From: John Arbash Meinel <john@arbash-meinel.com>
To: Steinar H. Gunderson <sgunderson@bigfoot.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance pb vs SQLServer.
Date: Sun, 14 Aug 2005 20:05:58 -0500
Message-ID: <42FFEA76.6070109@arbash-meinel.com> (raw)
In-Reply-To: <20050815010159.GA27795@uio.no>
References: <20050811131911.3175A1C0031B@mwinf0401.wanadoo.fr>
<42FFE17A.6010208@arbash-meinel.com>
<20050815010159.GA27795@uio.no>
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
=:->
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To: pgsql-performance@postgresql.org
Cc: john@arbash-meinel.com, sgunderson@bigfoot.com
Subject: Re: Performance pb vs SQLServer.
In-Reply-To: <42FFEA76.6070109@arbash-meinel.com>
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