Where the line gets blurry is that there can be layers of sharding inside the database even if it is never exposed to the end user. At the limit, disk pages are

Where the line gets blurry is that there can be layers of sharding inside the database even if it is never exposed to the end user. At the limit, disk pages are essentially a shard and treated similarly. Some database kernels do true sharding and resharding within a single server transparently for performance reasons, it isn't just for having data on multiple servers or cores. There is no intrinsic performance penalty for automatic sharding, it can be as fast or faster than any other scheme. The "faster" cases are when the sharding structure must be adaptive to have uniform distribution of data and load, such as when the key distribution is intrinsically unpredictable or when you are doing multi-attribute sharding. In these cases, effective sharding can only be created by software at runtime.