--
-- UPDATE ... SET <col> = DEFAULT;
--
CREATE TABLE update_test (
    a   INT DEFAULT 10,
    b   INT
);
INSERT INTO update_test VALUES (5, 10);
INSERT INTO update_test VALUES (10, 15);
SELECT * FROM update_test;
 a  | b  
----+----
  5 | 10
 10 | 15
(2 rows)

UPDATE update_test SET a = DEFAULT, b = DEFAULT;
SELECT * FROM update_test;
 a  | b 
----+---
 10 |  
 10 |  
(2 rows)

DROP TABLE update_test;