FIX: A partitioned table may generate an inefficient query plan when you run a Transact-SQL query that uses the Top n clause on a partitioned table in SQL Server 2005
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.
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SUMMARY
This article describes the following about this hotfix
release:
•
The issues that are fixed by this hotfix package
•
The prerequisites for applying the hotfix
package
•
Whether you must restart the computer after you apply the
hotfix package
•
Whether the hotfix package is replaced by any other hotfix
package
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Whether you must make any registry changes after you apply the
hotfix package
•
The files that are contained in the hotfix
package
When you run a Transact-SQL query that uses the Top n clause on a partitioned table in Microsoft SQL Server 2005, the partitioned table may generate an inefficient query plan. The plan may perform a clustered index scan instead of performing a more efficient non-clustered index seek. For example, you experience this problem if the following conditions are true:
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You run a query against a tblTest table. The query may resemble the following:
SELECT top 1 * FROM tblTest WHERE OrderID = 'ABCDXYZ' ORDER BY OrderDateTime DESC
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The tblTest table is partitioned on the OrderDateTime column.
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A clustered index is created on the OrderDateTime column.
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A non-clustered index is created on the OrderID column.
Notes
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A clustered index scan is still performed without the use of the ORDER BY clause.
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When the table and the indexes are not partitioned, a non-clustered index seek plan is generated.
A supported hotfix is 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 software update that contains this hotfix.
If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, 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:
Note If additional issues occur or if 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:
Note The "Hotfix download available" section and the online request forms display the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.
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 (http://support.microsoft.com/kb/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 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, 32-bit x86 versions
For the query in the "Symptoms" section, a query plan that uses a partitioned table may resemble the following:
SELECT top 1 * FROM tblTest WHERE ORDERID = 'AE5PL-14' ORDER BY OrderDateTime DESC
|--Top(TOP EXPRESSION:((1)))
|--Nested Loops(Inner Join, OUTER REFERENCES:([PtnIds1004]) PARTITION ID:([PtnIds1004]))
|--Constant Scan(VALUES:(((32)),((31)),((30)),((29)),((28)),((27)),((26)),((25)),((24)),((23)),((22)),((21)),((20)),((19)),((18)),((17)),((16)),
((15)),((14)),((13)),((12)),((11)),((10)),((9)),((8)),((7)),((6)),((5)),((4)),((3)),((2)),((1))))
|--Clustered Index Scan(OBJECT:([part2].[dbo].[tblTest].[IX_tblTest_RT]), WHERE:([part2].[dbo].[tblTest].[OrderID]='AE5PL-14')
ORDERED BACKWARD PARTITION ID:([PtnIds1004]))
A query plan that uses a non-partitioned table may resemble the following:
SELECT top 1 * FROM tblTest WHERE ORDERID = 'AE5PL-14' ORDER BY OrderDateTime DESC
|--Top(TOP EXPRESSION:((1)))
|--Nested Loops(Inner Join, OUTER REFERENCES:([Uniq1002], [nonpart2].[dbo].[tblTest].[OrderDateTime], [Expr1004]) WITH
ORDERED PREFETCH)
|--Index Seek(OBJECT:([nonpart2].[dbo].[tblTest].[IX_tblTest]), SEEK:([nonpart2].[dbo].[tblTest].[OrderID]='AE5PL-14') ORDERED
BACKWARD)
|--Clustered Index Seek(OBJECT:([nonpart2].[dbo].[tblTest].[IX_tblTest_RT]), SEEK:([nonpart2].[dbo].[tblTest].[OrderDateTime]=
[nonpart2].[dbo].[tblTest].[OrderDateTime] AND [Uniq1002]=[Uniq1002]) LOOKUP ORDERED FORWARD)
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
824684 (http://support.microsoft.com/kb/824684/)
Description of the standard terminology that is used to describe Microsoft software updates
For more information about a related issue, click the following article number to view the article in the Microsoft Knowledge Base:
923849 (http://support.microsoft.com/kb/923849/)
FIX: When you run a query that references a partitioned table in SQL Server 2005, query performance may decrease
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