I work on https://github.com/xtdb/xtdb which is broadly Postgres-compatible with a few key SQL extensions (SQL:2011 bitemporal tables + immutability, first-class nested data, pipeline syntax, etc). Built on Arrow and the JVM but is otherwise mostly from scratch.

XTDB is perhaps not directly relevant to the topic at hand, but I am a firm believer that ML workflows can benefit from robust temporal modelling.