complete recovery of datafile using RMAN

You can perform following experiment to perform complete recovery of datafile in archivelog mode.
1. Assume database is up and running.
2. Delete datafile 4.
3. Valid backup must exist.
4. All the redo logs after the last valid backup exist.

To restore it,
1. RMAN> SQL'alter database datafile 4 offline';
2. RMAN> restore datafile 4;
3. RMAN> recover datafile 4;
4. RMAN>SQL'alter database datafile 4 online';

If an active redo log file is lost, then incomplete recovery of a datafile is not possible . Instead we have to perform Tablespace point in time recovery.