96.Examine the following command:
SQL> ALTER TABLE booking SHRINK SPACE COMPACT;
Which activity is performed when the preceding command is executed?
A. The shrink operation touches every block in the BOOKING table
B. The high-water mark (HWM) for the BOOKING table is shifted from its original position
C. The progress of the shrink operation is saved in the bitmap blocks of the BOOKING table
D. The data manipulation language (DML) triggers on the BOOKING table are executed because the shrink operation is internally handled by the INSERT/DELETE operation
Answer: C
参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_3001.htm#SQLRF53
SQL> ALTER TABLE booking SHRINK SPACE COMPACT;
Which activity is performed when the preceding command is executed?
A. The shrink operation touches every block in the BOOKING table
B. The high-water mark (HWM) for the BOOKING table is shifted from its original position
C. The progress of the shrink operation is saved in the bitmap blocks of the BOOKING table
D. The data manipulation language (DML) triggers on the BOOKING table are executed because the shrink operation is internally handled by the INSERT/DELETE operation
Answer: C
参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_3001.htm#SQLRF53
上图描述了表收缩操作的两个阶段。第一个阶段执行压缩。在此阶段，行将尽可能移动到段的左侧部分。在内部，行将由数据包移动，以避免锁定问题。移动完行后，将启动收缩操作的第二阶段。在此阶段中，将调整高水位标记(HWM)，并释放未使用的空间。
如果有长时间运行的查询，而且这些查询可能跨越收缩操作，并尝试从已回收的块中读取数据，则COMPACT子句会很有用。指定SHRINK SPACE COMPACT子句时，收缩操作的进度将保存在相应段的位图块中。这意味着下次在同一个段上执行收缩操作时，Oracle DB Server 可以记住已经执行过的操作。然后你就可以在非峰值时段重新发出SHRINK SPACE子句，无需使用COMPACT子句即可完成第二阶段。
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/81227/viewspace-2119551/ ，如需转载，请注明出处，否则将追究法律责任。
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