How to identify sessions and Sqls for UNDO allocated / used (UNEXPIRED) ?

Hi Oracle Support experts,

How to identify sessions and Sqls for UNDO allocated / used size (UNEXPIRED) ?

Thanks and regards
Ezhil

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Tablespace                 Total_MB    Alloc_MB   Avail_MB  Big_slot    (%) Severity
------------------------------ ----------       ----------        ----------      --------        ---- --------
UNDO1                        27,000        21,279        5,721         614           79 NORMAL

NAME                                    TYPE    VALUE
------------------------------------ ----------- ------------------------------
undo_retention                      integer  21500

SELECT STATUS, round(SUM(BYTES)/1048576) Size_MB, COUNT(*) FROM DBA_UNDO_EXTENTS GROUP BY STATUS;

STATUS     SIZE_MB  COUNT(*)
---------            ----------      ----------
ACTIVE               265             6
EXPIRED         2,580        272
UNEXPIRED  18,434     2153

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