I had been hinted by someone in a comment before (wasn't able to find it), but indeed, the WinFS idea (combine a file system with a relational database system) isn't new. It was already available in the now dead (I believe) BeOS as BFS (BeOS File System). OSNews: "For the everyday user, though, BFS has much more tangible advantages. Any file or file type on a BFS volume can have arrays of metadata associated with it, in the form of "attributes." There is no limit to the amount, size, or type of attributes, and attributes can be displayed and edited, sifted, sorted, and queried for directly in the Tracker (Be's equivalent of the Finder). Because most attributes are indexed, search results are nearly instantaneous, regardless the size of the volume or the number of files being searched through. By default, BeOS ships with reasonable sets of attributes for common file types, but users are allowed to extend and customize these, and to create entirely new file types with entirely new arrays of attributes. In other words, the Be File System doubles as a database."
I wasn't able to find much more information about this, of course I also wanted to know if this had been implemented by another OS (such as Linux), but the closest I got was this attempts to port BFS to Linux but this showed no clear results.
If anybody knows more about this, let me know.