Posted by Erland Sommarskog on 01/25/08 09:02

Jared (blacktoe.the.crippler@gmail.com) writes:
> Consider the following two functionally identical example queries:
>
> Query 1:
>
> DECLARE @Name VARCHAR(32)
> SET @Name = 'Bob'
> SELECT * FROM Employees
> WHERE [Name] = CASE WHEN @Name IS NULL THEN [Name] ELSE @Name END
>
> Query 2:
>
> SELECT * FROM Employees WHERE [Name] = 'Bob'
>
> I would expect SQL Server to construct an identical QEP under the hood
> for these two queries, and that they would require essentially the
> same amount of time to execute. However, Query 1 takes much longer to
> run on my indexed table of ~300,000 rows. By "longer", I mean that
> Query 1 takes about two seconds, while Query 2 returns almost
> instantly.

SQL Server builds the query plan for the entire batch, and thus at
compile time the value of @Name is not known. Therefore the plan must
be such that it yields a correct result in either case.

> Is there a way to implement a conditional WHERE clause without
> suffering this performance hit? I want to avoid using the IF...THEN
> method because I frequently require several optional parameters in the
> WHERE clause.

I have an article on my web site that discusses a number of possible
approaches to this problem, see http://www.sommarskog.se/dyn-search.html.


--
Erland Sommarskog, SQL Server MVP, esquel@sommarskog.se

Books Online for SQL Server 2005 at
http://www.microsoft.com/technet/prodtechnol/sql/2005/downloads/books.mspx
Books Online for SQL Server 2000 at
http://www.microsoft.com/sql/prodinfo/previousversions/books.mspx

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