| From: | Joshua Drake <jd(at)commandprompt(dot)com> |
|---|---|
| To: | William Garrison <postgres(at)mobydisk(dot)com> |
| Cc: | Adrian Klaver <aklaver(at)comcast(dot)net>, Postgres General List <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: Simple query not using index: why? |
| Date: | 2008-09-03 20:01:30 |
| Message-ID: | 20080903130130.0a9ba092@jd-laptop |
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| Lists: | pgsql-general |
On Wed, 03 Sep 2008 15:55:17 -0400
William Garrison <postgres(at)mobydisk(dot)com> wrote:
> So I expected scanning the index was faster, and still had everything
> it needed to do the count. Or perhaps it was because I said COUNT(*)
> so it needs to look at the other columns in the table? I really just
> wanted the number of "hits" not the number of records with distinct
> values or anything like that. My understanding was that COUNT(*) did
> that, and didn't really look at the columns themselves.
We do not have visibility information in the index, so we have to scan
the pages to see what tuples are live or dead (and thus count them).
Sincerely,
Joshua D. Drake
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