Microsoft KB Archive/244230

= FIX: Slow Query Performance Using Correlated Subquery with "Or Exists" Clause =

Article ID: 244230

Article Last Modified on 3/14/2006

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APPLIES TO


 * Microsoft SQL Server 7.0 Standard Edition

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This article was previously published under Q244230



BUG #: 55001 (SQLBUG_70)



SYMPTOMS
A query that contains a correlated subquery with an OR EXISTS clause may perform slowly.



WORKAROUND
You can rewrite the query and include a redundant clause in the subquery as shown in the following code example: select A.col1 from TableA A, TableB B where A.col1 = B.col1 and B.col2 = 'X' and (A.col2 = 'Y'  or  exists (select * from tableC C                 where A.col3 = C.col3 -- Add redundant clause from outer query and B.col2 = 'X'                ) )



STATUS
Microsoft has confirmed this to be a problem in SQL Server 7.0. This problem has been corrected in U.S. Service Pack 1 for Microsoft SQL Server 7.0. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

232570INF: How to Obtain Service Pack 1 for Microsoft SQL Server 7.0 and Microsoft Data Engine (MSDE) 1.0

For more information, contact your primary support provider.



MORE INFORMATION
The following sample code demonstrates the problem: select A.col1 from TableA A, TableB B where A.col1 = B.col1 and B.col2 = 'X' and (A.col2 = 'Y'  or  exists (select * from tableC C                 where A.col3 = C.col3 --and B.col2 = 'X'                ) ) The SHOWPLAN output for this query shows the optimizer is picking a hash match join strategy.

Keywords: kbbug kbfix kbqfe KB244230

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