No, again, ART was Hyper's specialty. Because you're right, ART specializes at in-memory workloads it is not amenable to paging.

I believe Umbra is heavily BTree based, just like its cousin LeanStore.

One of its specific innovations is its buffer pool which uses virtual memory overcommit and multiple possible buffer sizes to squeeze better performance out of page management.

The talk at https://www.youtube.com/watch?v=pS2_AJNIxzU is delightful.

My understanding is the research projects LeanStore & Umbra -- and now I assume the product CedarDB based on the people involved, etc. -- are systems based on the observation that a) existing on-disk systems aren't built well with the characteristics of nVME/SSD drives in mind b) RAM prices up to this year were not dropping at the same rate as they were early in the 2010s, meaning that pure in-memory databases were not so competitive, so it's important to look at how we can squeeze performance out of systems that perform paging. And of course in the last 6 months this has become extremely relevant with the massive spike in RAM prices.

That and the query compilation stuff, I guess, which I know less about.