Might be the next big thing. But if that is a valuable feature it is _really_ easy to implement. App, sqlite, scan file system once, build up a database of metadata then put 3 indexes on it. I expect a lot of photo apps can already do that.

But all that still isn't really leveraging the power of the relational model. That is simply indexing files on a lot of different attributes, ie, leveraging an RDBMS implementation detail where they use trees. The point of the relational model is relational algebra (SELECT, JOIN & WHERE in SQL terms). And WHERE isn't the interesting one out of those 3, it is JOIN.

If the use case for a relational filesystem is interesting filters then it sounds a lot like a false start.