Re: On-disk bitmap index patch

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Dann Corbit <DCorbit(at)connx(dot)com>, Luke Lonergan <llonergan(at)greenplum(dot)com>, "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, Jie Zhang <jzhang(at)greenplum(dot)com>, Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>, Josh Berkus <josh(at)agliodbs(dot)com>, Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: On-disk bitmap index patch
Date: 2006-07-28 20:25:23
Message-ID: 200607282025.k6SKPNB08324@momjian.us
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What we don't want to happen is for us to release bitmapped indexes, and
find out later that btree is better in all cases. Then we have to tell
people not to use bitmapped indexes until we fix it in the next major
releasse. FYI, that is basically where we are right now with hash
indexes.

---------------------------------------------------------------------------

Tom Lane wrote:
> "Dann Corbit" <DCorbit(at)connx(dot)com> writes:
> > Others have looked into the usefulness of bitmap indexes. Here is what
> > they found:
> > http://www.oracle.com/technology/pub/articles/sharma_indexes.html
>
> I like this guy's style of argument: he admits a bitmap index on a
> unique column will be much bigger than a btree, and then airily
> dismisses it as not a problem. Not real convincing.
>
> > http://citeseer.ist.psu.edu/stockinger02bitmap.html
>
> Both of these pages say up front that they are considering read-only
> data. So one of the questions that has to be answered (and the
> submitters have been entirely mum about) is exactly how bad is the
> update performance? If it's really awful that's going to constrain
> the use cases quite a lot, whereas merely "a bit slower than btree"
> wouldn't be such a problem.
>
> In any case, arguing that other DBs find it's a win will cut no ice
> with me. See adjacent discussion about hash indexes --- those *ought*
> to be a win, but they aren't in Postgres, for reasons that are still
> guesses. The translation gap between other DBs' experience and ours
> can be large.
>
> regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 5: don't forget to increase your free space map settings

--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

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