[arjunnarayan]

[disclosure, former cockroachdb engineer]

you can get expected "single shard" performance in CockroachDB by manually splitting the shards (called "ranges" in CockroachDB) along the lines of the expected single shard queries (what you call a "properly shared database"). This is easy to do with a single SQL command. (This is what we do today; we use CockroachDB for strongly consistent metadata).

The difference between CockroachDB and a manually sharded database is that when you _do_ have to perform some cross-shard transactions (which you inevitably have to do at some point), in CockroachDB you can execute them (with a reasonable performance penalty) with strong consistency and 2PC between the shards, whereas in your manually sharded database... good luck! Hope you implement 2PC correctly.

[dravita]

The point about strong consistency and 2PC cross-shard is a good one. Even among other "auto-sharding" distributed relational databases, not all of them provide that.