Datomic

Datomic is a distributed database and implementation of Datalog.[2][3] It has ACID transactions, joins, and a logical query language, Datalog.[4][5] A distinguishing feature of Datomic is that time is a basic feature of data entities. It has been designed for first-class use with JVM languages such as Java and Clojure. https://en.wikipedia.org/wiki/Datomic

Not open source.

• Apr'2023: now free, but still not open source.

Available in AWS.

cf Triple, Quad, or Quint

It also happens to be effective at modeling graph data and was a great fit for performing graph traversal. (graph database)