I’m having a hard time understanding the author’s model of “scalability” such that unique indexes are a problem, but joins are not. An arbitrary join (even an e

I’m having a hard time understanding the author’s model of “scalability” such that unique indexes are a problem, but joins are not. An arbitrary join (even an equijoin) between two relations sharded on distinct keys needs at least the kind of 1:N broadcast coordination required to ensure uniqueness.