ORA-00393: log string of thread string is needed for recovery of offline ...

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ORA-00393: log string  of thread  string  is needed for recovery  of offline datafiles
Cause: Log cannot be cleared because the redo in it is needed to recover offline
datafiles. It has not been archived so there  is no other copy available. If the log is
cleared the tablespaces containing the files will have to be dropped.
Action:   Archive the log then repeat the clear command. If archiving is not
possible, and dropping the tablespaces is acceptible, then add the clause
UNRECOVERABLE DATAFILE at the end of the clear command.