OpenLink Virtuoso implements a lot of this functionality (xml storage, column store, graphs, rdf, json), optimized for speed and open source. I know because I was lucky enough to have had conversations about it with the guy responsible for it, Orri Erling <a href="http:&#x2F;&#x2F;sites.computer.org&#x2F;debull&#x2F;A12mar&#x2F;p3.pdf" rel="nofollow">http:&#x2F;&#x2F;sites.computer.org&#x2F;debull&#x2F;A12mar&#x2F;p3.pdf</a>