Reading Actual vs Estimated Execution Plans

Reading Actual vs Estimated Execution Plans in SQL Server: A Comprehensive Guide

Introduction

In SQL Server, execution plans are an essential part of understanding and optimizing query performance. They provide insights into how SQL Server executes queries and how resources are used during that execution. When analyzing execution plans, you can compare Actual Execution Plans and Estimated Execution Plans. These plans give you detailed information about query processing, and understanding their differences can help you identify performance bottlenecks and optimize queries effectively.

This guide will explore Actual Execution Plans and Estimated Execution Plans, their importance, and how to read and interpret them for better query optimization.

1. What are Execution Plans?

An execution plan is a set of steps that SQL Server takes to retrieve data for a query. Execution plans can be visualized in various formats, including graphical and textual formats. These plans show how SQL Server will access data and which operations will be performed during the execution of a query.

There are two types of execution plans:

Estimated Execution Plan: This is the plan SQL Server generates before actually running the query. It is based on statistics and assumptions about the data, providing an estimate of how SQL Server will execute the query.
Actual Execution Plan: This plan is generated after the query has been executed. It shows the real-time execution details, including the number of rows processed, actual resource usage (e.g., CPU, memory), and other runtime metrics.

2. Estimated Execution Plan

2.1 Definition and Purpose

An Estimated Execution Plan is created when SQL Server plans to execute a query but has not yet executed it. This plan is based on the database’s statistics, indexes, and query structure. SQL Server uses query optimization to estimate the most efficient way to execute the query.

An Estimated Execution Plan allows you to:

Visualize SQL Server’s assumptions about query execution before running the query.
Identify potential performance bottlenecks.
Suggest missing indexes or changes to query structure.
Preview query execution costs without actually running the query.

2.2 How to View Estimated Execution Plans

To view the Estimated Execution Plan in SQL Server Management Studio (SSMS), follow these steps:

Open a query window in SSMS.
Write your query.
Click on the Display Estimated Execution Plan button (the graphical icon resembling a plan) on the SSMS toolbar, or press Ctrl + L.

Once you run the Estimated Execution Plan, SSMS will show a graphical representation of the operations SQL Server intends to perform.

2.3 Key Features of Estimated Execution Plans

Estimated I/O Cost: Displays how much disk I/O SQL Server expects to perform for the query.
Estimated CPU Cost: Represents the CPU cost, including the estimated time to process each operator in the query.
Index Usage: Shows which indexes SQL Server plans to use.
Join Type: Indicates which type of join SQL Server intends to use (e.g., Nested Loops, Merge Join, Hash Join).
Scan or Seek: Indicates whether SQL Server plans to scan the entire table or perform an index seek (which is generally more efficient).

3. Actual Execution Plan

3.1 Definition and Purpose

An Actual Execution Plan is generated after SQL Server executes a query. Unlike the Estimated Execution Plan, which relies on statistics and assumptions, the Actual Execution Plan provides actual data about how the query was executed. It includes runtime metrics such as:

The actual number of rows processed.
The actual time taken for each operation.
The actual number of page reads/writes.
Any wait statistics that occurred during query execution.

The Actual Execution Plan can be used to:

Confirm how closely the Estimated Execution Plan matched the actual execution.
Identify discrepancies between expected and actual performance.
Analyze performance bottlenecks or areas for optimization.

3.2 How to View Actual Execution Plans

To view the Actual Execution Plan in SSMS:

Open a query window in SSMS.
Write your query.
Click the Include Actual Execution Plan button on the SSMS toolbar (the button looks like a plan icon) or press Ctrl + M.
Execute the query.

The Actual Execution Plan will appear in a separate tab alongside your query results.

3.3 Key Features of Actual Execution Plans

Actual Number of Rows: The Actual Execution Plan shows how many rows were actually processed by each operator, which can differ from the Estimated Execution Plan.
Actual Execution Time: This indicates the real time taken by SQL Server to process each operation.
Resource Consumption: It shows actual CPU, I/O, and memory usage during query execution.
Warnings: It highlights warnings such as missing indexes or insufficient memory, which can affect performance.

4. Key Differences Between Actual and Estimated Execution Plans

Feature	Estimated Execution Plan	Actual Execution Plan
Creation Time	Generated before the query is executed	Generated after the query execution
Data Used	Uses statistics, assumptions, and estimations	Uses actual runtime data, including row counts and resource usage
Row Counts	Estimated based on statistics	Actual row counts processed by SQL Server
Resource Usage	Does not include actual resource usage	Includes actual CPU, memory, and I/O usage
Execution Time	Estimated time for each operation	Actual time taken for each operation
Warnings	May not show issues unless statistics are outdated	Shows runtime issues, such as missing indexes

5. Interpreting Execution Plans

5.1 Key Operators in Execution Plans

Whether in an Estimated or Actual Execution Plan, the key operators in a query’s execution plan can include:

Clustered Index Scan: A full table scan using the clustered index.
Index Seek: Efficient query operation that uses an index to find matching rows.
Table Scan: A full scan of the table without using an index.
Nested Loops Join: A join where SQL Server compares each row of one table with rows from another table.
Merge Join: A join that assumes both tables are sorted and merges them efficiently.
Hash Join: A join that uses a hash table to perform the join, typically used when tables are not sorted.

5.2 Looking for Bottlenecks

When analyzing execution plans, you should be looking for operations that consume high amounts of resources. Common performance bottlenecks include:

Table Scans: These are expensive operations, especially on large tables. Look for places where an index could improve performance.
Sorts: Sorting data can be resource-intensive. If a sort operation is present, check whether it’s necessary or can be optimized.
Expensive Joins: Nested loops joins are fast with small datasets but can be slow for large datasets. Merge joins and hash joins may perform better for large datasets.

5.3 Missing Indexes

If SQL Server believes that a missing index could improve query performance, it will suggest the creation of the index in both the Actual and Estimated Execution Plans. The suggestion typically appears as a Missing Index warning.

6. Practical Example: Analyzing Execution Plans

Let’s say you have the following query:

SELECT Name, Department
FROM Employees
WHERE DepartmentId = 3;

Step 1: Viewing the Estimated Execution Plan

Run the query with Display Estimated Execution Plan enabled. The Estimated Execution Plan will suggest whether SQL Server plans to perform a Table Scan or Index Seek.

Step 2: Running the Query with Actual Execution Plan

Now, run the query with Include Actual Execution Plan enabled. After execution, check if the actual execution was similar to what was estimated. Look for discrepancies, such as an Index Seek in the Estimated Execution Plan turning into a Table Scan in the Actual Execution Plan. This could indicate that SQL Server couldn’t find an appropriate index or the query was not optimized as expected.

Step 3: Analyzing the Plan

If there was a Table Scan, this could be a sign that an index on DepartmentId is missing. Based on this, you could create the following index:

CREATE INDEX idx_DepartmentId ON Employees (DepartmentId);

You can then rerun the query to compare the Estimated and Actual Execution Plans again.

7. Best Practices for Using Execution Plans

Always compare the Estimated and Actual Execution Plans: This allows you to verify whether SQL Server is executing the query as expected.
Pay attention to warnings in the Actual Execution Plan: These warnings, such as missing indexes or bad joins, can highlight areas for improvement.
Optimize queries based on execution plans: Look for operations that consume the most resources and try to rewrite the query or add indexes.
Regularly update statistics: Outdated statistics can lead to suboptimal execution plans. Use UPDATE STATISTICS or enable Auto Update Statistics to keep them current.

Understanding and reading Actual vs. Estimated Execution Plans is a vital skill for SQL Server administrators and developers. By comparing these plans, you can gain valuable insights into how SQL Server executes queries and identify areas for performance improvement. Regularly analyzing execution plans can lead to more efficient query execution, better use of resources, and improved overall database performance.

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