STEP -1
=======

$ 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.

STEP -2
=======

— Create a new undo tablespace

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

Tablespace created.

STEP -3
=======

— Switch the database to the new UNDO tablespace.

ALTER SYSTEM SET UNDO_TABLESPACE=UNDOTBS2 SCOPE=BOTH;

System altered.

STEP -4
=======

— Try to drop the tablespace but failed.

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

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.

STEP -5
=======

— The 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.

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

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.