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Chapter 14: Indexing Strategies for Quer... > Special Index Considerations

Special Index Considerations

As always, there are some exceptions to the rules. While most indexing strategies provide support for standard optimizer behavior, there are always special cases and they can have both positive and negative impacts on performance. Being aware of these special cases allows you to take advantage of performance opportunities and avoid potential pitfalls.

In this section, we will discuss a few of these special issues. We will first address the concept of index covering and then look at logical operator processing.

Covering a Query

Covering is the process of retrieving all necessary data from an index instead of going to the data page. This behavior is specifically related to nonclustered indexes. The concept is simple. Nonclustered index leaf pages contain more rows per page than the data page does. Because your performance objective is to reduce page I/O, reading pages with more rows means that overall you will read fewer pages. Therefore, if all the data you need is on the index page, why read the data page at all?


  

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