With a typical sharded relational database setup, each sharding key value maps to exactly one shard. There should be replicas of that shard, which can be promo

With a typical sharded relational database setup, each sharding key value maps to exactly one shard. There should be replicas of that shard, which can be promoted in case of a master failure. But in rare cases all replicas may also be degraded or inconsistent and therefore non-promotable. When this happens to a giant monolithic non-sharded database, the impact is far more catastrophic than when it happens to a single smaller shard. In any case, replication is a separate concern from sharding. Each sharded database replica set (traditionally 1 master + N replicas) has a data set that is unique / independent of other sharded database replica sets. That said, some of the "NewSQL" distributed databases may arrange their data in more complex ways. [Edit to clarify: I mean in respect to the combination of both sharding and replication. With a traditional sharded relational database, you have replica sets where each replica contains the exact same shard or shards; with a NewSQL distributed DB, data may be replicated in a more complex arrangement]