Grand Jury trial is a show wholly controlled by DA.

No one can control or challenge  DA's discretion of picking a case to go forward or dropping a case for now.

DA presents his case to the grand jury, backed by all kinds of evidence, some of which may not even be allowed in the eventual court trial.

The defense attorney is barred from the grand jury trial, although the defendant himself may become his own and only witness to argue against DA's charge. But this is done with a price: the defendant testifies in the absence of his attorney and may be asked all kinds of questions by the DA or the grand jury.

Further, DA decides whether to extend such an opportunity to the defendant to testify in front of the grand jury.