They are. Just not in a way which would help. If a subtransaction commits the changes are still not visible to other connections until the top level transaction

They are. Just not in a way which would help. If a subtransaction commits the changes are still not visible to other connections until the top level transaction commits. The subtransactions are exposed to SQL as savepoints but my point still holds. I am not sure what would be needed to implement concurrent index build but subtransactions would not help.