Yes, this is true. I chose a nested-maps example because while it is inaccurate for actual DB applications, it's very helpful for explaining the limits of index ordering and the limits that range queries have when interacting with indexes. It's helpful to use maps as a first explanation because they're one of the most used datastructures that nearly all languages have built in (yes, I know, C, I'm talking about the other 9 of top 10 languages), and many work-a-day developers and dabblers will use nearly _only_ lists and maps (and objects/classes/structs) in their language of choice. I could've used Python's OrderedDict but I figured if I was going to stray away from "the most straightforward possible case using the datastructures everyone is already using daily", I'd have been better off jumping straight to a custom tree, like you've done.

That's a great miniature example of such a tree!