FIX: Slow Query Performance Using Correlated Subquery with "Or Exists" Clause (244230)

The information in this article applies to:

Microsoft SQL Server 7.0

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.

Modification Type:	Major	Last Reviewed:	3/14/2006
Keywords:	kbBug kbfix kbQFE KB244230