Microsoft® SQL Server 2008 R2 Unleashed

Included Columns

A feature available for nonclustered indexes in SQL Server 2008 is included columns. Included columns allow you to add nonkey columns to the leaf level of a nonclustered index for the purpose of index covering.

One advantage of included columns is that because the nonkey columns are stored only in the leaf level of the index, the nonleaf rows of the index are smaller, which helps reduce the overall size of the index, thereby helping reduce the I/O cost of using the index. Another advantage is that this feature allows you to exceed the SQL Server maximum limits of 16 index key columns and 900-byte index key size. The included nonkey columns are not factored in when calculating the number of index key columns or index key size. ...