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L Server Execution Plans: Understanding Query Processing
Introduction
SQL Server execution plans pave the road map that SQL Server follows to retrieve the data required by your query. Understanding query processing and SQL Server execution plans isn’t just for DBAs. It’s crucial for any SQL developer or data analyst driving valuable insights from querying databases.
What are Execution Plans?
Execution plans show you exactly what method the SQL Server query optimizer chose to retrieve the necessary data for the query under consideration. The plan describes the detailed steps, or operations, that must be performed to satisfy requests submitted in a query. Each operation involves obtaining a group of rows, performing an action, and handing off the result to the next operator – much like an assembly line in a car manufacturing plant.
Viewing Execution Plans
The simplest way to view the SQL Server Execution plan used for your query is to enable the feature in SQL Server Management Studio before running your query. You can do so by pressing Ctrl + M or going to Query > Include Actual Execution Plan.
Example:
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-- This is a simple SELECT statement SELECT * FROM Customers |
This will include an additional tab titled ‘Execution plan’ in your results, from where you can view the execution plan used for your query.
Understanding Execution Plans
Execution plans may seem quite intricate at first, but understanding them comes down to learning a few crucial operations and how they work. Among the most common operators you will find in your execution plans are Table Scan, Index Seek, Index Scan, Hash Match, and Nested Loops.
Each operator has an associated cost, indicating the amount of work required executing that operator. The goal of the SQL Server optimizer is to find the plan with the least overall cost.
Example:
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-- This query is selecting customers from a specific city SELECT * FROM Customers WHERE city='New York' |
In the execution plan, you might see an Index Seek operator if you have an index on the ‘city’ column, as it’s a more efficient operation than scanning the entire table.
Conclusion
Understanding SQL Server execution plans is an essential skill for anyone who frequently deals with SQL, as this knowledge is crucial in writing efficient queries and diagnosing performance issues. If you’re comfortable with the basics, further details provide an even more profound view of how SQL Server deals with your data.