如何缩小undo表空间的大小

SELECT a.name, b.name
FROM v$tablespace a, v$datafile b
WHERE a.name = (SELECT VALUE
FROM v$parameter
WHERE name = 'undo_tablespace')
AND a.ts# = b.ts#;

Create UNDO TABLESPACE UNDOTBS2 DATAFILE <'/opt/oracle/oradata/orcl/undotbs02.dbf'> SIZE 100M REUSE AUTOEXTEND ON;
Alter SYSTEM SET UNDO_TABLESPACE=UNDOTBS2;
Drop TABLESPACE <UNDOTBS1> INCLUDING CONTENTS AND DATAFILES;
Select * FROM DBA_TABLESPACES;
Create UNDO TABLESPACE <UNDOTBS1> DATAFILE <'/opt/oracle/oradata/orcl/undotbs01.dbf'> SIZE <100M> REUSE AUTOEXTEND ON;
Alter SYSTEM SET UNDO_TABLESPACE=UNDOTBS1;
Drop TABLESPACE UNDOTBS2 INCLUDING CONTENTS AND DATAFILES;
Select * FROM DBA_TABLESPACES;