Message-ID: <42496248.4080407@numerixtechnology.de>
Date: Tue, 29 Mar 2005 15:12:24 +0100
From: T E Schmitz <mailreg@numerixtechnology.de>
Reply-To: mailreg@numerixtechnology.de
User-Agent: Mozilla Thunderbird 1.0 (Windows/20041206)
MIME-Version: 1.0
To: Alvaro Herrera <alvherre@dcc.uchile.cl>
Cc: 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>
	<4249564B.1000807@numerixtechnology.de>
	<20050329135823.GA23852@dcc.uchile.cl>
In-Reply-To: <20050329135823.GA23852@dcc.uchile.cl>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 8bit

Alvaro Herrera wrote:
> On Tue, Mar 29, 2005 at 02:21:15PM +0100, T E Schmitz wrote:
> 
>>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?
> 
> That (GROUP BY using a seqscan) may be caused by the small size of the
> table.  Try populating it some more.
> 

Shall do.
But am I correct in assuming that I should place an index on the group 
by (TRANSAKTION.ORIGIN) column?


-- 


Regards/Gruß,

Tarlika Elisabeth Schmitz