Software Architect's Handbook

Secondary/non-clustered indexes

In addition to specifying a primary index, many database systems provide the ability to create secondary indexes, also known as non-clustered indexes. The performance of a database can benefit from having secondary keys available for data access.

Non-clustered indexes are defined by one or more columns that are ordered logically and serve as pointers to find the rest of the data for a given record. The order of a non-clustered index does not match the physical order of how the records are stored on disk.

Non-clustered indexes provide a way to specify an alternate key other than the primary key for accessing records in a table. The key could be a foreign key or any column that will be frequently used in joins, ...