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MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
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Figure 7-4 Object Explorer looking at indexes in the PersonContact table
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Figure 7-5 Adding a column to the index
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7: Optimizing Databases
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6 Click Options to access the Options properties page We ll accept the defaults, but I want to point out a couple of these settings You can also view them in Figure 7-6
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Automatically Recompute Statistics is checked Store Intermediate Sort Results In tempdb is checked Set Fill Factor is not checked Use Index is checked, indicating this index will be enabled
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Figure 7-6 Index options
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7 Click OK to create the index
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Indexes can be composed of more than one column When it does use more than one column, we refer to it as a composite index We might use a composite index to increase the selectivity of the index (make the entries more unique) or to cover a query When creating a composite index, one column is referred to as the key column, and the other columns are referred to as the included columns When creating a composite index, it is best to use the less common column as the key column because statistics are only derived and maintained on this column Statistics aren t maintained on included columns We ll discuss statistics more fully later in this chapter
Covering a Query
Covering a query with an index is done to increase the performance of a specific query One index will include all the referenced columns of the covered query
MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
For example, consider a Customer table that s 800MB in size Salespeople frequently run a query that pulls a contact name (first name and last name) and phone number from the Customer table based on the CustomerID Let s say the Customer table is actually 15 columns wide However, we are referencing only four columns in the query: CustomerID, FirstName, LastName, and Phone We could create a composite index of all four columns referenced in the query Now whenever this query is run, instead of searching through the entire 800MB table, the smaller composite index (perhaps 500KB) could be searched Figure 7-7 shows the comparison of the table and the index Clearly, the query optimizer can search the smaller index for data quicker than it can search the entire table
Figure 7-7 A comparison of a full table and a covering index
Covering index 500 KB Customer table over 800 MB in size
Covering a query can provide significant performance gains for specific frequently run queries However, since both the data in the index and the data in the underlying table must be updated, there is a maintenance cost involved Only the most frequently run queries should be covered by a composite index EXAM TIP A covering index can be used to optimize the most frequently used queries Covering a query with an index involves adding all of the referenced columns in the query to the index This allows the query optimizer to look at the smaller index only and not the larger table The T-SQL statement used to include additional columns in the index is INCLUDE In this next exercise, we ll cover a query by creating a composite index using T-SQL statements The columns we ll include in the index are LastName, FirstName, and EmailAddress Since LastName is less common than FirstName and is likely to be included in the WHERE clause to narrow the search, the LastName column will be the key column Exercise 72: Cover a Query with a Composite Index Using T-SQL 1 Launch a New Query window in SSMS Press CTRL+M or select the Query menu and then choose Include Actual Execution Plan
7: Optimizing Databases
2 Enter and execute the following query This is the query we are planning to optimize by covering it with an index
USE AdventureWorks; GO SELECT LastName, FirstName, EmailAddress FROM PersonContact;
3 Select the Execution Plan tab and observe the execution plan the query optimizer used Your display should look similar to Figure 7-8
Figure 7-8 The execution plan of the e-mail address query
4 Hover over the clustered index scan icon in the plan to observe the details of this plan step Your display should look similar to Figure 7-9
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