Using Included Columns in Visual Studio .NET

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Using Included Columns
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In addition to considering the performance degradation caused by write operation, keep in mind that indexes are limited to a maximum of 900 bytes. This limit can create a challenge in constructing more complex covering indexes. An interesting new indexing feature in SQL Server 2005 called included columns helps you deal with this challenge. Included columns become part of the index at the leaf level only. Values from included columns do not appear in the root or intermediate levels of an index and do not count against the 900-byte limit for an index.
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What are the two most important things to consider for nonclustered indexes The number of indexes must be balanced against the overhead required to maintain them when rows are added, removed, or modified in the table. You need to make sure that the order of the columns defined in the index match what the queries need, ensuring that the first column in the index is used in the query so that the query optimizer will use the index.
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Create Nonclustered Indexes
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In this practice, you will add a nonclustered index to the tables that you created in 3. 1. If necessary, launch SSMS, connect to your instance, and open a new query window. 2. Because users commonly search for a customer by city, add a nonclustered index to the CustomerAddress table on the City column, as follows:
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CREATE NONCLUSTERED INDEX idx_CustomerAddress_City ON dbo.CustomerAddress(City);
Lesson Summary
You can create up to 249 nonclustered indexes on a table. The number of indexes you create must be balanced against the overhead incurred when data is modified. An important factor to consider when creating indexes is whether an index can be used to satisfy a query in its entirety, thereby saving additional reads from either the clustered index or data pages in the table. Such an index is called a covering index. SQL Server 2005 s new included columns indexing feature enables you to add values to the leaf level of an index only so that you can create more complex index implementations within the index size limit.
Lesson 3: Creating Nonclustered Indexes
Lesson Review
The following questions are intended to reinforce key information presented in this lesson. The questions are also available on the companion CD if you prefer to review them in electronic form.
NOTE
Answers
Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
1. Which index option causes an index to be created with empty space on the intermediate levels of the index A. PAD_INDEX B. FILLFACTOR C. MAXDOP D. IGNORE_DUP_KEY
4 Review
Review
To further practice and reinforce the skills you learned in this chapter, you can
Review the chapter summary. Review the list of key terms introduced in this chapter. Complete the case scenario. This scenario sets up a real-world situation involving the topics of this chapter and asks you to create a solution. Complete the suggested practices. Take a practice test.
Summary
Indexes on SQL Server tables, just like indexes on books, provide a way to quickly access the data you are looking for even in very large tables. Clustered indexes cause rows to be sorted according to the clustering key. In general, every table should have a clustered index. And you can have only one clustered index per table, usually built on the primary key. Nonclustered indexes do not sort rows in a table, and you can create up to 249 per table to help quickly satisfy the most common queries. By constructing covering indexes, you can satisfy queries without needing to access the underlying table.
Key Terms
Do you know what these key terms mean You can check your answers by looking up the terms in the glossary at the end of the book.
B-tree clustered index clustering key covering index intermediate level leaf level nonclustered index online index creation
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