the actual size
===========

SQL> select tablespace_name,sum(bytes) from dba_data_files group by tablespace_name;

TABLESPACE_NAME GB
------------------------------ ----------
UNDO 3

space occupied
===========
SQL> select tablespace_name,sum(bytes) from dba_segments group by tablespace_name;

TABLESPACE_NAME MB
------------------------------ ----------
UNDO 198

now when i am resizing the datafile to 1GB i am getting this error..

SQL> ALTER DATABASE DATAFILE '/oradata/u01/undo01.dbf' RESIZE 1024M;
ALTER DATABASE DATAFILE '/oradata/u01/undo01.dbf' RESIZE 1024M
*
ERROR at line 1:
ORA-03297: file contains used data beyond requested RESIZE value


can someone help me with this.