You'd be in a better position to stop the attack and it's likely just the presentation of the firearm will prevent the attack. But it would still have to be an incapacitating shot. Cops have this drilled into their heads in the academies. Knife attacks are not easy to overcome because the target can move quickly and when you're under stress, it's difficult to put solid hits on a moving target. So they always preach "distance is your friend."