Обсуждение: 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

pascal+postgres wrote
> Hi,
> 
> I want to update some values in a table, and need to count the number of
> values actually changed; but ROW_COUNT returns the number 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't it 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,

The following should work...

WITH do_uodate AS (
UPDATE ... WHERE value IS NULL
RETURNING value
) 
SELECT count(*) FROM do_update WHERE value IS NOT NULL

I don't know why it doesn't work in subquery form but other than syntax this
and your first form are equivalent.

David J.




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pascal+postgres@ensieve.org writes:
> 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?
 

An INSERT/UPDATE/DELETE RETURNING can be used as a CTE, not as a
sub-select.  Sub-selects aren't sufficiently independent of the outer
query, eg they might be evaluated more than once, or not at all, or
only partially.
        regards, tom lane