To me, this means that planetscale (and vitess) immediately become less useful for a large class of applications: existing apps that use an RDBMS that you want to scale. Because most existing apps will expect FKs. I haven't tested my employer's application (which has FKs) to see what will happen, but I doubt it will be smooth sailing.
But there's probably plenty of greenfield development where teams would prefer to stick with standards like SQL rather than a proprietary noSQL language, and that seems like a great fit for this solution.
There is not and will never be a solution to the problem of horizontally scaling an application and database designed jointly for a single-box ACID RDBMS. You have to compromise something to get the scaling.
Depends on your definition of horizontally scaling and the inherent size limitations you would be willing to accept.
CitusDB for example handles this by allowing FKeys between tables that share the same partition key. However it's also not -as- scalable as Vitess or as georeplication friendly but for many uses that need "more than a single box" is probably enough scalability.
Heavy toll on performance and is not really postgres. For the same workload pg can be up to 30x faster or more than cockroach. At some point of course, pg won't be able to keep up and the horizontal scaling of crdb will beat it.
I believe they use some statistical analysis and declared locality tags (from config) to group related cells of data on the same nodes and regions, but I don't know how effective that proves. Pure performance isn't a strong suit of CRDB as far as I can tell, but sometimes that works fine, and it does scale well horizontally.
It's not entirely accurate to say that Vitess "doesn't have" foreign keys. It doesn't enforce FK constraints cross-shard, but it can use FKs to group referential data together within a shard, and the MySQL backing that shard can enforce FK constraints.
To me, this means that planetscale (and vitess) immediately become less useful for a large class of applications: existing apps that use an RDBMS that you want to scale. Because most existing apps will expect FKs. I haven't tested my employer's application (which has FKs) to see what will happen, but I doubt it will be smooth sailing.
But there's probably plenty of greenfield development where teams would prefer to stick with standards like SQL rather than a proprietary noSQL language, and that seems like a great fit for this solution.