In table columns we don't allow to use UDTs from other keyspaces.
We should also make sure that the following is not allowed:
- UDFs taking UDTs from other keyspaces as arguments or return types
- UDAs using UDFs from other keyspaces as its subfunctions
The only exception should be made for system keyspace. UDAs and UDFs from any keyspace should be able to reference and reuse those.
Having no dependencies between keyspaces makes this consistent with the way we treat UDTs, which is important, but also simplifies auth in multi-tenant environments, and is also crucial to upcoming 3.X strongly consistent schema work (the ability to treat keyspace as a unit of change and not having to worry about cross-keyspace dependencies).
P.S. Should probably still allow using other keyspaces UDFs and UDAs in SELECT statements. Strict isolation in schema dependencies is what matters to me here.