From: Bruno Wolff III <bruno@wolff.to>
To: Ken Geis <kgeis@speakeasy.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: bad estimates / non-scanning aggregates
Date: Thu, 28 Aug 2003 22:01:56 -0500
Message-ID: <20030829030156.GF4373@wolff.to> (raw)
In-Reply-To: <3F4EBF7E.8020602@speakeasy.org>
References: <3F4E99F7.7080604@speakeasy.org>
<20030829025109.GD4373@wolff.to>
<3F4EBF7E.8020602@speakeasy.org>
On Thu, Aug 28, 2003 at 19:50:38 -0700,
Ken Geis <kgeis@speakeasy.org> wrote:
> Bruno Wolff III wrote:
> >On Thu, Aug 28, 2003 at 17:10:31 -0700,
> > Ken Geis <kgeis@speakeasy.org> wrote:
> >
> >>The query I want to run is
> >>
> >>select stock_id, min(price_date) from day_ends group by stock_id;
> >
> >The fast way to do this is:
> >
> >select distinct on (stock_id) stock_id, price_date
> > order by stock_id, price_date;
>
> Not according to the optimizer! Plus, this is not guaranteed to return
> the correct results.
For it to be fast you need an index on (stock_id, price_date) so that
you can use an index scan.
The answers are guarenteed to be correct. See:
http://developer.postgresql.org/docs/postgres/sql-select.html#SQL-DISTINCT
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To: pgsql-performance@postgresql.org
Cc: bruno@wolff.to, kgeis@speakeasy.org
Subject: Re: bad estimates / non-scanning aggregates
In-Reply-To: <20030829030156.GF4373@wolff.to>
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