Microsoft KB Archive/920206

= FIX: System performance may be slow when an application submits many queries against a SQL Server 2005 database that uses simple parameterization =

Article ID: 920206

Article Last Modified on 11/20/2007

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APPLIES TO


 * Microsoft SQL Server 2005 Standard Edition
 * Microsoft SQL Server 2005 Standard X64 Edition
 * Microsoft SQL Server 2005 Standard Edition for Itanium-based Systems
 * Microsoft SQL Server 2005 Enterprise Edition
 * Microsoft SQL Server 2005 Enterprise X64 Edition
 * Microsoft SQL Server 2005 Enterprise Edition for Itanium-based Systems
 * Microsoft SQL Server 2005 Developer Edition

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Microsoft distributes Microsoft SQL Server 2005 fixes as one downloadable file. Because the fixes are cumulative, each new release contains all the hotfixes and all the security fixes that were included with the previous SQL Server 2005 fix release.



SUMMARY
This article describes the following about this hotfix release:
 * The issues that are fixed by this hotfix package
 * The prerequisites for installing the hotfix package
 * Whether you must restart the computer after you install the hotfix package
 * Whether the hotfix package is replaced by any other hotfix package
 * Whether you must make any registry changes
 * The files that are contained in the hotfix package



SYMPTOMS
Consider the following scenario: An application submits many queries against a Microsoft SQL Server 2005 database. The queries have identical query text and declare the same parameters. However, the queries also have different combinations of precision and scale for the values of the numeric data type in the parameter declaration string. The SQL Server 2005 database uses simple parameterization. In this scenario, SQL Server 2005 system performance may be slow. Additionally, you may experience high CPU usage.

This problem occurs more frequently when the SQL Server 2005 database is large and is in a high-stress scenario.

Note The default behavior of a SQL Server 2005 database is to use simple parameterization.



CAUSE
This problem occurs because SQL Server must examine many queries to find the one query that is a match to the query text, the parameters, and the parameter declaration string.



Hotfix information
A supported hotfix is now available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next service pack that contains this hotfix.

To resolve this problem, submit a request to Microsoft Online Customer Services to obtain the hotfix. To submit an online request to obtain the hotfix, visit the following Microsoft Web site:

http://go.microsoft.com/?linkid=6294451

Note If additional issues occur or any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. To create a separate service request, visit the following Microsoft Web site:

http://support.microsoft.com/contactus/?ws=support

Prerequisites
To apply this hotfix, you must have SQL Server 2005 Service Pack 1 (SP1) installed on the computer.

For more information about how to obtain SQL Server 2005 Service Pack 1, click the following article number to view the article in the Microsoft Knowledge Base:

913089 How to obtain the latest service pack for SQL Server 2005

Restart information
You do not have to restart the computer after you apply this hotfix.

Registry information
You do not have to change the registry.

Hotfix replacement information
This hotfix does not replace any other hotfixes.

Hotfix file information
This hotfix contains only those files that are required to correct the issues that this article lists. This hotfix may not contain of all the files that you must have to fully update a product to the latest build.

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

SQL Server 2005 hotfix, 32-bit

SQL Server 2005 hotfix, 64-bit

SQL Server 2005 hotfix, 64-bit for Itanium processors



STATUS
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the &quot;Applies to&quot; section.



WORKAROUND
To work around this problem, use explicit parameterization in the client application. When you use explicit parameterization in the client application, only one execution plan is created and stored in the procedure cache. In this workaround, SQL Server 2005 system performance runs as expected.

For more information about explicit parameterization and execution plan reuse, visit the following Microsoft Developer Network (MSDN) Web site:

http://msdn2.microsoft.com/en-us/library/ms175580.aspx



MORE INFORMATION
When an application submits many queries against a SQL Server 2005 database, the execution plan for each query must be present in the procedure cache before the query can be executed.

Note If the execution plan for a specific query is not present in the procedure cache, SQL Server compiles the query, writes the execution plan for the query to the procedure cache, and then executes the query.

When SQL Server 2005 retrieves a cached execution plan for a specific query from the procedure cache, SQL Server identifies the corresponding execution plan by using a hash key that was derived when the query was compiled. The original release version of SQL Server 2005 derives hash keys from the query text only and skips the parameter declaration string. When SQL Server 2005 derives hash keys from the query text only, SQL Server 2005 puts the hash keys for all queries that have identical query text and the same parameters in the same hash bucket. This behavior occurs even if each query has a different combination of precision and scale for the numeric values in the parameter declaration string. When SQL Server 2005 tries to retrieve a cached query execution plan from the procedure cache for a specific query, SQL Server must examine all the query execution plans in the matching hash bucket to find the one query execution plan that matches the query text, the parameters, and the parameter declaration string.

After you apply this hotfix, SQL Server 2005 derives hash keys from the query text and the parameter declaration string. When a new query has a combination of precision and scale for the numeric values in the parameter declaration string that does not match another existing query, SQL Server 2005 puts the hash key for the new query in a new hash bucket. SQL Server 2005 puts the hash key for the new query in a new hash bucket even when the new query has identical query text and the same parameters as another existing query. Therefore, when SQL Server 2005 tries to retrieve the cached execution plans from the procedure cache for the new query, the matching hash bucket contains only one query for SQL Server to examine.

In this hotfix resolution scenario, SQL Server 2005 still has many execution plans. This occurs because a separate execution plan is created for each query that has a new combination of precision and scale for the numeric values in the parameter declaration string. Each execution plan requires SQL Server 2005 system resources to create the execution plan. Each execution plan requires SQL Server 2005 system resources to store the execution plan in the procedure cache. Because the hotfix resolution scenario requires lots of SQL Server 2005 system resources, SQL Server 2005 system performance may still be slower than expected.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

824684 Description of the standard terminology that is used to describe Microsoft software updates

Keywords: kbtshoot kbbug kbfix kbqfe kbpubtypekc kbhotfixserver KB920206

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