第 18 题 (多选题)： Which three statements are true about indexes and their administration in an Oracle database.

A. An INVISIBLE index is not maintained when Data Manipulation Language (DML) is performed on its underlying table. B. A descending index is a type of function-based index. C. An index can be created as part of a CREATE TABLE statement. D. If a query filters on an indexed column then it will always be used during execution of the query. E. A UNIQUE and non-unique index can be created on the same table column. F. A DROP INDEX statement always prevents updates to the table during the drop operation. 显示答案