query question

I am new to using postgresql and have the following question.  if i submit
a query (through a perl script), what values (and in what order) will be
returned to me?  here is my query:

        SELECT p.ids_name, m.start_time, m.end_time
        FROM mail_schedule m, personnel p
        WHERE p.ids_int = m.ids_int AND
        m.dow = $weekday AND
        $timestamp BETWEEN m.start_time AND m.end_time
        UNION
        SELECT p.ids_name, c.on_start, c.on_end
        FROM mail_changes c, personnel p
        WHERE p.ids_int = c.ids_int AND
        $timestamp BETWEEN c.on_start AND c.on_end
        EXCEPT
        SELECT p.ids_name, c.off_start, c.off_end
        FROM mail_changes c, personnel p
        WHERE p.ids_int = c.ids_int AND
        $timestamp BETWEEN c.off_start AND c.off_end

thanks in advance for any assistance you can provide.
chris mutchler
davron@leibnizcreations.com