Offline backup
If what you need to do is to perform an offline backup of one or more tablespaces, you should first notice that you cannot take the SYSTEM tablespace or a tablespace with any active UNDO segments offline. Also, when using this method always take into consideration whether the tablespace is completely self-contained before performing this type of backup, in other words, you should first check if any logical or physical dependencies between objects exist first, for example, if any index related to any table in the tablespace that will become offline is stored in a different tablespace, in such case both the tablespaces (DATA and INDEX) should be taken offline and backed up together.
Tip
Never perform offline backups of your database if it's running in the NOARCHIVELOG mode.
You can easily check whether a tablespace is self-contained using the very useful DBMS_TTS.TRANSPORT_SET_CHECK procedure (part of the DBMS_TTS package), and the TRANSPORT_SET_VIOLATIONS view as per the following...
