To answer this question, we need to understand the purpose and functionality of an index in a database.
An index is a data structure that improves the speed of data retrieval operations on a database table. It allows the database system to locate and access specific rows more quickly, based on the values stored in one or more columns.
Let's go through each option to understand why it is correct or incorrect:
Option A) The indexed column is declared as NOT NULL.
This option is incorrect because declaring a column as NOT NULL does not directly affect the usefulness of an index. The purpose of an index is to improve data retrieval speed, regardless of whether the indexed column allows NULL values or not.
Option B) The indexed columns are used in the FROM clause.
This option is incorrect because using indexed columns in the FROM clause does not necessarily determine the usefulness of an index. The FROM clause is used to specify the tables from which data is retrieved, and an index is not specifically tied to this clause.
Option C) The indexed columns are part of an expression.
This option is incorrect because using indexed columns as part of an expression does not directly determine the usefulness of an index. While an index can still be used in certain cases when columns are part of an expression, it is not the primary factor that determines the usefulness of an index.
Option D) The indexed column contains a wide range of values.
This option is correct because an index is most useful when the indexed column contains a wide range of values. When a column has a wide range of values, an index allows for faster data retrieval by organizing the values in a way that makes it easier to search and locate specific rows.
The correct answer is Option D. This option is correct because having a wide range of values in the indexed column makes the index more useful in improving the speed of data retrieval operations.