To get to the point – i understand why these are true

select (null, null) is null;
select null is null as null_is_null;

I also understand why these are all false, resp. return null

select (null, null) is not null as null_null_is_not_null;
select null is not null as null_is_not_null;
select (null, null) = (null, null) as null_null_eq_null_null;
select (null, null) = null as null_null_eq_null_null;
select (null, null) != (null, null) as null_null_neq_null_null;
select (null, null) != null as null_null_neq_null_null;

But there are two aspects that puzzle me:

• All this implies that as a user, I have no way to distinguish between null and (null, null) – which in my representation may have different meanings.
• If all of the above is valid…then explain this to me:

select coalesce((null, null), (10, 20)) as magic;

returns

 magic 
-------
 (,)
(1 row)

One would expect that as per documentation

The COALESCE function returns the first of its arguments that is not null.

But we just established that (null, null) is null.