How do you optimize queries you don't know yet?

You don't. The engine does. A ghost is the slice of data xyzDB precomputes and keeps alive on every write — a filter, an ordering, an aggregate — and it decides which slice is worth it by watching what you actually query, not what you guessed you'd query months ago.

What would be three decisions in another database — view, index, JOIN — here is one you don't make.