试试看：

UPDATE TABLE_A A
SET 
  last_mod_timestamp =
NULLIF
(
  MAX 
  (
    COALESCE (A.create_timestamp, '0001-01-01'::TIMESTAMP)
  , COALESCE (A.edit_timestamp, '0001-01-01'::TIMESTAMP)
  , COALESCE (A.closed_timestamp, '0001-01-01'::TIMESTAMP)
  , COALESCE (B.create_timestamp, '0001-01-01'::TIMESTAMP)
  )
, '0001-01-01'::TIMESTAMP
)
FROM
(
  SELECT table_a_fk, MAX (create_timestamp) create_timestamp
  FROM TABLE_B
  GROUP BY table_a_fk
) B
WHERE A.a_id = B.table_a_fk