BUG: Wrong Index Chosen When Query References Values Beyond Statistics

ID: Q234883

The information in this article applies to:

Microsoft SQL Server version 7.0

BUG #: 55366 (SQLBUG_70)

SYMPTOMS

The optimizer may select the wrong plan for processing a query if the values in a search argument are not contained within the range of statistics for an expected index. This can cause significantly slower performance in queries.

This situation is usually encountered when nonclustered index growth is extending the index in a particular direction, in contrast to being randomly distributed throughout the values for the index. For a query that wants to use the nonclustered index in the outlying range, the clustered index may be improperly selected over the nonclustered index since the latter inserted values are not contained in the statistics for the nonclustered index (that is, the optimizer has no information that the outlying values exist).

RESOLUTION

A supported fix that corrects this problem is now available from Microsoft, but it has not been fully regression tested and should be applied only to systems experiencing this specific problem. If you are not severely affected by this specific problem, Microsoft recommends that you wait for the next SQL Server Service Pack that contains this fix.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:

http://www.microsoft.com/support/supportnet/overview/overview.asp

The English version of this fix should have the following file attributes or later:



   Version    Size      File name     Platform
   -------------------------------------------

   7.00.706   9578933   S70706I.EXE   (x86)
   7.00.706   15957581  S70706A.EXE   Alpha

NOTE: Due to file dependencies, the most recent hotfix or feature that contains the preceding files may also contain additional files.

NOTE: If this product was already installed on your computer when you purchased it from the Original Equipment Manufacturer (OEM) and you need this fix, please call the Pay Per Incident number listed on the above Web site. If you contact Microsoft to obtain this fix, and if it is determined that you only require the fix you requested, no fee will be charged. However, if you request additional technical support, and if your no-charge technical support period has expired, or if you are not eligible for standard no-charge technical support, you may be charged a non-refundable fee.

For more information about eligibility for no-charge technical support, see the following article in the Microsoft Knowledge Base:

Q154871 Determining If You Are Eligible for No-Charge Technical Support

WORKAROUND

Here are two workarounds:

Run UPDATE STATISTICS and execute sp_recompile on the affected table to alleviate the problem.

Consider making the extending index the clustered index.

STATUS

Microsoft has confirmed this to be a problem in SQL Server version 7.0.

Additional query words:


Keywords          : 
Version           : winnt:7.0
Platform          : winnt 
Issue type        : kbbug

Last Reviewed: July 16, 1999