1、SET UNUSED COLUMNS的若干问题

1.1 UNUSED后，所有的索引、约束和静态定义都被移除。

All constraints, indexes, and statistics defined on the column are also removed.

2、external table 外部表的特点

Oracle Database allows you read-only access to data in external tables.

You can select, join, or sort external table data. You can also create views and synonyms for external tables. However, no DML operations (UPDATE, INSERT, or DELETE) are possible, and no indexes can be created， on external tables.

3、FLASHBACK TABLE

3.1 Use the FLASHBACK TABLE statement to restore an earlier state of a table in the event of human or application error. The time in the past to which the table can be flashed back is dependent on the amount of undo data in the system. Also, Oracle Database cannot restore a table to an earlier state across any DDL operations that change the structure of the table.

3.2 You cannot roll back a FLASHBACK TABLE statement. However, you can issue another FLASHBACK TABLE statement and specify a time just prior to the current time. Therefore, it is advisable to record the current SCN before issuing a FLASHBACK TABLE clause.

4、index的其他问题

4.1 当指定主键后，不能再在主键上建索引，因为ORACLE 已经自动建了索引。即不能在一个列上建多个索引。

例子如下，table ord 的主键是ord_no

SQL> create index ord_idx on ord(ord_no);
create index ord_idx on ord(ord_no)
                            *
ERROR at line 1:
ORA-01408: such column list already indexed

5、数据库非正常关机中的sequence

The database might skip sequence numbers if you choose to cache a set of sequence numbers. For example, when an instance abnormally shuts down (for example, when an instance failure occurs or a SHUTDOWN ABORT statement is issued), sequence numbers that have been cached but not used are lost. Also, sequence numbers that have been used but not saved are lost as well. The database might also skip cached sequence numbers after an export and import

sequence numbers that have been used but not saved are lost as well.

那些已经被使用但还没有被保存的序列数字也会丢失。(问题：这里的保存怎么理解)

6、private synonym 和public synonym 区别

private synonym:只有有该对象访问权限的用户才能访问这个对象的synonym

public synonym：所有用户都能访问该synonym

示例：

SQL> conn test/test;
Connected to Oracle Database 10g Express Edition Release 10.2.0.1.0
Connected as testSQL> select count(*) from hr.employees;COUNT(*)
----------107SQL> create synonym employees for hr.employees;Synonym createdSQL> select count(*) from employees;COUNT(*)
----------107SQL> conn sys/111 as sysdba;
Connected to Oracle Database 10g Express Edition Release 10.2.0.1.0
Connected as SYSSQL> select count(1) from employees;select count(1) from employeesORA-00942: 表或视图不存在SQL> conn test/test;
Connected to Oracle Database 10g Express Edition Release 10.2.0.1.0
Connected as testSQL> drop synonym employees;Synonym droppedSQL> create public synonym employees for hr.employees;Synonym createdSQL> select count(*) from employees;COUNT(*)
----------107SQL> conn sys/111 as sysdba;
Connected to Oracle Database 10g Express Edition Release 10.2.0.1.0
Connected as SYSSQL> select count(*) from employees;COUNT(*)
----------107SQL>