Difference between Actual & Estimated Execution Plan

The Actual Execution Plan and the Estimated Execution Plan in SQL Server provide insights into how a query will or has been executed. They differ in their purpose, generation, and details:

1. Purpose

• Estimated Execution Plan:
  • Shows SQL Server’s prediction of how it plans to execute a query.
  • Useful for understanding the query structure and potential optimization without running the query.
• Actual Execution Plan:
  • Shows the real execution path taken by the query.
  • Includes runtime statistics such as the actual number of rows processed.

2. Execution Requirement

• Estimated Execution Plan:
  • Generated without executing the query.
  • Fast and lightweight.
  • Ideal for analyzing query plans for large or potentially resource-intensive queries.
• Actual Execution Plan:
  • Requires the query to be executed.
  • Suitable for identifying runtime issues, such as incorrect row estimates or memory spills.

3. Information Provided

• Estimated Execution Plan:
  • Contains estimated values for row counts and costs.
  • Based on the optimizer’s assumptions using current statistics and metadata.
• Actual Execution Plan:
  • Includes actual values for rows processed, execution times, and any deviations from the estimate.
  • Helpful for debugging performance issues caused by outdated statistics or data distribution changes.

4. Use Cases

• Estimated Execution Plan:
  • Query optimization during the development phase.
  • Evaluating potential performance without running resource-heavy queries.
• Actual Execution Plan:
  • Post-execution troubleshooting for slow or inefficient queries.
  • Validating the accuracy of the optimizer’s predictions.

5. Tools to Generate Plans

• Estimated Execution Plan:
  • In SQL Server Management Studio (SSMS), use “Display Estimated Execution Plan” or SET SHOWPLAN_XML ON;.
• Actual Execution Plan:
  • Enable “Include Actual Execution Plan” in SSMS or use SET STATISTICS PROFILE ON; before running the query.

6. Common Scenarios

By using both plans effectively, you can gain a deeper understanding of SQL Server’s query execution and optimize performance based on detailed insights.