[Jweb_Guru]

Joins seriously have nothing to do with the CAP theorem, except inasmuch as multi-key read/write transactions do. Consistent multi-key read-only transactions and write-only transactions are actually not terribly difficult to do under serializability across partitions. Additionally, from a sheer performance and data perspective, centralized relational databases work just fine for real workloads. Unfortunately, pretty much zero real-world apps don't need multi-key read/write transactions, and modern businesses expect uptime that is unrealistic for a centralized system, so people are forced to replicate across datacenters. Ultimately what most people end up doing (regardless of database) is partitioning into small enough key groups that they can afford the highly expensive latency cost for maintaining high availability (across multiple datacenters) while maintaining consistency within each group for read/write transactions, and giving up on consistency across partitions for such operations (but usually maintaining consistency on read-only and write-only transactions). NoSQL can sometimes give you a clearer understanding of the cost/consistency tradeoff you're making, and hierarchical keys can make it much easier to partition, but joins really hardly enter into it.