From: Gavin Sherry <swm@linuxworld.com.au>
To: Bruce Momjian <bruce@momjian.us>
Cc: Jie Zhang <jzhang@greenplum.com>
Cc: Hannu Krosing <hannu@skype.net>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org, Luke Lonergan <LLonergan@greenplum.com>
Subject: Re: On-disk bitmap index patch
Date: Tue, 25 Jul 2006 11:51:05 +1000 (EST)
Message-ID: <Pine.LNX.4.58.0607251149250.445@linuxworld.com.au> (raw)
In-Reply-To: <200607250104.k6P14St25388@momjian.us>
References: <200607250104.k6P14St25388@momjian.us>
On Mon, 24 Jul 2006, Bruce Momjian wrote:
> Jie Zhang wrote:
> > > IIRC they quoted the cardinality of 10000 as something that is still
> > > faster than btree for several usecases.
> > >
> > > And also for AND-s of several indexes, where indexes are BIG, your btree
> > > indexes may be almost as big as tables but the resulting set of pages is
> > > small.
> >
> > Yeah, Hannu points it out very well -- the bitmap index works very well when
> > columns have low cardinalities and AND operations will produce small number
> > of results.
>
> What operations on columns of low cardinality produce a small number of
> results? That seems contradictory.
WHERE a = 1 and b = 2
a = 1 may be 5% of the table and b = 2 may be 5% of the table but their
intersection may be .001%.
Luke: the URL you sent to the bitmap slides was internal to Greenplum.
Would you be able to put them on a public site?
Thanks,
Gavin
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To: pgsql-hackers@postgresql.org
Cc: swm@linuxworld.com.au, bruce@momjian.us, jzhang@greenplum.com, hannu@skype.net, tgl@sss.pgh.pa.us, LLonergan@greenplum.com
Subject: Re: On-disk bitmap index patch
In-Reply-To: <Pine.LNX.4.58.0607251149250.445@linuxworld.com.au>
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