PRB: Using WHERE with OR or IN Can Slow Down PerformanceID: Q68470
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It takes a long time to execute a query that uses a WHERE clause with multiple IN or OR options. However, if the same query is broken up into shorter separate queries, and the OR or IN options are not used, the performance improves dramatically.
Before SQL Server executes a query, it first attempts to optimize
the query. In the optimization step, it looks at what, if any,
indexes exist on a table, and decides whether those indexes will be
useful in the execution of the query. In addition, it examines the
conditions in the WHERE clause to estimate how many rows will be
affected by the query.
It is sometimes beneficial to avoid using the OR and IN options in
a WHERE clause. In certain situations, the query may not be able to
be optimized as well as it could be when the OR or IN option is
used.
In these instances, better performance can be obtained by breaking the query into separate, smaller queries. This is not to say, however, that the OR and IN options should never be used. There are situations where using an OR or IN option will give better performance than using separate queries.
Additional query words: Transact-SQL dblib Windows NT
Keywords : kbprg SSrvDB_Lib SSrvTran_SQL SSrvServer SSrvWinNT
Version : 4.2 | 4.2
Platform : OS/2 WINDOWS
Issue type :
Last Reviewed: March 10, 1999