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ORA-01307:

no LogMiner session is currently active
Cause: A select was issued from v$logmnr_contents without first invoking the dbms_logmnr.start_logmnr() procedure. Otherwise, dbms_logmnr.end_logmnr() was called without a prior call to dbms_logmnr.start_logmnr() or dbms_logmnr.add_logfile()
Action: Invoke the dbms_logmnr.start_logmnr() procedure before issuing a select from the v$logmnr_contents view.