very slow performance when using sum and group by

Hi,

I have a query which runs very very slow, it is doing a full table scan on tab1:

select sum(col1)
from tab1
where col2 = 'x'
group by col3

There is an index on (col2, col3), the following query runs very fast:

select *
from tab1
where col2 = 'x'

I tried to change the index to (col1, col2, col3), but there is no improvement by doing that, how can I improve the performance of this query?

Thanks

Have you noticed you are grouping in the first query? you certainly can expect this query to be slower than the second one.

By the way...
What's the cardinality of "x" on col2?
Do you have up-to-date stats?
Please post Explain Plan for your two queries.