Zhang <jzhang@greenplum.com>,
	Hannu Krosing <hannu@skype.net>,
	Gavin Sherry <swm@linuxworld.com.au>, pgsql-hackers@postgresql.org,
	Luke Lonergan <LLonergan@greenplum.com>
Subject: Re: On-disk bitmap index patch
Message-ID: <20060725050855.GA31414@mark.mielke.cc>
References: <C0EAB84F.8FB4%jzhang@greenplum.com>
	<200607250104.k6P14St25388@momjian.us>
	<20060725015805.GA30087@mark.mielke.cc>
	<5085.1153802202@sss.pgh.pa.us>
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On Tue, Jul 25, 2006 at 12:36:42AM -0400, Tom Lane wrote:
> mark@mark.mielke.cc writes:
> > Reading 1/4, for a larger table, has a good chance of being faster than
> > reading 4/4 of the table. :-)
> Really?
> 
> If you have to hit one tuple out of four, it's pretty much guaranteed
> that you will need to fetch every heap page.  So using an index provides
> zero I/O savings on the heap side, and any fetches needed to read the
> index are pure cost.  Now you have to demonstrate that the CPU costs
> involved in processing the index are significantly cheaper than the cost
> of just testing the WHERE qual at every heap tuple --- not a bet that's
> likely to win at a one-in-four ratio.

Haha. Of course - but that's assuming uniform spread of the values.
Next I would try clustering the table on the bitmap index... :-)

My databases aren't as large as many of yours. Most or all of them
will fit in 1 Gbytes of RAM. The I/O cost isn't substantial for these,
but the WHERE clause might be.

But yeah - we don't know. Waste of code or performance boost.

Cheers,
mark

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