Is a class unique within a database?
In db_class, I can select classes (tables) along with their "owner". I reckon that "owner" and "user" are the same thing in the CUBRID database. Can the same class exist for several owners in the same database?
In other words: does the owner correspond to what other databases call a "schema", or are there no schemata in CUBRID databases?
CUBRID does not support schema in the sense in which PostgreSQL or SQL Server use it. In that sense, a CUBRID database has only one schema.
Each table name is unique, the owner field in db_class points to the creator of that class. However you can grant access to other users to that table using the GRANT syntax.
You can use SHOW GRANTS to see authorizations. To see the users that have access to table t you can also write this:
select owner.name from db_authorization where class t in grants;