When absolutely necessary, we would solve this by keeping global (centralized) ID's in an autoincrement table either on the master or on a vertical shard. It di

When absolutely necessary, we would solve this by keeping global (centralized) ID's in an autoincrement table either on the master or on a vertical shard. It didn't come up that often, because most tables (in our app) are really dedicated to one specific entity (ie a customer's inventory items).