num_db_backups parameter (db level) controls the number of database backups to be retained for a database.This parameter specifies the number of backups that should be retained in the recovery history file.Once this number is reached , older backups are marked as expired in history file.The next database backup would prune these expired entries from the recovery history file.It should be noted that the actual backup images are not removed and it is only the entries in the recovery history file that are removed.
Entries in recovery file can have different types of status:Active,Inactive,Expired
Example for Active entry:
Start Time: 20071218191004 End Time: 20071218191016 Status: A
Active-->Using a active backup and current logs the database can be restored to current state
Inactive-->Restoring from inactive will take the db to a previous state than the current state
Expired-->It means that the backup is no longer needed as more recent backups are available
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