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SQL Server Execution Plans, 2nd Edition by Grant Fritchey

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Chapter 1: Execution Plan Basics

An execution plan, simply put, is the result of the query optimizer's attempt to calculate the most efficient way to implement the request represented by the T-SQL query you submitted.

Execution plans can tell you how SQL Server may execute a query, or how it did execute a query. They are, therefore, the 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, your query may be reading an entire table-worth of data when, by removing a function in your WHERE clause, it could simply retrieve ...

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