Is NULL equal to NULL or not?

Hi,

My first message on this list:

Using PostgreSQL 7.1.3.

I've noticed that
select NULL=NULL;
yields TRUE. It is also possible to select rows in this manner.

At the same time in PL/pgSQL:

> drop function demo1();
create function demo1() returns boolean as '
declare
a boolean:=NULL;
b boolean:=NULL;
begin
return a=b;
end;
' language 'plpgsql';

> select demo1();
yields NULL. Yet if I'd written "return NULL=NULL" I would have gotten the
same result as in the simple query;

Is there any logical explanation for this inconsistency? ... or should I
have been posting a bug? :-)

Regards
/Jørn Cornelius Olsen