GROUP BY disables indexes and parallel query

Here is what I've got:
SELECT A, B, C from MYTABLE WHERE A<=5 and B=9701; runs for 10 secs on 12-million row MYTABLE;

But when I do
SELECT A,B,COUNT(*) FROM MYTABLE WHERE A<=5 AND B=9701 GROUP BY A,B; runs for 20 mins.

Looks like GROUP BY clause disables both indexes and parallel query on MYTABLE.
Why is that and how to avoid it?

Thanks,
Sergei Received on Mon Apr 07 1997 - 00:00:00 CDT