To: Daniel CAUNE <d.caune@free.fr>
cc: pgsql-sql@postgresql.org
Subject: Re: Evaluation of if conditions 
In-reply-to: <001601c6d204$5d2cd840$0b00a8c0@tedy> 
References: <001601c6d204$5d2cd840$0b00a8c0@tedy>
Comments: In-reply-to Daniel CAUNE <d.caune@free.fr>
	message dated "Wed, 06 Sep 2006 18:32:40 -0400"
Date: Thu, 07 Sep 2006 00:10:18 -0400
Message-ID: <28985.1157602218@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Daniel CAUNE <d.caune@free.fr> writes:
> How does the IF statement evaluate conditions?  Does it evaluate conditions
> following their declaration order from left to right?  In case of
> or-conditions, does the IF statement stop evaluating conditions whenever a
> first or-condition is true?

> The following snippet seems to be invalid, which let me think that PL/PGSQL
> evaluates all the conditions:

>   IF (TG_OP = 'INSERT') OR
>      (OLD.bar = ...) THEN

It's not that all the conditions get evaluated by an OR, it's that
plpgsql needs to send all the parameter values that the IF-expression
needs down to the core SQL engine.  So it fails on "OLD.bar" not being
defined, long before the expression evaluator gets to think about
whether TG_OP = 'INSERT' or not.

So, yeah, you want to rewrite it as two separate IF-tests.

			regards, tom lane