They're claiming that the runtime will figure it out for you.

It wouldn't be too surprising if this is the sort of thing that an optimizing compiler or query planner could do better than a human.

If not, you're probably at the scale where performance regressions are caught and rolled back at early phases of rollout.

Like most magic, it's either going to make things 100x better or 100x worse, depending on how leaky the abstraction is at its current state of maturity.