FIX: Build 8.00.0837: A query that contains a correlated subquery runs slowly

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SYMPTOMS

If you have a set of predicates in the following form

(a = b) && (a = constant)

there is an implied redundant predicate (b = constant) that can be added. The absence of this implied predicate can make the query run slowly.

An enhancement has been made to SQL Server to add this redundant predicate for semi joins, anti-semi joins, inner joins, and outer joins.

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RESOLUTION

Service pack information

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

290211 (http://support.microsoft.com/kb/290211/ ) How to obtain the latest SQL Server 2000 service pack

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Hotfix information

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 tool in Control Panel.

   Date         Time   Version          Size             File name 
   ------------------------------------------------------------------------- 
   31-May-2003  18:45  2000.80.818.0       78,400   Console.exe 
   25-Jun-2003  01:01  2000.80.818.0       33,340   Dbmslpcn.dll 
   25-Apr-2003  02:12                     786,432   Distmdl.ldf 
   25-Apr-2003  02:12                   2,359,296   Distmdl.mdf 
   30-Jan-2003  01:55                         180   Drop_repl_hotfix.sql 
   23-Jun-2003  22:40  2000.80.837.0    1,557,052   Dtsui.dll 
   23-Jun-2003  22:40  2000.80.837.0      639,552   Dtswiz.dll 
   24-Apr-2003  02:51                     747,927   Instdist.sql 
   03-May-2003  01:56                       1,581   Inst_repl_hotfix.sql 
   08-Feb-2003  06:40  2000.80.765.0       90,692   Msgprox.dll 
   01-Apr-2003  02:07                       1,873   Odsole.sql 
   05-Apr-2003  01:46  2000.80.800.0       62,024   Odsole70.dll 
   07-May-2003  20:41  2000.80.819.0       25,144   Opends60.dll 
   02-Apr-2003  21:48  2000.80.796.0       57,904   Osql.exe 
   02-Apr-2003  23:15  2000.80.797.0      279,104   Pfutil80.dll 
   22-May-2003  22:57                      19,195   Qfe469571.sql 
   12-Jun-2003  16:37                   1,083,989   Replmerg.sql 
   04-Apr-2003  21:53  2000.80.798.0      221,768   Replprov.dll 
   08-Feb-2003  06:40  2000.80.765.0      307,784   Replrec.dll 
   05-May-2003  00:05                   1,085,874   Replsys.sql 
   01-Jun-2003  01:01  2000.80.818.0      492,096   Semobj.dll 
   31-May-2003  18:27  2000.80.818.0      172,032   Semobj.rll 
   29-May-2003  00:29                     115,944   Sp3_serv_uni.sql 
   01-Jun-2003  01:01  2000.80.818.0    4,215,360   Sqldmo.dll 
   07-Apr-2003  17:44                      25,172   Sqldumper.exe 
   19-Mar-2003  18:20  2000.80.789.0       28,672   Sqlevn70.rll 
   02-Jul-2003  00:18  2000.80.834.0      180,736   Sqlmap70.dll 
   08-Feb-2003  06:40  2000.80.765.0       57,920   Sqlrepss.dll 
   23-Jun-2003  22:40  2000.80.837.0    7,553,105   Sqlservr.exe 
   08-Feb-2003  06:40  2000.80.765.0       45,644   Sqlvdi.dll 
   25-Jun-2003  01:01  2000.80.818.0       33,340   Ssmslpcn.dll 
   01-Jun-2003  01:01  2000.80.818.0       82,492   Ssnetlib.dll
   01-Jun-2003  01:01  2000.80.818.0       25,148   Ssnmpn70.dll 
   01-Jun-2003  01:01  2000.80.818.0      158,240   Svrnetcn.dll 
   31-May-2003  18:59  2000.80.818.0       76,416   Svrnetcn.exe 
   30-Apr-2003  23:52  2000.80.816.0       45,132   Ums.dll 
   02-Jul-2003  00:19  2000.80.834.0       98,816   Xpweb70.dll

Note Because of file dependencies, the most recent hotfix or feature that contains the files may also contain additional files.

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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.

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MORE INFORMATION

When you apply a hotfix or service pack that contains this fix, the SQL Server optimizer automatically introduces the implied predicate and may remove the corresponding join predicate.

The example that is mentioned in the "Symptoms" section shows a join between a and b. In this example, after optimization the predicate looks like the following:

(a = constant) and (b = constant)

When the original predicate is from a multicolumn key, the simplified expression may remove one of the original join keys. For example, the predicate t1.a = t2.a && t1.b = t2.b && t1.c = constant is changed to t1.b = t2.b && t1.a = constant && t2.a = constant. If column a is the leading column of a multicolumn index, you cannot use a multicolumn density match for cardinality estimation. Additionally, this may cause query plans where the query plan seeks on only the leading index column and applies the remaining filter criteria as residual filters in a WHERE clause on the index seek line or in a WHERE clause for the join operator. For example, the query plan seeks on t1.a = constant. In this scenario, there may be greater disk input/output load and increased CPU utilization. In SQL Server 2000 SP4, you can disable this optimization through trace flag 9061.

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