######脚本###########

SELECT TABLESPACE_NAME, ROUND ( (USED / TOTAL_SIZE) * 100, 2) USED_RATE
  FROM (SELECT A.TABLESPACE_NAME, TOTAL_SIZE, USED
          FROM (  SELECT TABLESPACE_NAME,
                         ROUND (SUM (BYTES) / 1024 / 1024 / 1024, 2) TOTAL_SIZE
                    FROM DBA_DATA_FILES
                   WHERE 1 = 1 AND TABLESPACE_NAME LIKE 'UNDOTBS%'
                GROUP BY TABLESPACE_NAME) A,
               (  SELECT TABLESPACE_NAME,
                         ROUND (SUM (BYTES) / 1024 / 1024 / 1024, 2) USED
                    FROM DBA_UNDO_EXTENTS
                   WHERE 1 = 1 AND STATUS = 'UNEXPIRED'
                GROUP BY TABLESPACE_NAME) B
         WHERE 1 = 1 AND A.TABLESPACE_NAME = B.TABLESPACE_NAME);

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The Expired blocks will be reused and hence this should be counted as 'available' space in the Undo segment.

You can check the status of the undo extents via the SQLs below.
select sum(bytes /(1024*1024)) from dba_undo_extents where status='EXPIRED';
select sum(bytes /(1024*1024)) from dba_undo_extents where status='ACTIVE';
select sum(bytes /(1024*1024)) from dba_undo_extents where status='UNEXPIRED';

For more detailed information, please refer to：
Master Note: High Undo Space Usage ( Doc ID 1578639.1 )

Above all, if the undo status is Expired, you can safely ignore the undo usage, as the space will be reused automatically.