I think I remember the FDB team developing one that was closed source back before their acquisition. I thought the business model was going to be open core and closed, paid, layers on top. I seem to remember them benchmarking the SQL layer and it being highly performant still, despite the complexity it added.

Maybe this thing still exists in close source form at Apple? It wouldn't surprise me if it does and forms the basis of a Spanner alternative, they're big enough to need it. Or maybe they canned it pre/post acquisition.

Edit: ah, you've already mentioned the closed source layer that exists at Apple. There we go!