With SQL Server 2005, Microsoft had introduced the feature of plan guides – these allow us to have a much better control over the query plans and thus have more predictability and better performance for some of those queries that are tough to tune either because they are canned queries in a vendor application or the data distribution could be heavily skewed for the indexed columns leading to un-predictable results.  In a way, these allow us to simply influence the plan for the optimizer and either use a fixed query plan or force a plan by the usage of hints without changing the query in the code.  Other RDBMS like Oracle also have had such features – in Oracle, it is called a stored outline or optimizer plan stability

Mas:

http://decipherinfosys.wordpress.com/2009/02/11/plan-freezing-for-plan-guides-in-sql-server-2008/