When working with SQL Server, understanding execution plans is crucial for optimizing query performance. In this article, we will explore various aspects of execution plans, including types and common options.

Logical vs Physical Query Operations

SQL Server uses a cost-based optimizer to generate execution plans. This involves translating logical query operations, such as an INNER JOIN, into physical operations, such as a nested loop JOIN. It’s important to note that what you write in your query may be different from what SQL Server actually executes.

To view the concept of logical query operations, you can enable the “Include Actual Execution Plan” option or use the CTRL+M shortcut in SQL Server Management Studio. This will show you the physical operations involved in executing your query.

Finding an Optimal Plan

SQL Server aims to find the balance between selecting the best plan and completing the task efficiently. It does this through cost optimization, which involves generating multiple execution plans and selecting the one with the lowest cost.

While you can provide explicit hints to SQL Server, it is generally advisable to let the cost-based optimizer choose the most optimal plan. Applying the 80-20 rule, SQL Server is efficient in determining the best plan quickly.

Estimated Execution Plans

The estimated execution plan uses statistics and other parameters to generate a plan based on available information. It is an estimation and not the actual plan generated by SQL Server.

There are several display options for estimated execution plans:

• SHOWPLAN_TEXT: This option provides the text of the execution plan without any graphical representation.
• SHOWPLAN_ALL: Similar to the estimated graphical execution plan, this option includes additional features such as the type of physical and logical operators.
• SHOWPLAN_XML: This option presents the query plan in XML format.

Actual Execution Plans

The actual execution plan shows what SQL Server actually does to execute your query. It provides insights into the execution process and resource usage.

There are several display options for actual execution plans:

• STATISTICS TIME: This option displays the amount of time spent executing the query.
• STATISTICS IO: This option shows the IO characteristics of the query, including logical and physical reads.
• STATISTICS PROFILE: Similar to SHOWPLAN_TEXT, this option retrieves the text of the actual execution plan.
• STATISTICS XML: This option presents the actual execution plan in XML format.

These display options provide valuable information for analyzing and optimizing query performance.

Conclusion

Execution plans are essential for understanding and optimizing query performance in SQL Server. By examining the logical and physical operations, as well as utilizing the various display options for estimated and actual execution plans, you can gain insights into how SQL Server executes your queries and make informed decisions for optimization.

Stay tuned for the next article in this series, where we will dive deeper into SQL Server query execution plans for beginners.