When the commissioner is authorized by law to make an investigation or hold a hearing, he shall have full power by subpoena to compel the attendance of witnesses before him, or before any of the employees of the department by him delegated or deputized to sit for him, and to compel the production before him or before such employee, of any contracts, books, papers, accounts and documents deemed by him relevant or material to the conduct of the investigation or hearing, or upon request of any party to the hearing.