From: Chris <dmagick@gmail.com>
To: David Kerr <dmk@mr-paradox.net>
Cc: Craig Ringer <craig@postnewspapers.com.au>
Cc: pgsql-performance@postgresql.org
Subject: Re: Optimizer + bind variables
Date: Wed, 04 Nov 2009 11:02:22 +1100
Message-ID: <4AF0C48E.8090100@gmail.com> (raw)
In-Reply-To: <20091103235245.GA77360@mr-paradox.net>
References: <20091103174729.GA55736@mr-paradox.net>
<4AF0C014.4090000@postnewspapers.com.au>
<20091103235245.GA77360@mr-paradox.net>
David Kerr wrote:
> On Wed, Nov 04, 2009 at 07:43:16AM +0800, Craig Ringer wrote:
> - David Kerr wrote:
> - > Does/is it possible for the PG optimizer come up with differnet plans when
> - > you're using bind variables vs when you send static values?
> -
> - Yes, if the bind variable form causes your DB access driver to use a
> - server-side prepared statement. Pg can't use its statistics to improve
> - its query planning if it doesn't have a value for a parameter when it's
> - building the query plan.
>
> hmm, that's a little unclear to me.
>
> let's assume that the application is using prepare:
>
> Assuming the database hasn't changed, would:
> PREPARE bla1 as SELECT * from users where username = '$1';
> explain execute bla1
>
> give the same output as
> explain select * from users where username = 'dave';
>
> ?
No.
This is explained in the notes here:
http://www.postgresql.org/docs/current/static/sql-prepare.html
--
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To: pgsql-performance@postgresql.org
Cc: dmagick@gmail.com, dmk@mr-paradox.net, craig@postnewspapers.com.au
Subject: Re: Optimizer + bind variables
In-Reply-To: <4AF0C48E.8090100@gmail.com>
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