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SQL>show parameter undo;
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS1

select * from u_power as of timestamp to_timestamp('2014-03-09 08:00:00', 'yyyy-mm-dd hh24:mi:ss');

flashback table u_power to timestamp to_timestamp ('2014-03-10 08:00:00', 'yyyy-mm-dd hh24:mi:ss');