Automatic Statistics Generation in Visual Studio .NET

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Automatic Statistics Generation
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When a DBA creates an index, the query optimizer stores statistical information about the indexed columns. Additionally, if the AUTO_CREATE_STATISTICS database option is set to ON, the database engine creates statistics on columns that are not contained in indexes but that are used in query predicates.
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Using Transact-SQL to Manage Databases
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An additional benefit to having the AUTO_UPDATE_STATISTICS database option set to ON is that the query optimizer also automatically updates statistical information periodically as the data in the tables changes. This statistics update operation is initiated whenever the statistics used in a query execution plan fail a test for current statistics. This test is a random sampling across data pages taken either from the table or the smallest nonclustered index on the columns needed by the statistics. Almost always, statistical information is updated when approximately 20 percent of the data rows have changed; however, the query optimizer ensures that a minimum number of rows are sampled, with tables smaller than 8 MB being fully scanned to gather statistics. This test is important because when data in a column changes, index and column statistics can become out of date. As a result, the query optimizer might make less-thanoptimal decisions about how to process a query, which causes those queries to execute with dramatically substandard performance.
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Manual Statistics Generation
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You can also manually create statistics. To create statistics on all eligible columns in all user tables in the current database by using just one statement, you can execute the sp_createstats system stored procedure. To create statistics on specific table or view columns, you can use the CREATE STATISTICS statement. To manually update statistics, you can execute the UPDATE STATISTICS statement or execute the sp_updatestats system stored procedure. And you can drop statistics by using the DROP STATISTICS statement. A key benefit of creating statistics manually is that you can create statistics that contain densities of values for a combination of columns. By having statistics for a combination of columns, the database engine could make a better estimate for query execution.
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Viewing Column Statistics Information
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SQL Server 2005 gives DBAs several ways to obtain information about column statistics:
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The sp_autostats system stored procedure displays or changes the automatic UPDATE STATISTICS setting for a specific index and statistics or for all indexes and statistics for a specified table or indexed view in the current database. The sys.stats catalog view displays a row for each statistic of a tabular object of the type U, V, or TF.
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Lesson 2: Managing Statistics
The sys.stats_columns catalog view displays a row for each column that is part of sys.stats statistics. The STATS_DATE function returns the date that the statistics for the specified index were last updated. The DBCC SHOW_STATISTICS statement displays the current distribution statistics for the specified target on the specified table.
Quick Check
Why are statistics important to query performance During its evaluation of a query, the query optimizer uses the statistical information to estimate the cost of using an index and determine the optimal query plan for a query.
Quick Check Answer
PRACTICE
Manually Creating and Updating Statistics
The following two practices walk you through the process of manually creating and updating statistics.
Practice 1: Create Statistics
In this practice, you create statistics by using the CREATE STATISTICS statement. 1. If necessary, start SSMS and connect to the instance containing the AdventureWorks sample database. Open the Query Editor pane. 2. In the Query Editor pane, type the following Transact-SQL statement to view which columns in the HumanResources.Employee table do not have statistics built on them:
USE AdventureWorks; --Determine which columns do not have statistics on them. SELECT c.name FROM sys.columns c LEFT OUTER JOIN sys.stats_columns sc ON sc.[object_id] = c.[object_id] AND sc.column_id = c.column_id WHERE c.[object_id] = OBJECT_ID( HumanResources.Employee ) AND sc.column_id IS NULL ORDER BY c.column_id
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