Oracle – Select the index for the column of the date range with 20% selectivity associated with another table

Select c.customer_name, c.customer_address, i.invoice_num, i.invoice_date, i.invoice_amount
from the customer c, --- 100 k rows
Bill i ---- 10 M rows
where c.cust_id = i.cust_id
AND c.customer_name = & # 39; Tarun Kumar & # 39; - exactly one line
AND i.invoice_date> = (sysdate-12) - 20% of all lines



customer table
------------------
customer_name, cust_id --- Composite index

invoice table
----------------
cust_id 

Should I add the index to Invoice_date? Composite index (invoice date, cust_id)