PostgreSQL upsert pattern from writable CTE

psql_upsert.sql

WITH upsert AS (
	UPDATE target 
	SET col1 = s.col1,
	    col2 = s.col2,
	    ...
	FROM source s
	WHERE s.key = target.key
	RETURNING s.*
)
INSERT INTO target ( ... )
SELECT key,
	col1,
	col2,
	...
FROM source s
WHERE NOT EXISTS (SELECT * FROM upsert u WHERE u.key = s.key);