This feature not only allows you to open your database without datafile foo...
SQL> startupBut what's really important about this is that offline drop doesn't really drops anything (it just updates the controlfile to say that file isn't there) and what you can do later is:
ORACLE instance started.
Total System Global Area 700448768 bytes
Fixed Size 1260844 bytes
Variable Size 310379220 bytes
Database Buffers 385875968 bytes
Redo Buffers 2932736 bytes
ORA-01157: cannot identify/lock data file 4 - see DBWR trace file
ORA-01110: data file 4: '/u01/oradata/XE/datafile/foo.dbf'
SQL> alter database datafile 4 offline drop;
SQL> alter database open;
[oracle@srm oradata]$ rmanIn other words, it can be used to facilitate your database restore in certain cases. Imagine that your database has transactional and reporting data and that that data is spread across different tablespaces. In case your transactional data volume is small compared to reporting stuff (which is usually the case), you can plan your restore like this:
Recovery Manager: Release 10.2.0.1.0 - Production on Tue Dec 30 19:12:22 2008
Copyright (c) 1982, 2005, Oracle. All rights reserved.
RMAN> connect target;
connected to target database: XE (DBID=2555430687)
RMAN> restore tablespace foo;
Starting restore at 30-DEC-08
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=73 devtype=DISK
channel ORA_DISK_1: starting datafile backupset restore
channel ORA_DISK_1: specifying datafile(s) to restore from backup set
restoring datafile 00004 to /u01/oradata/XE/datafile/foo.dbf
channel ORA_DISK_1: reading from backup piece /u01/oradata/fra/XE/backupset/2008_12_30/o1_mf_nnnd0_TAG20081230T190854_4oog0pt0_.bkp
channel ORA_DISK_1: restored backup piece 1
piece handle=/u01/oradata/fra/XE/backupset/2008_12_30/o1_mf_nnnd0_TAG20081230T190854_4oog0pt0_.bkp tag=TAG20081230T190854
channel ORA_DISK_1: restore complete, elapsed time: 00:00:02
Finished restore at 30-DEC-08
RMAN> recover tablespace foo;
Starting recover at 30-DEC-08
using channel ORA_DISK_1
starting media recovery
media recovery complete, elapsed time: 00:00:01
Finished recover at 30-DEC-08
RMAN> sql 'alter tablespace foo online';
sql statement: alter tablespace foo online
Depending on the data volumes, the first step might take only a fraction of time compared to restoring everything in one shot.