When a standby database falls behind the primary database in time because of any interruption in redo transport or apply, the database can be synchronized again by applying the archived logs produced in the no-synchronization period. However, even if one of the necessary archived logfiles is not accessible, there is nothing to do for Data Guard to close the gap.
In such a case, we have to restore these archived logfiles from backups if they exist. If not, the only way to close the gap is by using an RMAN incremental backup taken from the primary database, especially to close the gap in question. We use the
BACKUP INCREMENTAL FROM SCN RMAN statement for this special-purpose backup.