Do foreign key triggers get ran even if the key's value doesn't change?

[Joe Van Dyk <joe@xxxxxxxxx>]

I came across http://bonesmoses.org/2014/05/14/foreign-keys-are-not-free/
which seems to indicate so.

When I run the following test script, having 50 foreign keys takes
about twice as long to do the update. Is there a reason for that?
Seems like the RI triggers wouldn't have to run on updates if the
value doesn't change.

begin;

set client_min_messages='warning';

CREATE OR REPLACE FUNCTION fnc_check_fk_overhead(key_count INT)
RETURNS INTERVAL AS
$$
DECLARE
  i INT;
  start_time TIMESTAMP;
  end_time TIMESTAMP;
BEGIN
  DROP TABLE if exists test_fk CASCADE;

  CREATE TABLE test_fk
  (
    id   BIGINT PRIMARY KEY,
    junk VARCHAR
  );

  INSERT INTO test_fk
  SELECT generate_series(1, 100000), repeat(' ', 20);

  CLUSTER test_fk_pkey ON test_fk;

  FOR i IN 1..key_count LOOP
    EXECUTE 'CREATE TABLE test_fk_ref_' || i ||
            ' (test_fk_id BIGINT REFERENCES test_fk (id) ON UPDATE NO ACTION)';
  END LOOP;

  start_time = clock_timestamp();

  FOR i IN 1..100000 LOOP
    UPDATE test_fk SET junk = '                    '
     WHERE id = i;
  END LOOP;

  end_time = clock_timestamp();

  FOR i IN 1..key_count LOOP
    EXECUTE 'DROP TABLE test_fk_ref_' || i;
  END LOOP;

  RETURN end_time - start_time;

END;
$$ LANGUAGE plpgsql VOLATILE;

SELECT fnc_check_fk_overhead(1);
SELECT fnc_check_fk_overhead(50);