Calcite. Support hidden columns

We should have ability to hide some columns. At first glance it's _key column. It really important for autogenerated PK.
Such columns couldn't be accessed for 'select * ' query, but available for explicit use in select statement.
Another example of hidden column is Oracle rownum.
Also another databases (Oracle, SQLSERVER, MySQL) have support hidden (invisible) columns which could be created user explicitly and amend the visibility through ALTER statement. Maybe we should consider possible extension it for future to support one.