update_with_condition.sql

CREATE TEMP TABLE beer(name text);
INSERT INTO beer VALUES ('Guinness');

SELECT * FROM beer;
--   name   | ctid  
------------+-------
-- Guinness | (0,1)
-- (1 row)

UPDATE beer SET name = 'Guinness' WHERE name != 'Guinness';
-- UPDATE 0

SELECT * FROM beer;
--   name   | ctid  
------------+-------
-- Guinness | (0,1)
-- (1 row)

SELECT n_dead_tup FROM pg_stat_user_tables WHERE relname = 'beer';
-- n_dead_tup 
--------------
--          0
-- (1 row)

update_wo_condition.sql

CREATE TEMP TABLE beer(name text);
INSERT INTO beer VALUES ('Guinness');

SELECT * FROM beer;
--   name   | ctid  
------------+-------
-- Guinness | (0,1)
-- (1 row)

UPDATE beer SET name = 'Guinness';
-- UPDATE 1

-- value hasn't been changed, but...
SELECT * FROM beer;
--   name   | ctid  
------------+-------
-- Guinness | (0,2)
-- (1 row)

-- ...we got a dead tuple and it should be vacuumized in future
SELECT n_dead_tup FROM pg_stat_user_tables WHERE relname = 'beer';
-- n_dead_tup 
--------------
--          1
-- (1 row)