[AdamProut]

That's true, I should have been a bit more specific.

Each individual query runs within a single shard as far I recall (its been a few years...). There maybe transactions (like new order) that do multiple queries such that the transaction does read and writes to more then one shard. This doesn't really change how easy the workload is to scale out too much though. Harder to scale workloads require a single query to run across many (often all) shards or require data reshuffling for joins (there is not one good shard key that keeps all joins shard local). In these cases adding more nodes or shards will make each query a bit slower.