Creating Indexes in VS .NET

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Creating Indexes
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contains the initial entry for the second half of the leaf pages. The root page now contains two rows, corresponding to the initial value for each of the two intermediatelevel pages. The next time SQL Server would have to introduce another intermediate level would occur when the 2,406,105th row of data is added to the table. As you can see, this type of structure allows SQL Server to very quickly locate the rows that satisfy queries, even in extremely large tables. In this example, finding a row in a table that has nearly 2.5 million rows requires SQL Server to scan only three pages of data. And the table could grow to more than 300 million rows before SQL Server would have to read four pages to find any row. Keep in mind that this example uses a char(60) column. If you created the index on an int column requiring 4 bytes of storage, SQL Server would have to read just one page to locate a row until the 2,016th row was entered. You could add a little more than 4 million rows to the table and still need to read only two pages to find a row. It would take more than 8 billion rows in the table before SQL Server would need to read three pages to find the data you were looking for.
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What structure guarantees that every search performed will always transit the same number of levels in the index and the same number of pages in the index to locate the piece of data you are interested in The B-tree structure that SQL Server uses to build its indexes.
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A SQL Server index is constructed as a B-tree, which enables SQL Server to search very large volumes of data without affecting the performance from one query to the next. The B-tree structure delivers this performance stability by ensuring that each search will have to transit exactly the same number of pages in the index, regardless of the value being searched on.
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Lesson 1: Understanding Index Structure
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At the same time, the B-tree structure results in very rapid data retrieval by enabling large segments of a table to be excluded based on the page traversal in the index.
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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 levels of the index can have multiple pages (Choose all that apply.) A. Root B. Intermediate C. Leaf D. B-tree
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Creating Indexes
Lesson 2: Creating Clustered Indexes
The first type of index you should create on a table is a clustered index. As a general rule of thumb, every table should have a clustered index. And each table can have only one clustered index. In this lesson, you will see how to create a clustered index by using the CREATE INDEX Transact-SQL command, including which options you can specify for the command. You will also learn how to disable and then reenable a clustered index.
After this lesson, you will be able to:
Implement clustered indexes. Disable and reenable an index.
Estimated lesson time: 20 minutes
Implementing Clustered Indexes
The columns you define for a clustered index are called the clustering key. A clustered index causes SQL Server to order the data in the table according to the clustering key. Because a table can be sorted only one way, you can create only one clustered index on a table. In addition, the leaf level of a clustered index is the actual data within the table. So when the leaf level of a clustered index is reached, SQL Server does not have to use a pointer to access the actual data in the table because it has already reached the actual data pages in the table.
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