To answer this question, 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 necessarily make an index more useful. A NOT NULL constraint ensures that the column cannot have null values, but it does not directly affect the usefulness of an index.
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 make an index more useful. The FROM clause is used to specify the tables from which to retrieve data, and while indexes can be used to optimize the retrieval of data, using indexed columns in the FROM clause alone does not guarantee the usefulness of an index.
Option C) The indexed columns are part of an expression - This option is incorrect because having indexed columns as part of an expression does not necessarily make an index more useful. While indexes can be used to optimize the evaluation of expressions, having indexed columns as part of an expression alone does not guarantee the usefulness of an index.
Option D) The indexed column contains a wide range of values - This option is correct because when the indexed column contains a wide range of values, an index can be more useful. Indexes are particularly effective when the indexed column has a large number of distinct values. A wide range of values in the indexed column allows for better selectivity, meaning that the index can filter out a larger portion of the data, resulting in improved query performance.
The correct answer is D. The indexed column contains a wide range of values. This option is correct because having a wide range of values in the indexed column makes an index more useful.