Q20：which three statements about rebuilding indexes are true (choose three)
a. the alter index rebuild command is used to change the storage characteristics of an index
b. using the alter index rebuild is usually faster than dropping and recreating an index because it uses the fast full scan feature;
c. oracle allows for the creation of an index or re-creation of an existing index while allowing concurrent operations on the base table
d. when building an index, the nologging and unrecoverable keywords can be used concurrently to reduce the time it takes to rebuild
a/b/c是对的，我怎么看着D也对呢？
RECOVERABLE or UNRECOVERABLE (RECOVERABLE is the default)
– UNRECOVERABLE is faster because it does not write redo log entries when
the index is created or rebuilt.
– This clause does not set a permanent attribute of the object, but is only
effective when the index is created; thus, it does not appear in the data
dictionary.
– This attribute cannot be updated.
– This attribute can only be used when the object is created or rebuilt.
– This attribute implies LOGGING by default, which means that additional
inserts are logged.
– The index is not recoverable if it needs to be re-created during a recovery
operation.
• LOGGING or NOLOGGING
– NOLOGGING is faster because it does not write any redo log entries during
the index life until NOLOGGING is changed to LOGGING.
– This attribute is permanent and thus appears in the dictionary.
– This attribute can be updated (ALTER INDEX NOLOGGING/LOGGING) at
any time.
UNRECOVERABLE and LOGGING are not compatible.
Note: The UNRECOVERABLE option can be used in early versions of Oracle8, but
will eventually be replaced by the NOLOGGING option.
To duplicate the semantics of the UNRECOVERABLE clause, create an object with
the NOLOGGING option and then use the ALTER command, specifying LOGGING.
To duplicate the semantics of a RECOVERABLE clause, create an object with the
LOGGING option.
