From: Bruno Wolff III <bruno@wolff.to>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: Postgresql Performance <pgsql-performance@postgresql.org>
Subject: Re: is it possible to make this faster?
Date: Thu, 25 May 2006 15:47:46 -0500
Message-ID: <20060525204746.GA26341@wolff.to> (raw)
In-Reply-To: <b42b73150605251331l85a1403pbf4c73656b6194bc@mail.gmail.com>
References: <b42b73150605251307id0e7998td4a3693f0d456f88@mail.gmail.com>
<20060525202314.GA20747@wolff.to>
<b42b73150605251331l85a1403pbf4c73656b6194bc@mail.gmail.com>
On Thu, May 25, 2006 at 16:31:40 -0400,
Merlin Moncure <mmoncure@gmail.com> wrote:
> On 5/25/06, Bruno Wolff III <bruno@wolff.to> wrote:
> >On Thu, May 25, 2006 at 16:07:19 -0400,
> > Merlin Moncure <mmoncure@gmail.com> wrote:
> >> been doing a lot of pgsql/mysql performance testing lately, and there
> >> is one query that mysql does much better than pgsql...and I see it a
> >> lot in normal development:
> >>
> >> select a,b,max(c) from t group by a,b;
> >>
>
> >SELECT DISTINCT ON (a, b) a, b, c FROM t ORDER BY a DESC, b DESC, c DESC;
>
> that is actually slower than group by in my case...am i missing
> something? (both essentially resolved to seq_scan)
If there aren't many c's for each (a,b), then a sort might be the best way to
do this. I don't remember if skip scanning ever got done, but if it did, it
would have been 8.1 or later.
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To: pgsql-performance@postgresql.org
Cc: bruno@wolff.to, mmoncure@gmail.com
Subject: Re: is it possible to make this faster?
In-Reply-To: <20060525204746.GA26341@wolff.to>
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