| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Jie Zhang <jzhang(at)greenplum(dot)com> |
| Cc: | Hannu Krosing <hannu(at)skype(dot)net>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>, pgsql-hackers(at)postgresql(dot)org, Luke Lonergan <LLonergan(at)greenplum(dot)com> |
| Subject: | Re: On-disk bitmap index patch |
| Date: | 2006-07-25 01:04:28 |
| Message-ID: | 200607250104.k6P14St25388@momjian.us |
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| Lists: | pgsql-hackers |
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.
> Also, the bitmap index is very small in low cardinality cases, where the
> btree tends to take up at least 10 times more space.
Also, are adding/changing rows is more expensive with bitmaps than
btrees?
--
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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