From: | T E Schmitz <mailreg(at)numerixtechnology(dot)de> |
---|---|
To: | Bruno Wolff III <bruno(at)wolff(dot)to> |
Cc: | Scott Marlowe <smarlowe(at)g2switchworks(dot)com>, pgsql-sql(at)postgresql(dot)org |
Subject: | Re: cost of CREATE VIEW ... AS SELECT DISTINCT |
Date: | 2005-03-29 13:21:15 |
Message-ID: | 4249564B.1000807@numerixtechnology.de |
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Lists: | pgsql-sql |
Bruno Wolff III wrote:
> On Tue, Mar 29, 2005 at 11:07:20 +0100,
> T E Schmitz <mailreg(at)numerixtechnology(dot)de> wrote:
>
>>Would the "SELECT DISTINCT origin" always cause a sequential table scan
>>regardless whether there is an index on the origin column or not?
>
>
> It's worse than that, SELECT DISTINCT cannot use a hash aggregate plan
> and will need to do a sort to eliminate duplicates. Unless the view
> is used in a way that restricts the candidate rows, this probably isn't going
> to be very fast. You might be better off changing the view to use GROUP BY
> instead of DISTINCT.
As far as I can see (via EXPLAIN), both DISTINCT and GROUP BY will lead
to a sequentail scan. Is that correct?
If that's the case, I should come up with a different concept to obtain
a list of ORIGINs.
--
Regards/Gruß,
Tarlika Elisabeth Schmitz
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