oracle undo tablespace schema

SELECT S.SID, S.USERNAME, round(sum(SS.VALUE) / 1e6, 2) AS UNDO_SIZE_MB,
       SQL.ELAPSED_TIME, SQL.SQL_TEXT
FROM V$SESSTAT SS
     JOIN V$SESSION S ON S.SID = SS.SID
     JOIN V$STATNAME STAT ON STAT.STATISTIC# = SS.STATISTIC#
     LEFT JOIN V$SQLAREA SQL 
     	ON S.SQL_ADDRESS = SQL.ADDRESS AND S.SQL_HASH_VALUE = SQL.HASH_VALUE
WHERE STAT.NAME = 'undo change vector size'
  AND S.TYPE <> 'BACKGROUND' AND S.USERNAME IS NOT NULL AND SS.VALUE >= 0.01 * 1e6
GROUP BY S.SID, S.USERNAME, SQL.ELAPSED_TIME, SQL.SQL_TEXT
ORDER BY S.USERNAME, round(sum(SS.VALUE) / 1e6, 2);