CREATE TABLE TEST_MERGE (KEY INT, VALUE VARCHAR (20), TS TIMESTAMP DEFAULT CURRENT TIMESTAMP);

WITH 
  P (KEY, VALUE) AS (VALUES (1, 'value 1'))
, U AS
(
  SELECT *
  FROM FINAL TABLE
  (
    UPDATE TEST_MERGE T
    SET (VALUE, TS) = (SELECT VALUE, CURRENT TIMESTAMP FROM P WHERE P.KEY = T.KEY)
    WHERE EXISTS (SELECT 1 FROM P WHERE P.KEY = T.KEY)
  )
)
, I AS
(
  SELECT *
  FROM FINAL TABLE
  (
    INSERT INTO TEST_MERGE (KEY, VALUE, TS)
    SELECT KEY, VALUE, CURRENT TIMESTAMP 
    FROM P
    WHERE NOT EXISTS (SELECT 1 FROM TEST_MERGE T WHERE T.KEY = P.KEY)
  )
)
SELECT 
  P.KEY
, COALESCE (U.VALUE, I.VALUE) AS VALUE
, COALESCE (U.TS, I.TS) AS TS 
FROM P
LEFT JOIN U ON U.KEY = P.KEY
LEFT JOIN I ON I.KEY = P.KEY
;