The semantics of (NULL,NULL) vs NULL

Hi,

I have a question.

As I noted that (null,null) is null, I thought why put (null,null) in an
array when that is the same as putting null in there.
However, when trying to unnest that array I got an error when using null
instead of the tuple. I experimented a bit, and
read the documentation on row and array comparison, but I could not find
documentation explaining the results below.

create type int_tuple as (a int, b int);
CREATE TYPE
=# select (null,null)::int_tuple is null;
?column?
----------
t
(1 row)

=# select array[null::int_tuple] = array[null::int_tuple];
?column?
----------
t
(1 row)

as the documentation states: 'Array comparisons compare the array contents
element-by-element'
Taking into account the results above I would expect the following to be
true

=# select array[(null,null)::int_tuple] = array[null::int_tuple];
?column?
----------
f
(1 row)

apparently (null,null) is has more information then just null:

=# select null::int_tuple;
int_tuple
-----------

(1 row)

=# select (null,null)::int_tuple;
row
-----
(,)
(1 row)

=# select * from unnest(array[null::int_tuple]);
ERROR: function returning set of rows cannot return null value

=# select * from unnest(array[(null,null)::int_tuple]);
a | b
---+---
|
(1 row)

Can anyone explain why:

create type int_tuple as (a int, b int);
CREATE TYPE
=# select (null,null)::int_tuple is null;
?column?
----------
t
(1 row)

and not

=# select array[(null,null)::int_tuple] = array[null::int_tuple];
?column?
----------
f
(1 row)

Thanks in advance,

Ingmar

version
--------------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.1 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.6.1
20110908 (Red Hat 4.6.1-9), 64-bit
(1 row)