CREATE OR REPLACE FUNCTION cleanNoteEventConnections() RETURNS TRIGGER AS $$
DECLARE
BEGIN
    EXECUTE 'update invoice set "noteEvent"=null where "noteEvent"=' || OLD.id;
RETURN NULL;
END;
$$ LANGUAGE 'plpgsql';

CREATE TRIGGER cleanNoteEventConnections BEFORE DELETE ON note_event FOR EACH ROW EXECUTE PROCEDURE cleanNoteEventConnections();

delete from note_event where id=34
result: Query returned successfully: 0 rows affected, 11 msec execution time.