If there is no assistant district attorney, the district attorney may employ a clerk to aid him; and in the northern, eastern, middle, Norfolk, Plymouth, Berkshire, Hampden and northwestern districts, the district attorney may appoint, for the sitting at which the appointment is made, a competent person to act as an assistant to the district attorney. The compensation of any person employed or appointed hereunder shall be paid by the commonwealth.
Mass. Gen. Laws ch. 12, § 18