CREATE SCHEMA logging;
CREATE TABLE logging.t_history (
  id serial,
  tstamp timestamp DEFAULT now(),
  schemaname text,
  tabname text,
  operation text,
  who text DEFAULT current_user,
  new_val json,
  old_val json
);

CREATE FUNCTION change_trigger() RETURNS trigger AS $$
BEGIN
  IF TG_OP = 'INSERT'
  THEN
    INSERT INTO logging.t_history (tabname, schemaname, operation, new_val)
      VALUES (TG_RELNAME, TG_TABLE_SCHEMA, TG_OP, row_to_json(NEW));
    RETURN NEW;
  ELSIF TG_OP = 'UPDATE'
  THEN
    INSERT INTO logging.t_history (tabname, schemaname, operation, new_val, old_val)
      VALUES (TG_RELNAME, TG_TABLE_SCHEMA, TG_OP,
      row_to_json(NEW), row_to_json(OLD));
      RETURN NEW;
  ELSIF TG_OP = 'DELETE'
  THEN
    INSERT INTO logging.t_history (tabname, schemaname, operation, old_val)
      VALUES (TG_RELNAME, TG_TABLE_SCHEMA, TG_OP, row_to_json(OLD));
      RETURN OLD;
  END IF;
END;
$$ LANGUAGE 'plpgsql' SECURITY DEFINER;

CREATE TABLE t_trig (id int, name text);
CREATE TRIGGER t BEFORE INSERT OR UPDATE OR DELETE ON t_trig
  FOR EACH ROW EXECUTE PROCEDURE change_trigger();