PostgreSQL includes one merge support function that may be used in the RETURNING list of a MERGE command to identify the action taken for each row; see Table 9.66.

Example:

MERGE INTO products p
 USING stock s ON p.product_id = s.product_id
 WHEN MATCHED AND s.quantity > 0 THEN
 UPDATE SET in_stock = true, quantity = s.quantity
 WHEN MATCHED THEN
 UPDATE SET in_stock = false, quantity = 0
 WHEN NOT MATCHED THEN
 INSERT (product_id, in_stock, quantity)
 VALUES (s.product_id, true, s.quantity)
 RETURNING merge_action(), p.*;
 merge_action | product_id | in_stock | quantity
--------------+------------+----------+----------
 UPDATE | 1001 | t | 50
 UPDATE | 1002 | f | 0
 INSERT | 1003 | t | 10

Note that this function can only be used in the RETURNING list of a MERGE command. It is an error to use it in any other part of a query.