From: Arjen van der Meijden <acm@tweakers.net>
Subject: Re: calculated expressions and index use
Date: Thu, 30 Jan 2003 17:13:40 +0100
Organization: Delft University of Technology
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Message-ID: <b1biv3$l4s$1@news.tudelft.nl>
References: <3E394AA6.8080400@gingerall.cz>
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In-Reply-To: <3E394AA6.8080400@gingerall.cz>
To: pgsql-sql@postgresql.org

A very dirty work around could be:
select * from foo where created+at >= (select now() - '1 hour'::interval);

Or perhaps this already works:
select * from foo where created+at >= (now() - '1 hour'::interval);

Pavel Hlavnicka wrote:
> Hi all,
> 
> I use simple sql statement like this
> 
> select * from foo
> where created_at >= 'now'::timestamp - '1 hour'::interval;
> 
> My table is indexed on created_at field. The query above doesn't use it, 
> but if I use
> 
> select * from foo where created_at >= 'now'
> 
> the index is used.
> 
> It looks like if the engine finds some expression to be evaluated it 
> gets rid of any index use (due the possible dependency?).
> 
> Is it possible to do something to 'precompute' some value in the query 
> condition, so planner understand is as a constant value?
> 
> (... and I've got some reason NOT to use a parameter in my Perl DBI code...)
> 
> Maybe my conclusions are wrong, of course.
> 
> Thank you very much in advance
> 
> Pavel
>