> Sample Code
> CREATE FUNCTION check_key(varchar,varchar) RETURNS OPAQUE AS '
> BEGIN
> IF COUNT(*) FROM $2 WHERE $2.$1=NEW.$1 THEN
> RAISE EXCEPTION ''integrity violation : duplicated key'';
> END IF;
> RETURN NEW;
> END;
> ' LANGUAGE 'plpgsql';
>
> CREATE TRIGGER trigger_check_key BEFORE INSERT OR UPDATE
> ON bar
> FOR EACH ROW EXECUTE PROCEDURE check_key('id','foo');
Trigger functions take no arguments and return opaque. The arguments given
on the create trigger statement are passed in a special fashion (for plpgsql
look at TG_NARGS and TG_ARGV array).
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