Message-ID: <4249564B.1000807@numerixtechnology.de>
Date: Tue, 29 Mar 2005 14:21:15 +0100
From: T E Schmitz <mailreg@numerixtechnology.de>
Reply-To: mailreg@numerixtechnology.de
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To: Bruno Wolff III <bruno@wolff.to>
Cc: Scott Marlowe <smarlowe@g2switchworks.com>,
	pgsql-sql@postgresql.org
Subject: Re: cost of CREATE VIEW ...  AS SELECT DISTINCT
References: <42487A6D.6020506@numerixtechnology.de>
	<1112047463.22988.25.camel@state.g2switchworks.com>
	<424928D8.7060705@numerixtechnology.de>
	<20050329132941.GA27251@wolff.to>
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Bruno Wolff III wrote:
> On Tue, Mar 29, 2005 at 11:07:20 +0100,
>   T E Schmitz <mailreg@numerixtechnology.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