FIX: SQL Server underestimates the cardinality of a query expression and query performance may be slow

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Article ID: 831302 - View products that this article applies to.
Bug #: 469592 (SQL Server 8.0)
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SUMMARY

Under certain conditions, Microsoft SQL Server 2000 may underestimate the cardinality of a query expression. This problem may cause slow query performance. To resolve this problem, install the hotfix that is described in the "Resolution" section, and then turn on trace flag 9207.


For more information about the latest service pack for Microsoft SQL Server 2000, click the following article number to view the article in the Microsoft Knowledge Base:
290211 How to obtain the latest SQL Server 2000 service pack

SYMPTOMS

SQL Server may underestimate the cardinality of a query expression, and query performance may be slow. This problem occurs if all the following conditions are true:
  • The predicate is made up of at least two conditions on different columns in the same table that are used together with the AND operator.
  • One of the predicates involves an inequality operator or a range expression.
  • The SQL Server query optimizer can obtain a multicolumn density from an index or from multicolumn statistics that involve the columns in the query expression.
  • A histogram that can be used to estimate the selectivity of the range expression exists, and the histogram's selectivity is less than or equal to the average selectivity of any arbitrary constant value.
Under these conditions, SQL Server may effectively apply the selectivity of the range expression two times by first using the multicolumn density for all the columns, and then by filtering based on the selectivity of the individual range expression. For example, the following query expression meets all the conditions:
SELECT a.col1, a.col2, a.col3, a.col4
WHERE a.col1 = ‘x’ AND a.col2 = ‘y’ AND a.col3 = ‘z’ 
AND a.col4 BETWEEN ‘a’ AND ‘b’

RESOLUTION

Service pack information

To resolve this problem, obtain the latest service pack for Microsoft SQL Server 2000. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
290211 How to obtain the latest SQL Server 2000 service pack

Hotfix information


The English version of this hotfix has the file attributes (or later) 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 tool in Control Panel.
   Date         Time   Version            Size    File name
   -----------------------------------------------------------------------
   31-May-2003  06:15  2000.80.818.0      78,400  Console.exe      
   02-Oct-2003  08:29  2000.80.867.0      33,340  Dbmslpcn.dll     
   24-Apr-2003  13:42                    786,432  Distmdl.ldf
   24-Apr-2003  13:42                  2,359,296  Distmdl.mdf
   29-Jan-2003  13:25                        180  Drop_repl_hotfix.sql
   11-Sep-2003  14:56  2000.80.859.0   1,905,216  Dtspkg.dll       
   26-Aug-2003  07:46  2000.80.854.0     528,960  Dtspump.dll      
   23-Jun-2003  10:10  2000.80.837.0   1,557,052  Dtsui.dll        
   23-Jun-2003  10:10  2000.80.837.0     639,552  Dtswiz.dll       
   23-Apr-2003  14:21                    747,927  Instdist.sql
   02-May-2003  13:26                      1,581  Inst_repl_hotfix.sql
   07-Feb-2003  18:10  2000.80.765.0      90,692  Msgprox.dll      
   31-Mar-2003  13:37                      1,873  Odsole.sql
   04-Apr-2003  13:16  2000.80.800.0      62,024  Odsole70.dll     
   07-May-2003  08:11  2000.80.819.0      25,144  Opends60.dll     
   02-Apr-2003  09:18  2000.80.796.0      57,904  Osql.exe         
   02-Apr-2003  10:45  2000.80.797.0     279,104  Pfutil80.dll     
   04-Aug-2003  05:47                    550,780  Procsyst.sql
   11-Sep-2003  12:07                     12,305  Qfe469315.sql
   22-May-2003  10:27                     19,195  Qfe469571.sql
   05-Sep-2003  13:06                  1,090,932  Replmerg.sql
   05-Sep-2003  18:48  2000.80.858.0     221,768  Replprov.dll     
   05-Sep-2003  18:48  2000.80.858.0     307,784  Replrec.dll      
   05-Sep-2003  11:30                  1,087,150  Replsys.sql
   13-Aug-2003  03:58                    986,603  Repltran.sql
   21-Oct-2003  11:38  2000.80.871.0      57,916  Semnt.dll        
   29-Jul-2003  07:43  2000.80.819.0     492,096  Semobj.dll       
   31-May-2003  05:57  2000.80.818.0     172,032  Semobj.rll
   05-Aug-2003  08:36                    127,884  Sp3_serv_uni.sql
   31-May-2003  12:31  2000.80.818.0   4,215,360  Sqldmo.dll       
   07-Apr-2003  05:14                     25,172  Sqldumper.exe    
   19-Mar-2003  05:50  2000.80.789.0      28,672  Sqlevn70.rll
   26-Sep-2003  16:12  2000.80.865.0     180,792  Sqlmap70.dll     
   02-Sep-2003  14:26  2000.80.857.0     188,992  Sqlmmc.dll       
   02-Sep-2003  10:33  2000.80.857.0     479,232  Sqlmmc.rll
   21-Oct-2003  11:38  2000.80.871.0     401,984  Sqlqry.dll       
   07-Feb-2003  18:10  2000.80.765.0      57,920  Sqlrepss.dll     
   21-Oct-2003  11:38  2000.80.871.0   7,602,257  Sqlservr.exe     
   25-Jul-2003  09:14  2000.80.845.0     590,396  Sqlsort.dll      
   07-Feb-2003  18:10  2000.80.765.0      45,644  Sqlvdi.dll       
   02-Oct-2003  08:29  2000.80.867.0      33,340  Ssmslpcn.dll     
   31-May-2003  12:31  2000.80.818.0      82,492  Ssnetlib.dll     
   31-May-2003  12:31  2000.80.818.0      25,148  Ssnmpn70.dll     
   31-May-2003  12:31  2000.80.818.0     158,240  Svrnetcn.dll     
   31-May-2003  06:29  2000.80.818.0      76,416  Svrnetcn.exe     
   30-Apr-2003  11:22  2000.80.816.0      45,132  Ums.dll          
   01-Jul-2003  11:49  2000.80.834.0      98,816  Xpweb70.dll      
Note Because of file dependencies, the most recent hotfix or feature that contains these files may also contain additional files.

STATUS

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

This problem was first corrected in Microsoft SQL Server 2000 Service Pack 4.

MORE INFORMATION

After you install the hotfix, you must turn on trace flag 9207 for the hotfix to take effect. To turn on trace flag 9207 and to make sure that the trace flag is set when the SQL Server service starts, add trace flag 9207 as a SQL Server startup parameter. To do so, follow these steps:
  1. Start SQL Server Enterprise Manager.
  2. Expand SQL Server Group, and then click your instance of SQL Server.
  3. Right-click your instance of SQL Server, and then click Properties.
  4. Click the General tab, and then click Startup Parameters.
  5. In the Parameter text box, type -T9207.
  6. Click Add, and then click OK.
  7. Click OK.
  8. Restart the SQL Server service so that the new startup parameter can take effect.
Note Do not use the DBCC TRACEON Transact-SQL statement as an alternative method to set the SQL Server startup parameter. The effect of the DBCC TRACEON Transact-SQL statement may be lost when the SQL Server service is stopped. Therefore, the trace flag may not be turned on when the SQL Server service is restarted.

When you turn on the trace flag, the SQL Server query optimizer does not include the column that has the range predicate as part of a multicolumn density match. Instead, the selectivity is applied individually by using the histogram information. In the example that is included the "Symptoms" section, if multicolumn density exists for the columns [col1, col2, col3, col4], the hotfix only uses the density of [col1, col2, col3]. The histogram that is available separately on column col4 is used appropriately.

For more information, 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: 831302 - Last Review: November 2, 2007 - Revision: 2.3
APPLIES TO
  • Microsoft SQL Server 2000 Desktop Engine (Windows)
  • Microsoft SQL Server 2000 Developer Edition
  • Microsoft SQL Server 2000 Enterprise Edition
  • Microsoft SQL Server 2000 Enterprise Edition 64-bit
  • Microsoft SQL Server 2000 Personal Edition
  • Microsoft SQL Server 2000 Standard Edition
Keywords: 
kbhotfixserver kbqfe kbqfe kbclient kbdatabase kbperformance kbquery kbsqlserv2000presp4fix kbfix kbbug KB831302

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