OCP-1Z0-052-V8 02-26题

川长思鸟来 2022-03-21 13:18 48阅读 0赞

26. Which two statements are true regarding undo tablespaces? (Choose two.)

A.The database can have more than one undo tablespace.

B.The UNDO\_TABLESPACE parameter is valid in both automatic and manual undo management.

C.Undo segments automatically grow and shrink as needed, acting as circular storage buffer for their

assigned transactions.

D.An undo tablespace is automatically created if the UNDO\_TABLESPACE parameter is not set and the

UNDO\_MANAGEMENT parameter is set to AUTO during the database instance start up.

Answer: AC

答案解析：

参考： [http://blog.csdn.net/rlhua/article/details/12259555][http_blog.csdn.net_rlhua_article_details_12259555]

A答案：可以有一个或多个undo表空间，A答案正确。

sys@TEST0924> select TABLESPACE\_NAME,CONTENTS from dba\_tablespaces;

TABLESPACE\_NAME CONTENTS

\------------------------------ ---------

SYSTEM PERMANENT

SYSAUX PERMANENT

UNDOTBS1 UNDO

TEMP TEMPORARY

USERS PERMANENT

UNDOTBS2 UNDO

EXAMPLE PERMANENT

LXTBS PERMANENT

FLA\_TBS1 PERMANENT

FLA\_TBS2 PERMANENT

10 rows selected.

B答案： UNDO\_TABLESPACE只能用于自动undo段模式，B答案错误

<table title="Undo Management Initialization Parameters"> 
 <tbody> 
  <tr> 
   <td align="left"><code>UNDO_TABLESPACE</code></td> 
   <td align="left">Optional, <strong>and valid only in automatic undo management mode</strong>. Specifies the name of an undo tablespace. Use only when the database has multiple undo tablespaces and you want to direct the database instance to use a particular undo tablespace.</td> 
  </tr> 
 </tbody> 
</table>

sys@TEST0924> show parameter UNDO\_TABLESPACE

NAME TYPE VALUE

\------------------------------------ ----------- ------------------------------

undo\_tablespace string UNDOTBS2

自动还原管理：

• 在专用还原表空间中对还原数据和空间进行完全自动的管理

• 用于所有会话

• 在AUTOEXTEND表空间中进行自优化，以满足长时间运行的查询的需求

• 在固定大小的表空间中进行自优化，以实现最佳的保留时间

用来支持闪回操作的DBA 任务：

• 配置还原保留时间

• 将还原表空间更改为固定大小

• 避免空间错误和“快照太旧”错误

Oracle DB 提供了“自动还原管理”功能，这是一种完全自动的机制，用于管理所有会话的专用还原表空间中的还原信息和空间。系统自动优化自身，从而尽可能地提供还原信息的最佳保留时间。更准确地说，对于自动扩展的表空间，会将还原保留时间优化为略长于运行时间最长的活动查询的持续时间。对于固定大小的还原表空间，数据库会进行动态优化，尽可能地获得最佳保留时间。

自动还原管理是Oracle Database 11g（和更高版本）中的默认功能。也支持手动还原管理，以便与Oracle8i及较早版本向后兼容，但是这种模式下DBA 需要进行更多的交互操作。

在手动还原管理模式下，通过回退段而不是还原表空间来管理还原空间。

注：Oracle 强烈建议使用自动还原管理。

尽管在默认情况下，Oracle DB 自动管理还原数据和空间，但如果数据库正在使用闪回操作，则可能需要执行某些任务。还原管理功能可防止空间错误、使用过多的空间和“快照太旧”错误。

C答案：原段根据需要可自动增长和收缩，它充当所分配的事务处理的循环存储缓冲区。故正确

D答案：错误。是由DBCA自动创建或者由create database时手动创建或者create unod tablespace手动创建。

### Creating an Undo Tablespace ###

Although Database Configuration Assistant (DBCA) automatically creates an undo tablespace for new installations of Oracle Database Release 11g, there may be occasions when you want to manually create an undo tablespace.

There are two methods of creating an undo tablespace. The first method creates the undo tablespace when the`CREATE DATABASE` statement is issued. This occurs when you are creating a new database, and the instance is started in automatic undo management mode (`UNDO_MANAGEMENT = AUTO`). The second method is used with an existing database. It uses the`CREATE UNDO TABLESPACE` statement.

You cannot create database objects in an undo tablespace. It is reserved for system-managed undo data.

REATE DATABASE rbdb1

CONTROLFILE REUSE.

UNDO TABLESPACE undotbs\_01 DATAFILE '/u01/oracle/rbdb1/undo0101.dbf';

CREATE UNDO TABLESPACE undotbs\_02

DATAFILE '/u01/oracle/rbdb1/undo0201.dbf' SIZE 2M REUSE AUTOEXTEND ON;

官方参考： [http://docs.oracle.com/cd/E11882\_01/server.112/e25494/undo.htm\#i1006309][http_docs.oracle.com_cd_E11882_01_server.112_e25494_undo.htm_i1006309]

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[http_blog.csdn.net_rlhua_article_details_12259555]: http://blog.csdn.net/rlhua/article/details/12259555
[http_docs.oracle.com_cd_E11882_01_server.112_e25494_undo.htm_i1006309]: http://docs.oracle.com/cd/E11882_01/server.112/e25494/undo.htm#i1006309
[https_blog.csdn.net_jiangjunshow]: https://blog.csdn.net/jiangjunshow/article/details/77338485