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BUG #: 55007 (SQLBUG_70)
On large tables, a nonclustered index covering the WHERE clause is not chosen if a varchar field is specified by a LIKE clause.
When possible, replace the nonclustered index covered by the WHERE clause with a clustered index. In the example given in the MORE INFORMATION section of this article, you would change the primary key clustered index by a nonclustered index. Re-create index [BUT000~SOT] as clustered.
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:
254561For more information, contact your primary support provider.
(https://support.microsoft.com/kb/254561/ )INF: How to Obtain Service Pack 2 for Microsoft SQL Server 7.0 and Microsoft Data Engine (MSDE) 1.0
The following script reproduces the problem:
Then run a query such as:
The query uses the primary key index by seeking through the CLIENT column, which has a selectivity of 1, instead of using the index BUT000~NAM, which is covering the WHERE clause.