Postgresql – How to trigger an event when row column crosses now()

Thanks for ll your help, it gave me alot of ideas to try. As i only wanted it to concentrate on the last entry from read_log I took WHERE s.Stu_no=r.Cardid and changed it to WHERE s.Stu_no=(SELECT MAX(id) from read_log) and it now only concentrates on the last entry in read_log.

cheers for the helps lads

^{(Obvious error in the trigger logic aside.)}
In Postgres 9.2 or later, use the function pg_trigger_depth() that @Akash already mentioned in a condition on the trigger itself (instead of the body of the trigger function), so that the trigger function is not even executed when called from another trigger (including itself - so also preventing loops).
This typically performs better and is simpler and cleaner:

CREATE TRIGGER set_history
BEFORE UPDATE ON field_data
FOR EACH ROW 
WHEN (pg_trigger_depth() < 1)
EXECUTE PROCEDURE gener_history();

The expression pg_trigger_depth() < 1 is evaluated before the trigger function is entered. So it evaluates to 0 in the first call. When called from another trigger, the value is higher and the trigger function is not executed.