Why is the need for you to include ACTIVE in the WHERE clause (I am assuming that your WHERE clause would look like: WHERE customer_id = 1234 AND ACTIVE = 'N'). Since customer_id is the PK and is unqiue in the table and also you know the customer_id so, you would always get one record.

If you have to include ACTIVE in the where clause then, I WOULD NOT consider creating an index on ACTIVE column.
Index on customer_id would be just enough.

Hope this helps!