Creating Indexes in VS .NET

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Creating Indexes
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Creating a Covering Index
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An index contains all the values contained in the column or columns that define the index. SQL Server stores this data in a sorted format on pages in a doubly linked list. So an index is essentially a miniature representation of a table. This structure can have an interesting effect on certain queries. If the query needs to return data from only columns within an index, it does not need to access the data pages of the actual table. By transiting the index, it has already located all the data it requires. For example, let s say you are using the Customer table that we created in 3 to find the names of all customers who have a credit line greater than $10,000. SQL Server would scan the table to locate all the rows with a value greater than 10,000 in the Credit Line column, which would be very inefficient. If you then created an index on the Credit Line column, SQL Server would use the index to quickly locate all the rows that matched this criterion. Then it would transit the primary key, because it is clustered, to return the customer names. However, if you created a nonclustered index that had two columns in it Credit Line and Customer Name SQL Server would not have to access the clustered index to locate the rows of data. When SQL Server used the nonclustered index to find all the rows where the credit line was greater than 10,000, it also located all the customer names. An index that SQL Server can use to satisfy a query without having to access the table is called a covering index. Even more interesting, SQL Server can use more than one index for a given query. In the preceding example, you could create nonclustered indexes on the credit line and on the customer name, which SQL Server could then use together to satisfy a query.
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Index selection
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SQL Server determines whether to use an index by examining only the first column defined in the index. For example, if you defined an index on FirstName, LastName and a query were looking for LastName, this index would not be used to satisfy the query.
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Balancing Index Maintenance
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Why wouldn t you just create dozens or hundreds of indexes on a table At first glance, knowing how useful indexes are, this approach might seem like a good idea. However, remember how an index is constructed. The values from the column that
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Lesson 3: Creating Nonclustered Indexes
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the index is created on are used to build the index. And the values within the index are also sorted. Now, let s say a new row is added to the table. Before the operation can complete, the value from this new row must be added to the correct location within the index. If you have only one index on the table, one write to the table also causes one write to the index. If there are 30 indexes on the table, one write to the table causes 30 additional writes to the indexes. It gets a little more complicated. If the leaf-level index page does not have room for the new value, SQL Server has to perform an operation called a page split. During this operation, SQL Server allocates an empty page to the index, moving half the values on the page that was filled to the new page. If this page split also causes an intermediatelevel index page to overflow, a page split occurs at that level as well. And if the new row causes the root page to overflow, SQL Server splits the root page into a new intermediate level, causing a new root page to be created. As you can see, indexes can improve query performance, but each index you create degrades performance on all data-manipulation operations. Therefore, you need to carefully balance the number of indexes for optimal operations. As a general rule of thumb, if you have five or more indexes on a table designed for online transactional processing (OLTP) operations, you probably need to reevaluate why those indexes exist. Tables designed for read operations or data warehouse types of queries generally have 10 or more indexes because you don t have to worry about the impact of write operations.
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