Once all the processing necessary to generate an execution plan has been completed, it would be crazy for SQL Server to throw away that work and do it all again each time a query gets called. Instead, it saves the plans created in a memory space on the server called the plan cache. This chapter will walk through how you can monitor the plan cache to see how SQL Server reuses execution plans.
How to analyze execution plan caching
Query plan hash and query hash as mechanisms for identifying queries to tune ...