FIX: Sampling Inconsistency Results in Access Violation or Slow-Running Queries

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Article ID: 251094 - View products that this article applies to.
This article was previously published under Q251094
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BUG #: 57007 (SQLBUG_70)
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When updating existing statistics using sampling, two things may happen:
  • An Access Violation occurs during Query Optimization and the user is unable to retrieve the contents of that table using regular SELECT statements that use this index.
  • In cases where an Access Violation does not occur, it is possible that the user works with sub-optimal plans.


When updating existing statistics for non-unique clustered indexes on variable-length columns using sampling, either default or explicit, with a sample size of less than 100%, statistics may become inconsistent. This problem may also occur if SQL Server auto-updates the statistics on this index because the Auto Update Statistics database option is set for that database.


The workaround is to implement one or more of the following:
  • Clear the Auto update statistics database option.
  • Update statistics for this index using the FULL SCAN option.
  • Drop and recreate the index.
  • Do not update statistics for this index.


Microsoft has confirmed this to be a problem in SQL Server 7.0. This problem has been corrected in U.S. Service Pack 2 for Microsoft SQL Server 7.0. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
254561 INF: How to Obtain Service Pack 2 for Microsoft SQL Server 7.0 and Microsoft Data Engine (MSDE) 1.0
For more information, contact your primary support provider.


For example, consider the following table with a clustered index:
create table t (c1 varchar(15), c2 varchar(25))
create clustered index t_c on t(c1, c2)
The statement that causes inconsistent data to be generated is:
UPDATE STATISTICS t(t_c) with sample 50 percent


For information on how to disable automatic updating of statistics for a specific index, see the Books Online topic sp_autostats (T-SQL).


Article ID: 251094 - Last Review: October 20, 2013 - Revision: 2.1
  • Microsoft SQL Server 7.0 Standard Edition
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