FIX: A query may take more time to finish in SQL Server 2005 if the query uses an inner join to join a derived table that uses the DISTINCT keyword

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Article ID: 949854 - View products that this article applies to.
Bug #: 50002496 (SQL Hotfix)
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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SYMPTOMS

Consider the following scenario. You have a query that uses an inner join. One of the tables that is included in the inner join is a derived table. The derived table uses the DISTINCT keyword. You run the query. In this scenario, the query may take more time to finish in Microsoft SQL Server 2005 than in SQL Server 2000.

Note A derived table is a SELECT statement after the FROM clause of a query.

CAUSE

This problem occurs because the query optimizer generates a bad execution plan. In SQL Server 2005, the query optimizer chooses an execution plan that contains the Top(TOP EXPRESSION:((1))) operator for the DISTINCT keyword. However, in SQL Server 2000, the execution plan for the same query uses the HASH MATCH aggregation operator for the DISTINCT keyword.

When the query optimizer optimizes a query in SQL Server 2005, the query optimizer uses the Top(TOP EXPRESSION:((1))) operator for the DISTINCT keyword to obtain the first row quickly. However, if the cardinality estimation of the rows for each operation of the Top(TOP EXPRESSION:((1))) operator is larger than the actual cardinality, SQL Server 2005 will take a long time to obtain the first row. Therefore, the query takes a long time to finish.

RESOLUTION

The fix for this issue was first released in Cumulative Update 7. For more information about how to obtain this cumulative update package for SQL Server 2005 Service Pack 2, click the following article number to view the article in the Microsoft Knowledge Base:
949095 Cumulative update package 7 for SQL Server 2005 Service Pack 2
Note Because the builds are cumulative, each new fix release contains all the hotfixes and all the security fixes that were included with the previous SQL Server 2005 fix release. Microsoft recommends that you consider applying the most recent fix release that contains this hotfix. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
937137 The SQL Server 2005 builds that were released after SQL Server 2005 Service Pack 2 was released
Microsoft SQL Server 2005 hotfixes are created for specific SQL Server service packs. You must apply a SQL Server 2005 Service Pack 2 hotfix to an installation of SQL Server 2005 Service Pack 2. By default, any hotfix that is provided in a SQL Server service pack is included in the next SQL Server service pack.

How to enable this hotfix

After you install Cumulative Update 7 for SQL Server 2005 Service Pack 2, you must turn on trace flag 4125. To do this, run the following statement in SQL Server Management Studio.
DBCC TRACEON(4125,-1)

STATUS

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

MORE INFORMATION

For more information about what files are changed, and for information about any prerequisites to apply the cumulative update package that contains the hotfix that is described in this Microsoft Knowledge Base article, click the following article number to view the article in the Microsoft Knowledge Base:
949095 Cumulative update package 7 for SQL Server 2005 Service Pack 2

Important information for SQL Server 2005 Analysis Services

A known issue exists in Cumulative Update 7 for SQL Server 2005 Analysis Services. Consider the following scenario. You install Cumulative Update 7 for SQL Server 2005 SP2. In SQL Server Profiler, you start a trace or stop a trace. Then, you run a query. The execution time of the query exceeds the value of the ForceCommitTimeout server setting. In this scenario, all the queries that are running on the server are canceled. To work around this issue, do not run MDX queries before you start a trace or stop a trace.

REFERENCES

For more information about the list of builds that are available after SQL Server Service Pack 2, click the following article number to view the article in the Microsoft Knowledge Base:
937137 The SQL Server 2005 builds that were released after SQL Server 2005 Service Pack 2 was released
For more information about the Incremental Servicing Model for SQL Server, click the following article number to view the article in the Microsoft Knowledge Base:
935897 An Incremental Servicing Model is available from the SQL Server team to deliver hotfixes for reported problems
For more information about how to obtain SQL Server 2005 Service Pack 2, 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
For more information about the new features and the improvements in SQL Server 2005 Service Pack 2, visit the following Microsoft Web site:
http://go.microsoft.com/fwlink/?LinkId=71711
For more information about the naming schema for SQL Server updates, click the following article number to view the article in the Microsoft Knowledge Base:
822499 New naming schema for Microsoft SQL Server software update packages
For more information about software update terminology, 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

Properties

Article ID: 949854 - Last Review: May 6, 2008 - Revision: 2.1
APPLIES TO
  • Microsoft SQL Server 2005 Developer Edition
  • Microsoft SQL Server 2005 Enterprise Edition
  • Microsoft SQL Server 2005 Standard Edition
  • Microsoft SQL Server 2005 Workgroup Edition
Keywords: 
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