Review execution plans

Reviewing execution plans is an essential task for database administrators (DBAs) when administering Microsoft Azure SQL solutions. Execution plans provide valuable insights into how queries are executed by the database engine and can help identify performance bottlenecks and optimize query performance. In this article, we will explore the process of reviewing execution plans and understand how to interpret the information they provide.

To review an execution plan, you can use SQL Server Management Studio (SSMS) or Azure Data Studio. Let’s consider an example where we have a simple query that retrieves data from a table:

SELECT *
FROM Customers
WHERE Country = 'USA'

To review the execution plan for this query, follow these steps:

1. Open SSMS or Azure Data Studio and connect to the Azure SQL database.
2. Open a new query window and paste the query in the editor.
3. Click on the “Include Actual Execution Plan” button, typically located in the toolbar or accessible through the “Query” menu.
4. Execute the query by clicking the “Execute” button or pressing F5.

Once the query execution is complete, the execution plan will be displayed in a separate tab or pane. The execution plan provides a graphical representation of how the query is processed and includes information about the various operations performed by the database engine.

The execution plan consists of a tree-like structure with multiple nodes representing the query operators. Each node represents an individual operation, such as scanning a table, performing a join, or applying a filter. These nodes can be expanded to view more details about the specific operation.

The most critical information in an execution plan is the estimated and actual number of rows processed by each operation. It helps identify potential performance issues such as table scans where a large number of rows are processed, indicating the need for proper indexing or query optimization.

Along with row estimates, the execution plan also provides information about the execution cost of each operation. The cost is an arbitrary unit used by the query optimizer to estimate resource consumption. Higher costs indicate higher resource requirements and can help pinpoint areas for optimization.

Additionally, the execution plan may contain warning icons or messages that highlight potential issues. For example, a missing index warning suggests that creating a specific index can significantly improve the query’s performance.

When reviewing execution plans, keep the following tips in mind:

1. Look for table scans or index scans: These operations indicate that the entire table or index is being scanned, which can be inefficient for large tables. Consider adding appropriate indexes or optimizing the query to use index seeks or seeks instead.
2. Analyze join operations: Ensure that the join operations are performed efficiently. Look for indications of hash or merge joins and ensure that the join columns are properly indexed.
3. Identify expensive operations: Pay attention to operations with high costs or a large discrepancy between estimated and actual row counts. These can be potential areas for optimization.
4. Validate index usage: Check if the execution plan utilizes existing indexes. If indexes are not used, it may indicate the need for index modifications or query optimization.
5. Consider leveraging query hints: In certain cases, you may need to enforce a specific execution plan using query hints. However, use them judiciously and only when necessary.

Understanding execution plans and leveraging them to optimize query performance is a crucial skill for DBAs. Regularly reviewing execution plans can help maintain optimal performance and ensure efficient resource utilization in Azure SQL solutions.