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  3. You are the administrator of SQL server 2000 computer. The server contains a database named MedicalRecords. Users access medical

You are the administrator of SQL server 2000 computer. The server contains a database named MedicalRecords. Users access medical

admin2009-05-19  51
问题 You are the administrator of SQL server 2000 computer. The server contains a database named MedicalRecords. Users access medical records by using the PatientID field. This field is the clustered primary key for the Patients table. When users try to access medical records, the database responds slowly. You examine the database options as shown in the exhibit.


You want to accelerate query response time and minimize administrative overhead. How should you reconfigure the database?
选项 A、Create a SQL Server Agent job to execute the UPDATE STATISTICS statement, and schedule the job to run weekly.
B、Select the Auto Update Statistics check box.
C、Run the database maintenance plan wizard, and accept the default settings.
D、Rebuild the primary key as a nonclustered primary key.
E、Clear the Auto Create Statistics check box.
答案B
解析 Explanation: The Auto Update Statistics option is selected by default but in the Exhibit it has been disabled. Apparently the statistics are out-of-date and the queries are running slowly. The Auto Update Statistics option should be enabled to increase performance and minimize administrative overhead.

Note: SQL Server keeps statistics about the distribution of the key values in each index and uses these statistics to determine which index would be the best index to use for a particular query process. As the data in a column changes, index and column statistics can become out-of-date.

This can hinder the query optimizer’s ability to make optimal decisions on how to process a query. As a result query performance will deteriorate. When statistics become out-of-date they can be updated automatically if the Auto Update Statistics check box on the table property dialog box is checked.

Incorrect Answers:
A: It would require some administrative effort to set up job that update the statistics. Just selecting the Auto Update Statistics check is a better solution.

C: The Database Maintenance Plan Wizard cannot be used to recreate the statistics, which are the main problem at the moment.

Note: The Database Maintenance Plan Wizard can be used to set up the core maintenance tasks necessary to ensure that a database performs well, is regularly backed up, and is checked for inconsistencies. The Database Maintenance Plan Wizard creates a SQL Server 2000 job that performs these maintenance tasks automatically at scheduled intervals and can be used to reorganize the data on the data and index pages by rebuilding indexes with a new fill factor; compress data files by removing empty database pages; update index statistics to ensure the query optimizer has up-todate information about the distribution of data values in the tables; perform internal consistency checks of the data and data pages within the database to ensure that a system or software problem has not damaged data; backup the database and transaction log files; and to set up log shipping.

D: Looking at the exhibit we see that this is a statistics problem, not a index problem.

Note: A nonclustered index is similar to an index in a textbook. With this type of index, the data is stored in one place and the index in another. These indexes, however, point to the storage location of the data. Nonclustered indexes should be used on columns that contain a large number of distinct values; for queries that do not return large result sets; and for columns that are frequently involved in search conditions of a query that return exact matches. A clustered index, on the other hand, determines the physical order of data in a table. The PRIMARY KEY index automatically creates a clustered index as it is used to identify a row unless a nonclustered index is specified.

E: When set the Auto Create Statistics check box is checked, statistics are automatically created on columns that are used in a predicate. Adding statistics improves query performance because the SQL Server query optimizer can better determine how to evaluate a query. If the statistics are not used, SQL Server automatically deletes them. When this check box is cleared, statistics are not automatically created.
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