A lot of what you're referring to is dictated by the sharding strategy. Vitess and Neki both let you configure this via the VSchema / data topology (That's what I'm getting at here)

https://planetscale.com/blog/making-768-servers-look-like-1#...

This decision matters a ton. I drilled into this specific point on sharding in one of my articles from awhile back:

https://planetscale.com/blog/database-sharding

This is something you face any time you shard data, no matter the system. Single-shard queries are preferred to cross-shard ones. We want 99% of what we do to be single shard, but occasionally cross-shard work is unavoidable.