Microsoft KB Archive/831302

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

Article ID: 831302

Article Last Modified on 11/2/2007

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APPLIES TO


 * Microsoft SQL Server 2000 Desktop Engine (Windows)
 * Microsoft SQL Server 2000 Developer Edition
 * 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

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Bug #: 469592 (SQL Server 8.0)



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 &quot;Resolution&quot; 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’



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 &quot;Applies to&quot; 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 &quot;Symptoms&quot; 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

Additional query words: update statistics performance sample scan default

Keywords: kbbug kbfix kbperformance kbqfe kbdatabase kbsqlserv2000presp4fix kbclient kbquery kbhotfixserver KB831302

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