Using Nonclustered Indexes

A nonclustered index is analogous to an index in a textbook. The data is stored in one place, the index in another, with pointers to the storage location of the data. The items in the index are stored in the order of the index key values, but the information in the table is stored in a different order (which can be dictated by a clustered index). If no clustered index is created on the table, the rows are not guaranteed to be in any particular order.

Similar to the way you use an index in a book, Microsoft® SQL Server™ searches for a data value by searching the nonclustered index to find the location of the data value in the table and then retrieves the data directly from that location. This makes nonclustered indexes the optimal choice for exact match queries because the index will contain entries describing the exact location in the table of the data values being searched for in the queries. If the underlying table is sorted using a clustered index, the location is the clustering key value; otherwise, the location is the row ID (RID) comprised of the file number, page number, and slot number of the row. For example, to search for an employee ID (emp_id) in a table that has a nonclustered index on the emp_id column, SQL Server looks through the index to find an entry that lists the exact page and row in the table where the matching emp_id can be found, and then goes directly to that page and row.

Multiple Nonclustered Indexes

Some books contain multiple indexes. For example, a gardening book can contain one index for the common names of plants and another index for the scientific names because these are the two most common ways in which the readers find information. The same is true for nonclustered indexes. You can define a nonclustered index for each of the columns commonly used to find the data in the table.

Considerations

Before creating nonclustered indexes, understand how your data will be accessed. Consider using nonclustered indexes for:

Columns that contain a high number of distinct values, such as a combination of last name and first name (if a clustered index is used for other columns). If there are very few distinct values, such as only 1 and 0, no index should be created.
Queries that do not return large result sets.
Columns frequently involved in search conditions of a query (WHERE clause) that return exact matches.
Decision Support System applications for which joins and grouping are frequently required. Create multiple nonclustered indexes on columns involved in join and grouping operations, and a clustered index on any foreign key columns.
Covered queries.