From: Richard Huxton <dev@archonet.com>
To: alvarezp@alvarezp.ods.org, pgsql-performance@postgresql.org
Subject: Re: Seq scan on zero-parameters function
Date: Fri, 6 Feb 2004 08:50:50 +0000
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On Friday 06 February 2004 07:19, Octavio Alvarez wrote:
> Hi!
>
>    I'd like to know if this is expected behavior. These are two couples of
> queries. In each couple, the first one has a WHERE field = function()
> condition, just like the second one, but in the form WHERE field =
> (SELECT function()). In my opinion, both should have the same execution
> plan, as the function has no parameters and, therefore, is constant.

Not necessarily constant - think about random() or timeofday().
Have you set the attributes on your function?

http://www.postgresql.org/docs/7.4/static/sql-createfunction.html

> pgdb=# explain analyze select count(*) from t_students where period =
> (select current_period_id());

It's not entirely clear to me why this form is different from the other form 
though.

-- 
  Richard Huxton
  Archonet Ltd