Have you actually studied the JSONB capabilities of PostgreSQL and related features such as partial indexes? These things can be used at scale and there is at l

Have you actually studied the JSONB capabilities of PostgreSQL and related features such as partial indexes? These things can be used at scale and there is at least one company that has millions of partial indexes in production. And you might want to look at how SalesForce has implemented a highly scalable architecture on top of Oracle RDBMS servers. That includes a document store as well as a dynamic denormalized schema layer that works real well for reporting. Heck, I've implemented a persistent memcache server on top of SQLITE and it scaled pretty well because modern servers and network infrastructure just happen to do most of the work needed. It is not easy to learn PostgreSQL well enough to evaluate it, but more and more people are doing so and finding that it is the ideal open source persistence solution for the vast majority of business uses.