Execution plans show you what's going on behind the scenes in SQL Server. They can provide you with a wealth of information on how your queries are being executed by SQL Server, including:
Which indexes are getting used and where no indexes are being used at all.
How the data is being retrieved, and joined, from the tables defined in your query.
How aggregations in GROUP BY queries are put together.
The anticipated load, and the estimated cost, that all these operations place upon the system.
All this information makes the execution plan a fairly important tool in the tool belt of database administrator, database developers, report writers, developers, and pretty much anyone who writes TSQL to access data in a SQL Server database.
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Source: SQL Server Execution Plans - SQLServerCentral
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