The following is the ORA-00393 error message, you can learn the cause and solution of the Oralce error.
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 It has not been archived so there is no other copy available.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.
Solution: Archive the log and then repeat the clear command. If archiving is not possible, and dropping the tablespaces is acceptable, then add the clause UNRECOVERABLE DATAFILE at the end of the clear command.
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