Execution Plan

   Execution plans can tell you how a query will be executed, or how a query was executed. They are, therefore, the DBA's primary means of troubleshooting a poorly performing query.  You can use the estimated execution plan to estimate how the query will run and perform. It may not be exact however it will be close. The reason it may not be exact is because when you execute your query it might have planned to go that way of the estimated plan but might have encountered a problem along the way and it might have to take a detour and go another way or use something different.  The Actual  Execution Plan is returned after the query has been executed and gives you the chance to see how well or badly it performed. It  could very well help you realize what you can replace and how to make your query perform better.