From: T E Schmitz <mailreg@numerixtechnology.de>
To: Alvaro Herrera <alvherre@dcc.uchile.cl>
Cc: pgsql-sql@postgresql.org
Subject: Re: cost of CREATE VIEW ... AS SELECT DISTINCT
Date: Tue, 29 Mar 2005 15:12:24 +0100
Message-ID: <42496248.4080407@numerixtechnology.de> (raw)
In-Reply-To: <20050329135823.GA23852@dcc.uchile.cl>
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>
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
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To: pgsql-sql@postgresql.org
Cc: mailreg@numerixtechnology.de, alvherre@dcc.uchile.cl
Subject: Re: cost of CREATE VIEW ... AS SELECT DISTINCT
In-Reply-To: <42496248.4080407@numerixtechnology.de>
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