The SQL Query Store is a powerful feature introduced in SQL Server 2016 that helps in managing and troubleshooting query performance. It stores historical query execution plans along with performance statistics, allowing database administrators to track and analyze changes over time. Query plan changes and regressions can occur due to various factors like data distribution changes, hardware upgrades, or software updates. In this blog post, we will explore some strategies for handling query plan changes and regressions effectively using the SQL Query Store.
Table of Contents
- Understanding Query Plan Changes
- Monitoring Query Plan Regressions
- Forcing a Specific Query Plan
- Updating Statistics
- Recompiling Problematic Queries
- Using Plan Guides
- Conclusion
Understanding Query Plan Changes
Query plan changes occur when the SQL Server optimizer estimates a different plan for a specific query. These changes can lead to performance regressions if the new plan is less efficient. The SQL Query Store captures these plan changes along with their associated performance metrics.
To understand query plan changes, you can analyze the different plans stored in the Query Store and compare their performance characteristics. This will help you identify when and why a particular plan changed. You can use various SQL Server built-in functions and dynamic management views (DMVs) to retrieve this information.
Monitoring Query Plan Regressions
Query regressions are performance deteriorations that occur when a new query plan performs worse than the previous plan. The SQL Query Store allows you to identify and monitor query regressions by comparing the performance metrics of different plans. You can use the sys.query_store_plan DMV to retrieve plan information, such as average execution time, average CPU time, and average logical and physical reads.
By regularly monitoring query plan regressions, you can proactively identify and address performance issues before they impact your application’s response time.
Forcing a Specific Query Plan
In some cases, you may come across a query regression where the newly selected query plan is significantly slower than the previous one. In such scenarios, you can force SQL Server to use a specific query plan that performed well in the past.
To force a specific query plan, you can use the USE PLAN hint or the FORCESEEK query hint. The USE PLAN hint allows you to specify the XML showplan for the desired plan, while the FORCESEEK query hint can be used to force the optimizer to use a particular index seek operation.
Updating Statistics
Outdated or incorrect statistics can lead to suboptimal query plans. By keeping your statistics up-to-date, you can help the SQL Server optimizer generate accurate and efficient plans.
To update statistics, you can use the UPDATE STATISTICS command. This command updates the statistics for the specified table or indexed view. It is recommended to schedule regular statistics updates, especially after significant data modifications.
Recompiling Problematic Queries
Recompiling a problematic query can sometimes lead to better query plans. By using the RECOMPILE query hint or the OPTION (RECOMPILE) query option, you can force SQL Server to generate a new query plan during each execution.
Recompiling can be useful when dealing with queries that have parameters with significant variations in values or when you suspect that query parameter sniffing is causing performance issues.
Using Plan Guides
Plan guides let you influence the query optimizer’s decisions and force it to use specific query plans. You can use plan guides to provide hints, enforce join order, or even use a fixed query plan for parameterized queries.
By creating plan guides, you can override the default behavior of the query optimizer and ensure consistent query performance irrespective of changes in data distribution or system configuration.
Conclusion
The SQL Query Store is a robust feature that can help in tracking and managing query plan changes and regressions. By understanding these strategies and leveraging the power of the Query Store, database administrators can effectively address performance issues and ensure optimal query performance.
Using techniques such as monitoring query plan regressions, forcing specific query plans, updating statistics, recompiling queries, and using plan guides, you can proactively manage and mitigate query performance issues in your SQL Server environment. With these strategies in place, you can optimize your database performance and deliver a seamless experience for your applications and end-users.
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