Message-ID: <20021101145630.50886.qmail@web13801.mail.yahoo.com>
Date: Fri, 1 Nov 2002 06:56:30 -0800 (PST)
From: Chris Gamache <cgg007@yahoo.com>
Reply-To: cgg007@yahoo.com
Subject: FIFO Queue Problems
To: pgsql-sql@postgresql.org
In-Reply-To: <20021101143348.A7426@goldfisch.at>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii

I'm having a race condition with a FIFO queue program that I've created...

CREATE TABLE fifo (
  id serial,
  data varchar(100),
  status int4 DEFAULT 0
);

I have a program that claims a row for itself

  my $processid = $$;
  my $sql_update = <<EOS;
    UPDATE fifo
      set status=$processid
    WHERE id = (SELECT min(id) FROM fifo WHERE status=0);
EOS

I can then

  my $sql_get = <<EOS
    SELECT data FROM fifo WHERE status=$processid
EOS

The problem occurrs when two of the processes grab the exact same row at the
exact same instant. It happens roughly 1 out of 1000 times. I'm not sure if
setting the transactions to serializable would fix the problem since it occurrs
between different postmasters.

My experience with Views and Triggers is minimal. Would they even help? Could
someone suggest a better FIFO queue method?


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