Update Returning as subquery

Hi,

I want to update some values in a table, and need to count the number of values actually changed; but ROW_COUNT returns
thenumber of total rows touched. 

But this gives a syntax error:

SELECT count(*) INTO my_count
FROM ( UPDATE stuff SET value = maybe_null(key)
--^ WHERE value IS NULL RETURNING value ) AS t
WHERE value IS NOT NULL;

Why is that forbidden? Isn't the purpose of a RETURNING clause to return values like a SELECT statement would, and
shouldn'tit therefore be allowed to occur in the same places? 



I switched it around using a CTE in this case:

WITH new_values AS ( SELECT key, maybe_null(key) AS value FROM stuff WHERE value IS NULL)
UPDATE stuff AS s
SET value = n.value
FROM new_values AS n
WHERE n.key = s.key
AND n.value IS NOT NULL;

Which only touches rows that will be changed and returns a useful ROW_COUNT, but needs a join.

Cheers,

--
Pascal Germroth