>>One more idea.
>>
>>Get the current result into a temp table, add index on PK and ChildPK fields (Or index on PK and include ChildPK - may be better) and then try group by.
>
>Yes, that is the next thing to do. I would have liked to avoid that as I would have wanted one SQL to do it all.

Can you compare the actual execution plans with and without group by to find out what takes that long?