For instance, suppose you want to join on a column that's not a primary key. No big deal in a typical RDBMS; just do an index lookup on the join column. But if your data is sharded across machines, that doesn't work, because you can't update the index without running the risk that readers will observe it in an inconsistent state. Unless you can do transactions, that is.

So I'm optimistic that Cockroach will get join support before too long. The necessary primitives are available, making it "merely" a matter of figuring out the engineering details.