How to Shrink the datafile of Undo Tablespace

-- Create new undo tablespace with smaller size.
SQL> create undo tablespace UNDO_RBS1 datafile 'undorbs1.dbf' size 100m;

-- Set new tablespace as undo_tablespace
SQL> alter system set undo_tablespace=undo_rbs1;

-- Drop the old tablespace.
SQL> drop tablespace undo_rbs0 including contents.

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