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In the world of SQL server, the engine behind the scenes that gets things done is a query optimizer. This optimizer determines the most efficient way to execute the SQL statements that you write. The ultimate blueprint for this engine is what we call an “Execution Plan”. Understanding how to read and relate to these execution plans is crucial in debugging, optimization, and enhancing the performance of your SQL server jobs. In today’s blog post, we take a deeper look into SQL Server Query Execution Plans and how you can start making sense out of them.
What is an Execution Plan?
An Execution Plan is essentially a map that SQL Server navigates to execute an SQL query. Whenever an SQL query is run, SQL Server creates an execution plan which it uses for that query in order to fetch the requested data. The purpose of this plan is to take your query and logically decide the fastest and least resource-intensive route to your answer. The optimizer takes into account various factors such as database schema, indexes, statistics, etc. to determine the best method.
Anatomy of an Execution Plan
A typical Execution Plan consists of several basic components, listed as nodes on a tree structure. These components include:
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* SELECT: This is the root node where the final result set is returned. * JOIN: This node represents the operation where two tables are combined. * SORT: Represents the operation which sorts the result set as per given criteria. * FETCH: This node fetches a row at a time from the table. |
Simple Execution Plan Example
Let’s take a simple example to understand. Consider the following SQL query:
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SELECT * FROM Employees WHERE DepartmentId = 10; |
The Execution Plan of this query will include scanning the index of the Employees table to find the rows which fulfil the WHERE clause condition and then fetching the respective rows. If the Employees table has an index on DepartmentId, only that index needs to be scanned. Otherwise, the entire table needs to be scanned.
How to Retrieve an Execution Plan
SQL Server provides a functionality to view the execution plan of any query. When you write a query in SQL Server Management Studio (SSMS), you can click on the “Display Estimated Execution Plan” button to view the execution plan. Here is an example of how you do it:
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-- Write your query SELECT * FROM Employees WHERE DepartmentId = 10; -- Click on 'Display Estimated Execution Plan' button |
Learning to understand and use Execution Plans effectively can greatly enhance your ability to optimize and troubleshoot your SQL queries. We hope this introduction has served as a helpful starting point, and encourage you to dive deeper and experiment with your own SQL Server environment to gain hands-on experience.