What is the effect of a subpoena being issued to a witness at a preliminary hearing?

Rather than attempt either to have the fees waived or the subpoena enforced, appellant merely objected at the conclusion of the hearing, claiming a denial of due process resulted from the officer's absence. Even where the right to compel the attendance of witnesses is constitutionally protected, appellant's actions would not amount to a sufficient showing of due diligence to support a claim that the failure of a witness to attend denied him a fair hearing. "Due diligence requires application be made to the court invoking its aid to secure such witnesses long before trial and certainly at a time which would not require an extended interruption of a trial, then almost completed. A defendant may not complain of the absence of a witness unless he had made a showing of due diligence to obtain the attendance of the witness." (People v. Du Bose, supra, 10 Cal.App.3d at p. 549, 89 Cal.Rptr. 134.)