| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Gavin Sherry <swm(at)linuxworld(dot)com(dot)au> |
| Cc: | Jie Zhang <jzhang(at)greenplum(dot)com>, pgsql-hackers(at)postgresql(dot)org, Luke Lonergan <LLonergan(at)greenplum(dot)com> |
| Subject: | Re: On-disk bitmap index patch |
| Date: | 2006-07-24 00:25:18 |
| Message-ID: | 28750.1153700718@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Gavin Sherry <swm(at)linuxworld(dot)com(dot)au> writes:
> On Sun, 23 Jul 2006, Tom Lane wrote:
>> However, the main problem I've got with this is that a new index AM is a
>> pretty large burden, and no one's made the slightest effort to sell
>> pghackers on taking this on.
> For low cardinality sets, bitmaps greatly out perform btree.
If the column is sufficiently low cardinality, you might as well just do
a seqscan --- you'll be hitting most of the heap's pages anyway. I'm
still waiting to be convinced that there's a sweet spot wide enough to
justify supporting another index AM. (I'm also wondering whether this
doesn't overlap the use-case for GIN.)
regards, tom lane
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