Conversely, you could have a NoSQL layer below Postgres, where PG stores and indexes metadata which tells it which, of many, small NoSQL dbs to find the actual data in. These data dbs then can be sharded/replicated across physical systems as you like. You loose some raw speed on reads, but avoid a global write lock and the system scales quite well. I've started playing around with such a system with https://github.com/cloudflare/SortaSQL