Suppose you have a database running in NOARCHIVELOG mode. Disk 1 is damaged due to a media failure, and datafile #1 on this disk is lost.
There are two online log files containing sequence numbers 75 and 76, with the current log sequence 76.
The most recent backup was made at log sequence 73. Is it possible to completely recover this database? No.
If the DBA replaces datafile #1 with a backup copy made at log sequence 73, it cannot be made current with the rest of the database, which is currently at 76.
There are not enough logs available.
Online redo logs are not being archived. The current two logs, 75 and 76, at one time held sequence number 73. But each log is over-written at every log switch.
Since the redo log 73 was over-written, a whole database restore is necessary. Remember, if a database is running in
NOARCHIVELOG mode, you have to restore the whole database backup even though only one file has been corrupted or lost.