c# gtin Lesson 3: Planning Indexes in C#

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Lesson 3: Planning Indexes
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SQL Server only considers an index based on the first column defined in the index. For example, if you defined an index on FirstName, LastName, and a query was looking for the LastName, SQL Server would not use this index to satisfy the query.
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You might be wondering why someone wouldn t just create dozens or hundreds of indexes on a table. Although at first glance, it might seem to be a good thing to do, remember how an index is constructed. The values from the column on which the index is created are used to build the index. The values within the index are also sorted. Now a new row is added to the table. Before the operation can complete, SQL Server must add the value from this new row to the correct location within the index. If there is 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 addi tional 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, an empty page is allocated to the index, and one-half of the values on the page that was filled are moved to the new page. If this also causes an intermediatelevel index page to overflow, a page split occurs at that level as well. If the new row also causes the root page to overflow, the root page is split into a new intermediate level, causing a new root page to be created. Indexes can improve query performance, but each index created also causes perfor mance degradation on all data manipulation. Therefore, you need to carefully bal ance 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 transaction processing (OLTP) operations, you probably need to reevaluate why those indexes exist. Tables designed for read operations or data warehouse types of queries usually have many more indexes because little consideration needs to be taken for the impact of write operations.
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In addition to the performance degradation because of write operations, indexes are limited to a maximum row size of 900 bytes. This limitation can create a challenge when constructing more-complex covering indexes. An interesting new feature of indexes in SQL Server 2005 is called an included col umn. 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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Quick Check
1. What are the two most important things to consider for nonclustered indexes 2. What requirement does a clustered index impose on storage
Quick Check Answer
1. The number of indexes must be balanced against the overhead required to maintain them when rows are added, removed, or modified in the table. The order of the columns defined in the index need to be considered with the queries that can use them. This ensures that the first column in the index is used in the query, so the Optimizer uses the index. 2. A clustered index forces rows on data pages as well as data pages within the doubly linked list to be sorted by the clustering key.
Practice: Creating Indexes
In these exercises, you create clustered and nonclustered indexes.
Exercise 1: Creating a Clustered Index
In this exercise, you create a clustered index. The index is then disabled and reenabled. 1. Launch SSMS, connect to your instance, and open a new query window. 2. Change context to the AdventureWorks database. 3. Create a clustered index on the PostTime of the DatabaseLog table by executing the following query:
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