| From: | "Steinar H(dot) Gunderson" <sgunderson(at)bigfoot(dot)com> |
|---|---|
| To: | Brian Herlihy <btherl(at)yahoo(dot)com(dot)au> |
| Cc: | Postgresql Performance <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: GROUP BY vs DISTINCT |
| Date: | 2006-12-20 11:00:07 |
| Message-ID: | 20061220105739.GB31739@uio.no |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-performance |
On Tue, Dec 19, 2006 at 11:19:39PM -0800, Brian Herlihy wrote:
> Actually, I think I answered my own question already. But I want to
> confirm - Is the GROUP BY faster because it doesn't have to sort results,
> whereas DISTINCT must produce sorted results? This wasn't clear to me from
> the documentation. If it's true, then I could save considerable time by
> using GROUP BY where I have been using DISTINCT in the past. Usually I
> simply want a count of the distinct values, and there is no need to sort
> for that.
You are right; at the moment, GROUP BY is more intelligent than DISTINCT,
even if they have to compare the same columns. This is, as always, something
that could be improved in a future release, TTBOMK.
/* Steinar */
--
Homepage: http://www.sesse.net/
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