Change or switch undo tablespace in Oracle database

This is a mostly asked question on different oracle forums; How do I change the UNDO tablespace for my database and drop the old one? and the answer often is; Its very simple daa. But I don't think its that simple. The complexity comes in if there were pending transactions in your old undo tablespace and you have already switched to the new one and when you try to drop the old one it says "Tablespace is in use".

How to switch the database to a new UNDO tablespace and drop the old one

$ sqlplus / as sysdba

SQL> show parameter undo

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS1
SQL> 

The current undo tablespace as suggested by the initialization parameter undo_tablespace is UNDOTBS1. Leave this sysdba as is, open another console, log in as user SCOTT and initiate a transaction.

$ sqlplus scott/tiger

update emp set sal = sal + 1000 where empno=7839;

1 row updated.

With an update on emp table we have initiated a transaction. The undo data is written to a segment in the UNDOTBS1 tablespace. Now leave this SCOTT's session intact and go back to the sysdba console without issuing any COMMIT or ROLLBACK.

CREATE UNDO TABLESPACE undotbs2
DATAFILE '/d01/apps/oradata/oraxpo/undotbs201.dbf'
         SIZE 50M AUTOEXTEND ON NEXT 5M;

Tablespace created.

-- We created a new UNDO tablespace named UNDOTBS2


ALTER SYSTEM SET UNDO_TABLESPACE=UNDOTBS2 SCOPE=BOTH;
System altered.

-- Switch the database to the new UNDO tablespace.

SQL> DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES;
DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES
*
ERROR at line 1:
ORA-30013: undo tablespace 'UNDOTBS1' is currently in use

-- Try to drop the tablespace but failed. 

SQL>

With the alter system set undo_tablespace=UNDOTBS2, the database UNDO tablespace is changed and any new transaction's undo data will go to the new tablespace i.e. UNDOTBS2. But the undo data for already pending transaction (e.g. the one initiated by SCOTT before the database UNDO tablespace switch) is still in the old tablespace with a status of PENDING OFFLINE. As far as it is there you cannot drop the old tablespace.

set lines 10000
column name format a10

SELECT a.name,b.status 
FROM   v$rollname a,v$rollstat b
WHERE  a.usn = b.usn
AND    a.name IN ( 
		  SELECT segment_name
		  FROM dba_segments 
		  WHERE tablespace_name = 'UNDOTBS1'
		 );

NAME       STATUS
---------- ---------------
_SYSSMU8$  PENDING OFFLINE

The above query shows the name of the UNDO segment in the UNDOTBS1 tablespace and its status. Now lets see which users/sessions are running this pending transaction.

column username format a6

SELECT a.name,b.status , d.username , d.sid , d.serial#
FROM   v$rollname a,v$rollstat b, v$transaction c , v$session d
WHERE  a.usn = b.usn
AND    a.usn = c.xidusn
AND    c.ses_addr = d.saddr
AND    a.name IN ( 
		  SELECT segment_name
		  FROM dba_segments 
		  WHERE tablespace_name = 'UNDOTBS1'
		 );

NAME       STATUS          USERNA        SID    SERIAL#
---------- --------------- ------ ---------- ----------
_SYSSMU8$  PENDING OFFLINE SCOTT         147          4

So this is SCOTT with SID=147 and SERIAL#=4. Since we know now the user, we can go to him/her and request to end the transaction gracefully i.e. issue a ROLLBACK or COMMIT. However, if this is not possible (say the user initiated the transaction and left for annual leave :) and trust me this happens) you may go ahead and kill the session to release the undo segments in the UNDOTBS1 tablespace.

SQL> alter system kill session '147,4';
 
System altered.
 
SELECT a.name,b.status , d.username , d.sid , d.serial#
FROM   v$rollname a,v$rollstat b, v$transaction c , v$session d
WHERE  a.usn = b.usn
AND    a.usn = c.xidusn
AND    c.ses_addr = d.saddr
AND    a.name IN ( 
		  SELECT segment_name
		  FROM dba_segments 
		  WHERE tablespace_name = 'UNDOTBS1'
		 );
 
no rows selected

As we can see once the session is kills we don't see anymore segments occupied in the UNDOTBS1 tablespace. Lets drop UNDOTBS1.

SQL> DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES;
DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES
*
ERROR at line 1:
ORA-30013: undo tablespace 'UNDOTBS1' is currently in use

If you are retaining undo data then you still won't be able to drop the tablespace because it is still in use by undo_retention. Let the UNDO_RETENTION time pass and then try to drop the tablespace. In my case it is 900 seconds i.e. 15 minutes.

-- After 15 minutes.
SQL> DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES;

Tablespace dropped.