Note
For a more detailed discussion of clustered tables versus heap tables (that is, tables with no clustered indexes) and more detailed descriptions of clustered and nonclustered index key structures and index key rows, as well as how SQL Server internally maintains indexes, see Chapter 34.
The efficiency of the index lookup and the types of lookups should drive the selection of nonclustered indexes. In the book index example, a single page reference is a very simple lookup for the book reader and requires little work. If, however, many pages are referenced in the index, and those pages are spread throughout the book, the lookup is no longer simple, and much more work is required to get all the information.
You should choose your nonclustered ...
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