Re: On-disk bitmap index patch

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