Received: from p1.selectacast.net (IDENT:root@p1.selectacast.net
	[206.41.1.251]) by hub.org (8.10.1/8.10.1) with ESMTP id e6D3pYW54486
	for <pgsql-general@hub.org>; Wed, 12 Jul 2000 23:51:34 -0400 (EDT)
Received: from selectacast.net (IDENT:jks@d1 [209.219.91.66])
	by p1.selectacast.net (8.9.3/8.9.3) with ESMTP id XAA16835
	for <pgsql-general@hub.org>; Wed, 12 Jul 2000 23:51:33 -0400
Message-ID: <396D3CC3.3B3BE82E@selectacast.net>
Date: Wed, 12 Jul 2000 23:51:31 -0400
From: Joseph Shraibman <jks@selectacast.net>
Organization: Xtenit
X-Mailer: Mozilla 4.72 [en] (X11; U; Linux 2.2.15 i686)
X-Accept-Language: en
MIME-Version: 1.0
To: "pgsql-general@hub.org" <pgsql-general@hub.org>
Subject: Re: select for update not locking properly.
References: <396D2AEA.5D19984A@selectacast.net>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Archive-Number: 200007/550

And I forgot to mention my version is:

PostgreSQL 7.0.1 on i686-pc-linux-gnu, compiled by gcc egcs-2.91.66


Joseph Shraibman wrote:
> 
> I have a program that does this:
> BEGIN;
> SELECT mystring FROM mytable WHERE x = 3 AND y = 4 FOR UPDATE;
> <my program takes mystring and appends to it>
> UPDATE mytable SET mystring = '<appended string here>' WHERE x = 3 AND y
> = 4;
> END;
> 
> But the locking isn't working properly.  I do something that should
> cause 3 different threads to try and do that append, and the first one
> goes through properly, but the second two append to the result of the
> first on only, meaning that the third one didn't see the result of the
> second's append.
> 
> I tried to make a simple example that reproduced this, but failed.
> 
> I'm guessing that the value of that the select is generated before the
> row is locked, and thus each of my last two threads saw the table after
> the first append, *then* one of them blocked because the other had
> locked the table. It's the only thing I can figure.