Aha, I’ve never seen those docs before.
But, it’s now clear that @search(by: [day]) means date up to the day (2020-01-01, 2020-01-02, …).
Now to answer your question:
Up to a certain point. Usually that point is just to have 1 index. After that, the cost of maintaining the multiple index would outweigh the increase in search performance (i.e. everytime you add a new mutation, you need to update multiple indices).
Probably true. Again, will be seeking to amend the docs