Which two statements are true regarding B-tree index? (Choose two.)
A.
The leaf blocks in the index are doubly linked.
B.
The leaf node stores a bitmap for each key value.
C.
The rows with NULL value in key columns also have entries in the index.
D.
The deletion of a row from the table causes a logical deletion in index leaf block and the
space becomes available for the new leaf entry.
source: http://docs.oracle.com/cd/E11882_01/server.112/e40540/indexiot.htm#CNCPT1170
Branch Blocks and Leaf Blocks
A B-tree index has two types of blocks: branch blocks for searching and leaf blocks that store values. The upper-level branch blocks of a B-tree index contain index data that points to lower-level index blocks.
A B-tree index is balanced because all leaf blocks automatically stay at the same depth. Thus, retrieval of any record from anywhere in the index takes approximately the same amount of time. The height of the index is the number of blocks required to go from the root block to a leaf block. The branch level is the height minus 1.
Branch blocks store the minimum key prefix needed to make a branching decision between two keys. This technique enables the database to fit as much data as possible on each branch block. The branch blocks contain a pointer to the child block containing the key. The number of keys and pointers is limited by the block size.
The leaf blocks contain every indexed data value and a corresponding rowid used to locate the actual row. Each entry is sorted by (key, rowid). Within a leaf block, a key and rowid is linked to its left and right sibling entries. The leaf blocks themselves are also doubly linked.