Re: FIFO Queue Problems

Do you need to unlock the table fifo when you are done?

On Fri, 2002-11-01 at 11:51, Tom Lane wrote:
> Chris Gamache <cgg007@yahoo.com> writes:
> > 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
> 
> > The problem occurrs when two of the processes grab the exact same row at the
> > exact same instant.
> 
> Probably the best fix is to do it this way:
> 
>     BEGIN;
>     LOCK TABLE fifo IN EXCLUSIVE MODE;
>     UPDATE ... as above ...
>     COMMIT;
> 
> The exclusive lock will ensure that only one process claims a row
> at a time (while not preventing concurrent SELECTs from the table).
> This way you don't need to worry about retrying.
> 
>             regards, tom lane
> 
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Wei Weng <wweng@kencast.com>