* Oracle Database provides a mechanism to make table structure modifications without significantly affecting the
availability of the table. The mechanism is called online table redefinition. Redefining tables online provides a
substantial increase in availability compared to traditional methods of redefining tables.
* To redefine a table online:
Choose the redefinition method: by key or by rowid
* By key-Select a primary key or pseudo-primary key to use for the redefinition. Pseudo-primary keys are unique
keys with all component columns having NOT NULL constraints. For this method, the versions of the tables before and
after redefinition should have the same primary key columns. This is the preferred and default method of redefinition.
* By rowid-Use this method if no key is available. In this method, a hidden column named M_ROW$$ is added to the
post-redefined version of the table. It is recommended that this column be dropped or marked as unused after the
redefinition is complete. If COMPATIBLE is set to 10.2.0 or higher, the final phase of redefinition automatically sets this
column unused. You can then use the ALTER TABLE ... DROP UNUSED COLUMNS statement to drop it.
You cannot use this method on index-organized tables.
Note:
* When you rebuild an index, you use an existing index as the data source. Creating an index in this manner enables
you to change storage characteristics or move to a new tablespace. Rebuilding an index based on an existing data
source removes intra-block fragmentation. Compared to dropping the index and using the CREATE INDEX statement,
re-creating an existing index offers better performance.
Incorrect:
Not E: Edition-based redefinition enables you to upgrade the database component of an application while it is in use,
thereby minimizing or eliminating down time.

Explanation/Reference:
Explanation:
B (not C): You can validate that all database files and archived redo logs can be backed up by running a command as follows:
RMAN> BACKUP VALIDATE DATABASE ARCHIVELOG ALL;
This form of the command would check for physical corruption. To check for logical corruption, RMAN> BACKUP VALIDATE CHECK LOGICAL DATABASE ARCHIVELOG ALL; D: You can use the VALIDATE keyword of the BACKUP command to do the following:
Check datafiles for physical and logical corruption
Confirm that all database files exist and are in the correct locations.
Note:
You can use the VALIDATE option of the BACKUP command to verify that database files exist and are in the correct locations (D), and have no physical or logical corruptions that would prevent RMAN from creating backups of them. When performing a BACKUP...VALIDATE, RMAN reads the files to be backed up in their entirety, as it would during a real backup. It does not, however, actually produce any backup sets or image copies (Not A, not E).

Explanation/Reference:
Explanation:

Explanation
Schema objects are referenced with varying usage patterns; therefore, their cache behavior may be quite different. Multiple buffer pools enable you to address these differences. You can use a KEEP buffer pool to maintain objects in the buffer cache and a RECYCLE buffer pool to prevent objects from consuming unnecessary space in the cache. When an object is allocated to a cache, all blocks from that object are placed in that cache. Oracle maintains a DEFAULT buffer pool for objects that have not been assigned to one of the buffer pools.