Re: FIFO Queue Problems

Chris Gamache <cgg007(at)yahoo(dot)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