undo tablespace存放undo数据块，可以建立多个undo 表空间，但处于active状态只有一个，active状态的undo tablespace是不能
offline 和drop。
如果未建立undo tablespace oracle 使用system undo segment。（如果没有建立undo表空间，就默认用system表空间作为undo表空间
，这是很不好的）
1、查看undo表空间
SQL> show parameter undo
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string rtbs
SQL> select tablespace_name,status,contents from dba_tablespaces;
TABLESPACE STATUS CONTENTS
---------- --------------------------- ---------------------------
SYSTEM ONLINE PERMANENT
RTBS ONLINE UNDO
SYSAUX ONLINE PERMANENT
TEMP ONLINE TEMPORARY
USERS ONLINE PERMANENT
TEXT ONLINE PERMANENT
LX01 ONLINE PERMANENT
7 rows selected.
2、创建undo表空间
SQL> create undo tablespace
2 undotbs datafile '/u01/app/oracle/oradata/anny/undotbs01.dbf' size 100m
3 autoextend on next 10m maxsize 500m;
Tablespace created.
SQL> select tablespace_name,status,contents from dba_tablespaces;
TABLESPACE STATUS CONTENTS
---------- --------------------------- ---------------------------
SYSTEM ONLINE PERMANENT
RTBS ONLINE UNDO
SYSAUX ONLINE PERMANENT
TEMP ONLINE TEMPORARY
USERS ONLINE PERMANENT
TEXT ONLINE PERMANENT
LX01 ONLINE PERMANENT
UNDOTBS ONLINE UNDO
09:03:06 SQL> show parameter undo
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string rtbs
09:03:25 SQL> select TABLESPACE_NAME,STATUS,CONTENTS from dba_tablespaces;
TABLESPACE_NAME STATUS CONTENTS
------------------------------ --------- ---------
SYSTEM ONLINE PERMANENT
RTBS ONLINE UNDO
SYSAUX ONLINE PERMANENT
TEMP ONLINE TEMPORARY
USERS ONLINE PERMANENT
EXAMPLE ONLINE PERMANENT
INDX ONLINE PERMANENT
OLTP ONLINE PERMANENT
TEST ONLINE PERMANENT
CUUG ONLINE PERMANENT
LX01 ONLINE PERMANENT
LX02 ONLINE PERMANENT
UNDO_TBS01 ONLINE UNDO
13 rows selected.
09:03:28 SQL> alter system set undo_tablespace=undo_tbs01;
System altered.
3、切换undo 表空间
09:04:11 SQL> show parameter undo;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDO_TBS01
CUUG
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