I am a huge fan of the Tinkerpop Stack (http://tinkerpop.incubator.apache.org http://tinkerpop.incubator.apache.org) -- they have written a common set of interf

I am a huge fan of the Tinkerpop Stack (http://tinkerpop.incubator.apache.org http://tinkerpop.incubator.apache.org) -- they have written a common set of interfaces for graph databases. There is an implementation that can use Postgres as its storage engine (https://github.com/pietermartin/sqlg https://github.com/pietermartin/sqlg), there is one that can use ElasticSearch, I even seen one that uses MongoDB. Check it out