[glittershark]

We've thought about that, actually! We have an experimental mode where multiple copies of the same query can be created (actually just multiple copies of the leaf node in the dataflow graph, so intermediate state is reused) with different subsets of keys materialized - the idea is then that these separate readers would be run on different regions, so eg the reader in the EU region gets keys for EU users, and the reader in the NA region gets keys for NA users.