Aggregates and Primary Keys

Hi,

I have this:

create table a ( x int4 primary key, dat int4, count int4 ) ;
create table b ( x int4 references a(x), count int4 ) ;

insert into a values ( 1, 1, 10 ) ;
insert into a values ( 2, 2, 20 ) ;
insert into b values ( 1, 2 ) ;
insert into b values ( 1, 3 ) ;
insert into b values ( 2, 3 );
insert into b values ( 2, 4 );

select * from a ;
select * from b ;

x | dat | count
---+-----+-------
1 | 1 | 10
2 | 2 | 20
(2 rows)

x | count
---+-------
1 | 2
1 | 3
2 | 3
2 | 4
(4 rows)

select a.x, a.dat, a.count - sum(b.count)
from a, b
where a.x = b.x
group by a.x, a.dat, a.count ;

x | ?column?
---+----------
1 | 5
2 | 13
(2 rows)

My concern is with the "group by" clause. Strictly speaking, it
shouldn't be necessary to *also* group by a.dat and a.count, since a.x
is a primary key, right? Is there some performance loss in specifying
a.dat and a.count in the group by? Should I be doing this some other
way?

Thanks,
-itai