获取undo表空间最大大小推荐的sql

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select
ur undo_retention,
 dbs db_block_size,
 ((ur * (ups * dbs)) + (dbs * 24)) / 1024 / 1024 as "M_bytes"
   from (select value as ur
           from v$parameter
          where name = 'undo_retention'),
        (select (undoblks / ((end_time - begin_time) * 86400)) ups
           from v$undostat where undoblks in (select max(undoblks) from  v$undostat ) ),
        (select value as dbs
           from v$parameter
          where name = 'db_block_size');