Full Table Scan on a Partition table

[Subha]

I am running a sql statement like

select * from
from dummy ad
where credit_acct not in (select dt.account_number
from Customer_Accounts_100 dt
where ad.credit_acct = dt.account_number));

The table customer_accounts has 40 partitions .The above querry goes for a full table scan on Customer_Accounts rather than the partition Customer_Accounts_100.

why ? what should we do to run only on the partition table .