Well, you are free to suggest a better advice. In the case of shards splitting into different servers, Quarry will have to implement a server / database selector [1], and processing data from two databases in a single query may not be possible at all, like currently in the case of tools-db and replicas, where all inter-host joins must happen in application space.
In any case, the question is how to do cross-wiki joins properly instead of emulating them with temporary variables, which what I have suggested may be how one would do it properly, when it works at least //in the foreseeable future//. (Yes, changing is a possibility but it’s not currently foreseeable whether or when it will happen.)
Zhuyifei1999