I think that might be counterproductive on cockroach - the ideal key has locality for data accessed together, but also has writes generally spread across the key range. If all the new rows have a similar key prefix then the shard covering the end of the key range will be hot which limits scalability.

This is a difference from PG, where having all the writes be concentrated in the key range is helpful as the Btree nodes covering that will be more likely to be highly cached

does Cockroach really use key prefix, vs a hash of the key? I would have thought it would be important to avoid unexpected performance hits resulting from key generation approach.

It is easy enough--arguably easier, right?--to generate random keys, but if you always work with hashes of the key (which I am not claiming cockroach doesn't do, only that I would be frustrated if they do) then you lose the ability to do range queuries.

Cockroach lets you hash-shard your keys n-ways if you choose to through a feature called hash-sharded indexes (https://www.cockroachlabs.com/docs/stable/hash-sharded-index...) That's a trade-off: costlier range scans (a range scan turns into n scans) for avoiding a throughput-limiting hot spot.