Which two statements are true about the B-tree Index?
A.
The leaf blocks in the index are doubly linked.
B.
The leaf node stores a bitmap for each key value.
C.
Rows with a 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 the index leaf block and the
space becomes available for a new leaf entry.
Explanation:
http://www.siue.edu/~dbock/cmis565/module12-indexes.htm
A,B
A,D
http://docs.oracle.com/database/121/CNCPT/indexiot.htm#CNCPT88836
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. In Figure 3-1 the leftmost leaf block (
0-10) is linked to the second leaf block (11-19).Correct Ans: A,D
https://docs.oracle.com/database/121/CNCPT/indexiot.htm#CNCPT88836
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
How the Database Maintains Indexes
The database automatically maintains and uses indexes after they are created. The database also automatically reflects changes to data, such as adding, updating, and deleting rows, in all relevant indexes with no additional actions required by users. Retrieval performance of indexed data remains almost constant, even as rows are inserted. However, the presence of many indexes on a table degrades DML performance because the database must also update the indexes.
A,D