Note that this was discussed during the latest face-to-face meeting, and we decided to put this feature on hold, since we will allow users to define composite projections at query time, at that should do the trick in most cases.

We might have to implement this feature later regardless, but it's not as important as it used to be since the lack of it can be worked around.

See https://docs.google.com/document/d/16PAa__LsxyLZcbW3q1MvgyIznh4ZnCYLupbNMXKMRds/edit?pli=1#heading=h.rc2zh4omy1m9