Assessment question on indexes

A certain OCA/OCP Introduction to Oracle9i SQL study guide contains the following question.

Which of the following statements could use an index on the columns PRODUCT_ID and WAREHOUSE_ID of the OE.INVENTORIES table? A. select count(distinct warehouse_id) from oe.inventories; B. select product_id, quantity_on_hand from oe.inventories where warehouse_id = 100;
C. insert into oe.inventories values (5,100,32); D. None of these statements could use the index

The following answer is provided:

1. The index contains all the information needed to satisfy the query in option A, and a full-index scan would be faster than a full-table scan. A leading subset of indexes columns is not specified in the WHERE clause of option B, and INSERT operations, as in option C, are slowed down by indexes. For more information on indexes, see Chapter 9.

Now I don't believe this is correct. I believe the correct answer is option D. The query in option A does not use a leading subset of the indexed columns (same as option B). I have run some tests that show the index is not used for option A or option B. I am running a 9.2 database on Windows.

Can anyone shed some light on this? What is the correct answer? Received on Wed Feb 09 2005 - 18:10:08 CST