The problem isn't so much the writes, it's the reads. Every time you join tables, you're using a PK 2-4x the size it needs to be, and at least that much slower.

The problem isn't so much the writes, it's the reads. Every time you join tables, you're using a PK 2-4x the size it needs to be, and at least that much slower. Even filtering on a secondary index may involve an internal lookup via PK to the main table. It doesn't take long to start noticing the performance difference. Since you'd have a secondary index for the public UUID, yes that one index suffers from the random-writes issue still, but it takes a lot of volume to notice. If it ever is a big deal, you can use a separate KV store for it. But if you picked UUID as the PK, it's harder to get away from it.