I've been trying figure this out for a week now, and I just can't see how this is true. How can a relational query "preserve keys" across relational operations like PROJECT and JOIN? It just doesn't work as far as I can tell, even SELECT is questionable. Granted, there may be some subtlety that I am missing that allows it for JOIN (if so please let me know). But PROJECT seem plainly impossible, given that one or more of the key columns could be removed and that you cannot validly derive keys merely from data inferencing.

:crazy: