Doesn't include everything that Horizon does, but it provides a good foundation for building it http://postgrest.com/
https://github.com/calebmer/postgraphql
That said, the real-time aspect is what I'm really dying for, plus a company supporting a BAAS. The ease of firebase is so nice, and you can make really cool real time apps with it very fast. I'm not sure if switching to a relational model would make impose some technical limitation that json stores don't (besides obvious complexity). Postgraphql + socket.io is kind of what i'm thinking is the start