Hi,
u can guide the optimizer to use index using the following parameters. This works even with 20 rows.

Alter session set optimizer_index_caching=90;

Alter session set optimizer_index_cost_adj=30;

Value 30 for optimizer_index_cost_adj is just to start with. Exact value could be arrived from the value of following events:

(db file scattered read/db file sequential read)*100

After setting this, execute and let us know the result.

Thanks