Earlier quoted context omitted.
> You'd probably agree that this is a typical usecase for a productivity app. Actually I can't think of any app that matches the abstract problem you describe. Can you give a specific example? Usually the answer to these problems becomes much clearer in the context of a specific app.
Pretty much anything you'd like to do with CouchDB or MongoDB. Replace "document based storage" with table based and you have pretty much any *SQL application. So I guess my question is: How do you expect people to use your fine grained model with databases? If the answer is "not at all" then I find the scope too limiting. If the answer is "1 grain == 1 db" then I find the claim that you are solving difficult permiss…
Wekan is a Trello clone that uses MongoDB for storage. On Sandstorm, each board lives in a grain, so there ends up being one MongoDB per board. This works fine. The only thing stand-alone Wekan ever did that queried multiple boards at once is display the user's list of all boards. On Sandstorm, displaying the user's grain list is Sandstorm's job, not Wekan's -- and indeed, usually the user is more interested in seeing the list of all their grains rather than just the Wekan boards, so delegating this to Sandstorm is a UX win.
If that is not the kind of example you have in mind, then you really need to give a specific example.