When customizing the database schema, you should only perform the following actions:
Do not perform the following actioons to customize the database schema:
Drop a built-in object.
Rename a built-in object.
Drop a column from a built-in table.
Rename a column in a built-in table.
Replace a built-in view, procedure, package, or trigger.
Built-in objects include tables, views, procedures, packages, indexes, triggers, and sequences.