best system design interview advice i got (and put to successful use in loops with goog and fb) was to not try to give the "right" answer -- it's like a professor grading papers on a topic they've taught countless times, you have more of an opportunity to fail than you do to succeed. instead, be decisive and give answers or approximations for any questions that arise, and bring your unique perspective to the problem -- in my case i have a background in mobile app development, so i spent a lot of time speaking to those use-cases.

if you try to cover every base and fret about everything being optimal, you're going to barely have time to get started.