Starting with Oracle Database 11g Release 1, the default value of the UNDO_MANAGEMENT parameter is AUTO so that automatic undo management is enabled by default. You must set the parameter to MANUAL to turn off automatic undo management, required. In Oracle Database 10g and Oracle9i , the UNDO_MANAGEMENT parameter is set to MANUAL by default. A PL/SQL procedure DBMS_UNDO_ADV.RBU_MIGRATION is provided to help properly size the Undo tablespace for each individual environment. This feature facilitates seamless migration to AUM from databases being upgraded to the new release so that they can start taking advantage of all the benefits of AUM.
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