About the behavior of array_length() for empty array

postgres=# select '{}'::integer[] = array[[]]::integer[][];
?column?
----------
t

postgres=# select '{}'::integer[] = array[[[]]]::integer[][][];
?column?
----------
t

From this point of view, seems N dimensions of empty array all are equivalent.

But the result of array_length of them always are null.

postgres=# select array_length('{}'::integer[],0);
array_length
--------------

postgres=# select array_length(array[[[]]]::integer[][][],0);
array_length
--------------

postgres=# select array_length(array[[[]]]::integer[][][],3) is null;
?column?
----------
t

I just think any "empty" array length should return 0, but not null. It's not a NULL array.

Is there standard definition of this behavior?

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