"RESTORE LOG" works even if filegroup doesn't exist?

Question

Hello, everyone!
   I’m working on a Log Shipping scenario where a filegroup  called “MyDatabaseFileStream” only exists on the Primary
database, not at the Secondary. The filegroup in question is huge, and I don’t need a copy on the Secondary. I just need a copy of the “PRIMARY” filegroup, so I use the “FILEGROUP”
when I try to restore the database from a ".bak" file (command below)…
   RESTORE DATABASE MyDatabase
   FILEGROUP = 'PRIMARY'
   FROM DISK = N'D:\MyDatabase.bak'
   WITH
      STANDBY = N'D:\ROLLBACK_UNDO_MyDatabase.BAK'
… and If the path of some file doesn't exist on the server, the restore fails:
Msg 5133, Level 16, State 1, Line 1
Directory lookup for the file "S:\ MyDatabase.MDF"
failed with the operating system error 3(The system cannot find the path specified.).
Msg 3156, Level 16, State 3, Line 1
File ' MyDatabase_Data'
cannot be restored to 'S:\ MyDatabase.MDF'. Use WITH MOVE to identify a valid location
for the file. Msg 5133, Level 16, State 1, Line 1
Directory lookup for the file "S:\ MyDatabase.LDF"
failed with the operating system error 3(The system cannot find the path specified.).
Msg 3156, Level 16, State 3, Line 1
File ' MyDatabase_Log'
cannot be restored to 'S:\ MyDatabase.LDF'. Use WITH MOVE to identify a valid location
for the file. …
   Then, I use "WITH MOVE"...
   RESTORE DATABASE MyDatabase
   FILEGROUP = 'PRIMARY'
   FROM DISK = N'D:\MyDatabase.bak'
   WITH
      STANDBY = N'D:\ROLLBACK_UNDO_MyDatabase.BAK',
      MOVE N'MyDatabase_Data' TO N'D:\MyDatabase.mdf',
      MOVE N'MyDatabase_Log' TO N'D:\MyDatabase.ldf'
... and everything goes fine. The database is restored (database in "Standby/ReadOnly") on the Secondary, without the filegroup
“MyDatabaseFileStream” (just the “PRIMARY”).
Processed 536 pages for database 'MyDatabase', file 'MyDatabase_Data' on file 1.
Processed 1 pages for database 'MyDatabase', file 'MyDatabase_Log' on file 1.
RESTORE DATABASE ... FILE=<name> successfully processed 537 pages in 0.377 seconds (11.128 MB/sec).
   And finally, the question is: when a try to restore a log (from a ".trn" file) using the command below...
RESTORE log MyDatabase
FROM Disk='D:\MyDatabase_20150220184253.trn'
WITH STANDBY = N'D:\ROLLBACK_UNDO_MyDatabase.BAK'
... and the restore succeeds. The command above does not have any filegroup specified, and the restore succeeds even when some transaction
inserts, updates or deletes data from tables based on the “MyDatabaseFileStream” filegroup.
Processed 0 pages for database ' MyDatabase',
file 'MyDatabase_Data' on file 1.
Processed 1 pages for database 'MyDatabase', file 'MyDatabase_Log' on file 1.
RESTORE LOG successfully processed 1 pages in 0.212 seconds (0.020 MB/sec).
   OK, ok… Restoring a log will restore only the log file (nothing at the “PRIMARY”, neither at the “MyDatabaseFileStream”),
but at the next log restore, the data inside
the log file is moved to the data files, correct? The “MyDatabaseFileStream” related data (inside the transaction log) goes where? I was expecting an error at some point (like when restoring the database without the “WITH
MOVE”).
   I'm not complaining. In fact, I'm glad it works. J  But,
at the same time, I'm unsure... Can I rely on this behavior? Am I missing something?

Answer 1

Thank you for your answer Erland.
   I really get an error (Msg 601, Level 12, State
3, Line 1 Could not continue scan with NOLOCK due to data movement.) when I try a select on the table that references the filegroup "MyDatabaseFileStream". And you
are right about the backup, too: if I restore the "Primary" and "MyDatabaseFileStream" filegroups and apply the log backups, I get the database "full" synchronized. So, the "MyDatabaseFileStream" related data are actually
inside the log backups, but as you said they are not applied anywhere if the filegroup is missing.
   Now I understanding better and feeling more confident about the solution for our scenario.