I've been looking at this, too, after the recent surge in HN Prolog headlines.

You're probably looking for Datalog. The Wikipedia topic has a list of systems that support it: https://en.m.wikipedia.org/wiki/Datalog

Also, I found that AllegroGraph is a production graph database that supports Prolog queries... I don't think you load it with Prolog syntax, though--probably RDF triples instead.