For this reason, SQL Server stores execution plans in a cache (called the plan cache), so that the stored execution plan is reused for subsequent executions of a given query.

Figure 1. Execution plan of a server query. As you can see in the above execution plan, SQL Server has highlighted a missing index that it thinks would help our query improve.

SQL Server has an optimizer that usually does a great job of optimizing code for the most effective execution. A query execution plan is the breakdown of how the optimizer will run (or ran) a query.