Surprising results from assigning multiple variables in FOR loop

Question

CREATE OR REPLACE FUNCTION my_function(input jsonb)
  RETURNS jsonb
  LANGUAGE plpgsql AS  -- language declaration required
$func$
DECLARE
   _key   text;
   _value text;
BEGIN
    FOR _key, _value IN
      SELECT jsonb_each_text($1)
    LOOP
        -- do some math operation on its corresponding value
        RAISE NOTICE '%: %', _key, _value;
    END LOOP;

    RETURN input;
END
$func$;

For the function, if I call:

my_function('{"a":1, "b":2}');

It will raise notice that looks like (a, 1), (b, 2) in the message box.

However, if I change the code:

        RAISE NOTICE '%: %', _key, _value;

into:

        RAISE NOTICE '%', _key;

The same results will pop up.

While the code:

        RAISE NOTICE '%', _value;

would pop up nothing. How did that happen?

Answer 1

Guess I am to blame for this confusion.
The code example is from my answer to your previous question, which had a subtle bug (now fixed).

What was going wrong?

jsonb_each_text() is a set-returning function, it returns a set of rows. When called with:

SELECT * FROM jsonb_each_text('{"a":1, "b":2}')

it returns:

 key | value 
-----+-------
 a   | 1
 b   | 2

But when called like above with:

SELECT jsonb_each_text('{"a":1, "b":2}')

it returns:

 jsonb_each_text 
-----------------
 (a,1)
 (b,2)

Each row is returned with a single value, a row value, the text representation of which is as displayed.

Now, the assignment in:

  FOR _key, _value IN SELECT jsonb_each_text($1) ...

assigns the first value per row to the first variable provided, which is _key in the example. The row value is cast to text in the assignment. Since there is no second value, _value is set to NULL. So, the later:

 RAISE NOTICE '%: %', _key, _value;

outputs the messages:

NOTICE:  (a,1): <NULL>
NOTICE:  (b,2): <NULL>

Which can be pretty misleading. (But note the dangling : <NULL>!)

Change to:

FOR _key, _value IN SELECT * FROM jsonb_each_text($1)

.. and the mists should clear.