The internal structure of a nonclustered index in SQL Server varies based on whether the table also has a clustered index. Tables that have a clustered index are clustered tables, and tables that don't have a clustered index are heap tables. Nonclustered indexes based on a heap table contain a row ID (RID) that lets SQL Server find a data page from the leaf level of the nonclustered index. This process generally improves performance because it dramatically reduces the overhead associated...