FIX: SQL Server 2005 may overestimate the cardinality of the JOIN operator when a SQL Server 2005 query contains a join predicate that is a multicolumn predicate

This article describes the following about this hotfix release:

• The issues that are fixed by the 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

Consider the following scenario:

• A Microsoft SQL Server 2005 query contains a join predicate that is a multicolumn predicate. For example, the query resembles the following.

  FROM t1 JOIN t2 ON t1.a = t2.a AND t1.b = t2.b

• The columns that are used in the join predicate have multicolumn statistics or a multicolumn index.
• One of the columns that is used in the join predicate has a skewed data distribution. The skewed data distribution shows that a specific value in the table occurs much more frequently than the other values in the table.
• The columns that are used in the join predicate for one of the two tables make up a unique key or an almost-unique key.

In this scenario, SQL Server 2005 may overestimate the cardinality of the JOIN operator.

When additional join predicates are involved, this problem may increase the estimated cost of the JOIN operator to the point where the query optimizer chooses a different join order. When the query optimizer chooses a different join order, SQL Server 2005 system performance may be slow.

Hotfix information

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, contact Microsoft Customer Service and Support to obtain the hotfix.

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. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays 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

• SQL Server 2005 Service Pack 1 (SP1)
  
  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 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 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

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

To enable this hotfix, you must turn on trace flag 4104 after you apply this hotfix.

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