FIX: Poor Performance with In Clause Using Multiple Correlated Subqueries Unioned Together (243211)


The information in this article applies to:

  • Microsoft SQL Server 7.0
This article was previously published under Q243211
BUG #: 56516 (SQLBUG_70)

SYMPTOMS

A query containing an IN clause of correlated subqueries that are unioned together performs slower on SQL Server 7.0 than on SQL Server 6.5 when the correlation clauses are redundant.

WORKAROUND

Remove the redundant correlation from the subqueries.

STATUS

Microsoft has confirmed this to be a problem in SQL Server 7.0. This problem has been corrected in U.S. Service Pack 2 for Microsoft SQL Server 7.0. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

254561 INF: How to Obtain Service Pack 2 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 is an example of the type of query that performs poorly: 
Select a from table1 where a  IN
(
    ( SELECT table2.a 
      FROM table2, table3 
      WHERE (table2.a = table1.a)
      AND (table2.b = table3.b)
    )
UNION
    ( SELECT C.Contact_Id 
      FROM table1 C, table4 
      WHERE (C.e = "value") 
      AND C.d = table4.d
      AND (table1.a = C.a)
    )
)
The above query can rewritten as follows: 
Select Contact_Id from table1 where contact_id  IN
(
    ( SELECT table2.a 
      FROM table2, table3 
      WHERE (table2.b = table3.b)
    )
UNION
    ( SELECT C.a 
      FROM table1 C
      WHERE (C.e = "value")
    )
)
Modification Type:MajorLast Reviewed:3/14/2006
Keywords:kbBug kbfix KB243211
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