1. Which two statements are true about space-saving features in an Oracle Database?

A.An index created with the UNUSABLE attribute has no segment.

B.Private Temporary Tables(PTTs)store metadata in memory only.

C.Private Temporary Tables(PTTs) when used, are always dropped at the next commit statement.

D.An index that is altered to be UNUSABLE will retain its segment.

E. A table that is truncated will always have all of its extents removed.

参考答案：AB

解析：
A:不可用索引会自动删除segment,但是索引分区后，某个分区的索引不可用，其它索引段还在的
B:私有临时表这个特性18C才引入(https://docs.oracle.com/en/database/oracle/oracle-database/18/admin/managing-tables.html#GUID-9B373086-0760-4B18-9688-BACFF07EC74B)
C:私有临时表和全局临时表都有ON COMMIT DELETE ROWS和ON COMMIT PRESERVE ROWS的两种分类(delete rows用于事务相关,也就在事务结束后truncate data in the temporary table,preserve rows表示在会话结束后清除临时表的数据)
D:不可用的索引会被删除段,已使用alter index unusable测试
E:普通表截断后会保留min_extents设置的初始区段大小