When you are depending on the fields in the rows as part of your audit record, you need to ensure that the values reflect reality. We were able to make sure that the last_changed_ * fields always contain the correct value, but how about the created_by and created_at values? These can easily be changed in later updates, but they should never change. Even initially, they can be set to false values, since default values can be easily overridden by giving any other value in the INSERT statement.
So, let's modify our changestamp() trigger function into a usagestamp() function, which makes sure that initial values are what they should be and that they stay like that:
CREATE OR REPLACE FUNCTION usagestamp()
RETURNS TRIGGER AS $$
BEGIN
IF TG_OP = 'INSERT' THEN
NEW.created_by = SESSION_USER;
NEW.created_at = CURRENT_TIMESTAMP;
ELSE
NEW.created_by = OLD.created_by;
NEW.created_at = OLD.created_at;
END IF;
NEW.last_changed_by...