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