To: Michael Fuhr <mike@fuhr.org>
cc: PostgreSQL SQL List <pgsql-sql@postgresql.org>,
	Joost Kraaijeveld <J.Kraaijeveld@Askesis.nl>
Subject: Re: Foreign key reference counting strategy? 
In-reply-to: <20061015001126.GA20713@winnie.fuhr.org> 
References: <1160828741.28838.9.camel@panoramix>
	<4530EB84.4050904@logix-tt.com>
	<20061014234844.GA20342@winnie.fuhr.org>
	<13525.1160870286@sss.pgh.pa.us>
	<20061015001126.GA20713@winnie.fuhr.org>
Comments: In-reply-to Michael Fuhr <mike@fuhr.org>
	message dated "Sat, 14 Oct 2006 18:11:26 -0600"
Date: Sat, 14 Oct 2006 20:20:10 -0400
Message-ID: <13667.1160871610@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Michael Fuhr <mike@fuhr.org> writes:
> On Sat, Oct 14, 2006 at 07:58:06PM -0400, Tom Lane wrote:
>> No, I don't think so, because the DELETE will already be holding
>> exclusive lock on the doomed PK row, which any would-be inserters of
>> matching FK rows will be blocked on.  AFAICS the DELETE should go
>> through and then the inserters will fail.

> Unless the inserters got there first.  I just tested both ways; if
> the insert acquires the lock first then the delete fails, but if the
> delete acquires the lock first then the insert fails.

Well, if the inserters get a lock on the PK row before the DELETE does,
then of course.  I was just disputing the assertion that doing IF EXISTS
in an after trigger would add a new way for the DELETE to fail.  At that
point, any uncommitted inserts must be blocked waiting for the delete.

			regards, tom lane