Message-ID: <42FFEA76.6070109@arbash-meinel.com>
Date: Sun, 14 Aug 2005 20:05:58 -0500
From: John Arbash Meinel <john@arbash-meinel.com>
User-Agent: Mozilla Thunderbird 1.0.6 (X11/20050727)
MIME-Version: 1.0
To: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance pb vs SQLServer.
References: <20050811131911.3175A1C0031B@mwinf0401.wanadoo.fr>
	<42FFE17A.6010208@arbash-meinel.com>
	<20050815010159.GA27795@uio.no>
In-Reply-To: <20050815010159.GA27795@uio.no>
Content-Type: multipart/signed; micalg=pgp-sha1;
	protocol="application/pgp-signature";
	boundary="------------enig600AA473DF119A810C53F557"

This is an OpenPGP/MIME signed message (RFC 2440 and 3156)
--------------enig600AA473DF119A810C53F557
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

Steinar H. Gunderson wrote:

>On Sun, Aug 14, 2005 at 07:27:38PM -0500, John Arbash Meinel wrote:
>  
>
>>My guess is that this is part of a larger query. There isn't really much
>>you can do. If you want all 3.2M rows, then you have to wait for them to
>>be pulled in.
>>    
>>
>
>To me, it looks like he'll get 88 rows, not 3.2M. Surely we must be able to
>do something better than a full sequential scan in this case?
>
>test=# create table foo ( bar char(4) );
>CREATE TABLE
>test=# insert into foo values ('0000');
>INSERT 24773320 1
>test=# insert into foo values ('0000');
>INSERT 24773321 1
>test=# insert into foo values ('1111');
>INSERT 24773322 1
>test=# select * from foo group by bar;
> bar  
>------
> 1111
> 0000
>(2 rows)
>
>I considered doing some odd magic with generate_series() and subqueries with
>LIMIT 1, but it was a bit too weird in the end :-)
>
>/* Steinar */
>  
>
I think a plain "GROUP BY" is not smart enough to detect it doesn't need
all rows (since it is generally used because you want to get aggregate
values of other columns).
I think you would want something like SELECT DISTINCT, possibly with an
ORDER BY rather than a GROUP BY (which was my final suggestion).

John
=:->


--------------enig600AA473DF119A810C53F557
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: OpenPGP digital signature
Content-Disposition: attachment; filename="signature.asc"

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.5 (GNU/Linux)
Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org

iD8DBQFC/+p2JdeBCYSNAAMRAnhQAJ4oOR2RxZyaLi4yJsrSCHJObxtfewCfQZIV
6mg+ebmDTj6dK5q4doe4hj4=
=6vR1
-----END PGP SIGNATURE-----

--------------enig600AA473DF119A810C53F557--