Explanation/Reference:
You can use files on a standby database to recover a lost datafile.
Start RMAN and take the following steps to copy the datafiles from the standby to the primary:
1. Connect to the standby database as the target database
2. (A) Connect to the primary database as the auxiliary database
3. Back up the datafile on the standby host across the network to a location on the primary host. (This step, E, is assumed to have been taken already).
4. Exit the RMAN client
5. Start RMAN and connect to the primary database as target, and to the recovery catalog
6. (B, D) Use the CATALOG DATAFILECOPY command to catalog this datafile copy so that RMAN can use it.
CATALOG DATAFILECOPY '/disk9/df2copy.dbf';
Then use the SWITCH DATAFILE command to switch the datafile copy so that /disk9/df2copy.dbf becomes the current datafile:
RUN {
SET NEWNAME FOR DATAFILE 2 TO '/disk9/df2copy.dbf';
SWITCH DATAFILE 2;
}
References: https://docs.oracle.com/cd/E11882_01/server.112/e41134/rman.htm#SBYDB4870