[GENERAL] select array_length(array_remove(ARRAY[NULL,NULL,NULL],NULL), 1); returns NULL instead of 0

[Alexander Farber]

Good evening,

I wonder, why the following returns NULL and not 0 in 9.5.3?

# select array_length(array_remove(ARRAY[NULL,NULL,NULL],NULL), 1);
 array_length
--------------

(1 row)


# select array_length(array_remove(ARRAY[3,3,3],3), 1);
 array_length
--------------

(1 row)

In a code for a word game (could be a card game too)
I remove played letter tiles from player's hand using
array_position and finally "compress" it using array_remove:

        FOR _tile IN SELECT * FROM JSONB_ARRAY_ELEMENTS(in_tiles)
        LOOP
                _letter :=  _tile->>'letter';
                _value  := (_tile->>'value')::int;
                _col    := (_tile->>'col')::int + 1;
                _row    := (_tile->>'row')::int + 1;

                IF _value = 0 THEN
                        _pos = ARRAY_POSITION(_hand, '*');
                ELSE
                        _pos = ARRAY_POSITION(_hand, _letter);
                END IF;

                IF _pos >= 1 THEN
                        _hand[_pos] := NULL;
                ELSE
                        RAISE EXCEPTION 'Tile % not found in hand %',
_tile, _hand;
                END IF;

                _letters[_col][_row] := _letter;
                _values[_col][_row]  := _value;
        END LOOP;

        -- remove played tiles from player hand
        _hand := ARRAY_REMOVE(_hand, NULL);
        -- move up to 7 missing tiles from pile to hand
        _hand_len := ARRAY_LENGTH(_hand, 1);   -- OOPS can be NULL
        _pile_len := ARRAY_LENGTH(_pile, 1);        -- OOPS can be NULL
        _move_len := LEAST(7 - _hand_len, _pile_len);
        _hand := _hand || _pile[1:_move_len];
        _pile := _pile[(1 + _move_len):_pile_len];

I understand that I have to wrap ARRAY_LENGTH calls
with COALESCE, but I am just curious why isn't 0 returned
in the first place...

Regards
Alex