To: dave.bath@unix.net
Cc: pgsql-sql@postgresql.org
Subject: Re: Yes,
	pg does triggers first before asserting check constraints! Was Re:
	why vacuum
In-reply-to: <200510271042.11487.dave.bath@unix.net> 
References: <200510261015.17623.lawgon@thenilgiris.com>
	<200510261622.40874.dave.bath@unix.net>
	<2827.1130310024@sss.pgh.pa.us>
	<200510271042.11487.dave.bath@unix.net>
Comments: In-reply-to "Bath, David" <dave.bath@unix.net>
	message dated "Thu, 27 Oct 2005 10:42:10 +1000"
Date: Wed, 26 Oct 2005 20:55:31 -0400
Message-ID: <13074.1130374531@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Bath, David" <dave.bath@unix.net> writes:
> you wrote (2005-10-26 17:00)
>>> Sybase/MS-SQL's check constraint model asserts the constraint
>>> BEFORE the trigger, which discourages you from attempting to>
>>> check and handle meaning of data! 

>> Er, doesn't PG do it that way too?

> Well, it works for me!

You're right of course.  I was confusing this with a related problem
that people occasionally complain about, which is that datatype-related
errors are thrown before the trigger can do anything about them.  For
instance, you can't stuff "abc" into a int4 field and hope that the
trigger will have a chance to replace it with something valid.  (Yes,
people have asked for that :-()  A less silly example is that domain
constraints on a field of a domain type get checked before the trigger
can run.

			regards, tom lane