results return slowly when use SUM and ORDER BY

Hi,

    I have a table with about 2 million records. This table has a column (call col_A) and it's foreign key . I create index on it.

    If I SELECT from this table, all works are normal. But when I SELECT and SUM with GROUP BY(col_A), it is very slowly !

    From askTom, I can increase sort_area_size to my sql faster. I do not try this but I think if I set work_area_size_policy=AUTO, Oracle will increase sort_area_size to sort data. And if increase value, it can effect to other sessions in DB ?