Explanation/Reference:
Explanation:
An index with nonkey columns can significantly improve query performance when all columns in the query
are included in the index either as key or nonkey columns. Performance gains are achieved because the
query optimizer can locate all the column values within the index; table or clustered index data is not
accessed resulting in fewer disk I/O operations.
Note: All data types except text, ntext, and image can be used as nonkey columns.
Incorrect Answers:
C: Redesign nonclustered indexes with a large index key size so that only columns used for searching and
lookups are key columns.
D: The most unique column should be the first in the index.
References: https://docs.microsoft.com/en-us/sql/t-sql/statements/create-index-transact-sql?view=sql-
server-2017