As I understand it, ETS is itself an in-memory database that comes with its own data model, but it doesn't let me build one out of my own data structures and algorithms, which is what I had in mind.

Also, I think, all data has to be copied in and out of ETS. It cannot be referenced in-place, which is going to slow things down quite a bit. (Correct me if I'm wrong. I have no first hand experience with ETS)