| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Greg Stark <gsstark(at)mit(dot)edu> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, pgsql-general(at)postgresql(dot)org, Ben Harper <rogojin(at)gmail(dot)com> |
| Subject: | Re: SELECT DISTINCT very slow |
| Date: | 2009-07-10 17:17:08 |
| Message-ID: | 1247246228.26589.318.camel@monkey-cat.sm.truviso.com |
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| Lists: | pgsql-general |
On Fri, 2009-07-10 at 01:36 +0100, Greg Stark wrote:
> Arguably the missing feature here is skip-scans where we scan the
> index but only pull out one record for each distinct value. I'm not
> sure there's anything particularly stopping Postgres from being able
> to do them, but it might be a lot of code for a narrow use case.
Hypothetically, would something like a "sort distinct" operator be of
any use? I wonder how much work it would save if the sort could save
steps by weeding out duplicate tuples while sorting. That might make
sort into a better plan in cases where don't have a good estimate of the
distinct values.
Regards,
Jeff Davis
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