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 (920346)

SYMPTOMS

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.

RESOLUTION

Hotfix information

A supported hotfix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Only apply it to systems that are experiencing this specific problem. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next SQL Server 2005 service pack that contains this hotfix.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the hotfix. For a complete list of Microsoft Product Support Services telephone numbers and information about support costs, visit the following Microsoft Web site:

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

Note In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

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 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

File name	File version	File size	Date	Time	Platform
Microsoft.sqlserver.sqlenum	9.0.2164.0	908,064	15-Jun-2006	03:32	Not Applicable
Ms.ss.mgdsqldumper.dll	2005.90.2164.0	75,552	15-Jun-2006	03:29	Not Applicable
Msgprox.dll	2005.90.2164.0	197,920	15-Jun-2006	03:29	Not Applicable
Replprov.dll	2005.90.2164.0	547,616	15-Jun-2006	03:31	Not Applicable
Replrec.dll	2005.90.2164.0	782,112	15-Jun-2006	03:32	Not Applicable
Sbmsmdlocal.dll	9.0.2164.0	15,661,856	15-Jun-2006	03:33	Not Applicable
Sqlaccess.dll	2005.90.2164.0	347,936	15-Jun-2006	03:31	Not Applicable
Sqlservr.exe	2005.90.2164.0	28,950,872	15-Jun-2006	03:33	Not Applicable

SQL Server 2005 hotfix, 64-bit

File name	File version	File size	Date	Time	Platform
Microsoft.sqlserver.sqlenum	9.0.2164.0	875,296	15-Jun-2006	04:12	Not Applicable
Ms.ss.mgdsqldumper.dll	2005.90.2164.0	75,552	15-Jun-2006	03:29	Not Applicable
Ms.ss.mgdsqldumper.dll	2005.90.2164.0	91,424	15-Jun-2006	04:10	Not Applicable
Msgprox.dll	2005.90.2164.0	259,360	15-Jun-2006	04:10	Not Applicable
Replprov.dll	2005.90.2164.0	745,248	15-Jun-2006	04:12	Not Applicable
Replrec.dll	2005.90.2164.0	1,008,416	15-Jun-2006	04:12	Not Applicable
Sbmsmdlocal.dll	9.0.2164.0	15,661,856	15-Jun-2006	03:33	Not Applicable
Sqlaccess.dll	2005.90.2164.0	355,104	15-Jun-2006	04:11	Not Applicable
Sqlservr.exe	2005.90.2164.0	39,251,232	15-Jun-2006	04:13	Not Applicable

SQL Server 2005 hotfix, 64-bit for Itanium processors

File name	File version	File size	Date	Time	Platform
Microsoft.sqlserver.sqlenum	9.0.2164.0	875,296	15-Jun-2006	03:29	Not Applicable
Ms.ss.mgdsqldumper.dll	2005.90.2164.0	75,552	15-Jun-2006	03:29	Not Applicable
Ms.ss.mgdsqldumper.dll	2005.90.2164.0	163,104	15-Jun-2006	03:29	Not Applicable
Msgprox.dll	2005.90.2164.0	542,496	15-Jun-2006	03:28	Not Applicable
Replprov.dll	2005.90.2164.0	1,617,184	15-Jun-2006	03:29	Not Applicable
Replrec.dll	2005.90.2164.0	2,141,472	15-Jun-2006	03:29	Not Applicable
Sbmsmdlocal.dll	9.0.2164.0	48,733,984	15-Jun-2006	03:30	Not Applicable
Sqlaccess.dll	2005.90.2164.0	349,472	15-Jun-2006	03:28	Not Applicable
Sqlservr.exe	2005.90.2164.0	72,208,672	15-Jun-2006	03:30	Not Applicable