California, United States of America
The following excerpt is from Falls v. Superior Court, 42 Cal.App.4th 1031, 49 Cal.Rptr.2d 908 (Cal. App. 1996):
The actions of petitioners consist of subpoenaing a witness for a judicial hearing and interviewing that witness prior to the hearing. Such actions are indivisible from and "intimately associated with" the act of initiating a criminal prosecution--for which a prosecutor has always enjoyed immunity. (Imbler v. Pachtman, supra, 424 U.S. 409, 421-422, 96 S.Ct. 984, 990-991.)
Prosecutions cannot proceed without witnesses and unless subpoenaed there would be no witnesses.