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. Rather than guess at why a given query is performing thousands of scans, putting your I/O through the roof, you can use the execution plan to identify the exact piece of SQL code that is causing the problem. For example, it may be scanning an entire table-worth of data when, with the proper index, it could simply backpack out only the rows you need. All this and more is displayed in the execution plan.