Table of Contents
- Introduction
- Understanding Query Plan Stability Issues
- Using Query Store to Identify Query Plan Changes
- Analyzing Query Store Data
- Resolving Query Plan Stability Issues
- Conclusion
1. Introduction
The Query Store is a powerful feature in Microsoft SQL Server that helps you identify and resolve query performance issues. One common challenge faced by database administrators is query plan instability, where the execution plan chosen by the query optimizer changes frequently. In this blog post, we will explore techniques for identifying and resolving query plan stability issues using the Query Store.
2. Understanding Query Plan Stability Issues
Query plan stability issues occur when the SQL Server query optimizer selects different execution plans for the same query over time. This can lead to inconsistent query performance, where sometimes a query runs fast and other times it runs slow. The Query Store captures and stores information about query plans and their performance, making it a valuable tool for troubleshooting query plan stability issues.
3. Using Query Store to Identify Query Plan Changes
To identify query plan changes, you can leverage the Query Store views and functions provided by SQL Server. The sys.query_store_plan view, for example, contains information about the different query plans used for a particular query. By querying this view, you can analyze the plan history and identify any plan changes that might be causing the stability issues.
4. Analyzing Query Store Data
Once you have identified query plan changes, it’s important to analyze the query store data to understand the factors contributing to the instability. You can use the sys.query_store_runtime_stats view to examine the execution statistics for each plan version. This can help you identify important metrics such as CPU time, duration, and query cost, which can guide you in determining the most efficient plan.
5. Resolving Query Plan Stability Issues
There are several techniques you can employ to resolve query plan stability issues. One approach is to force a specific execution plan using a query hint, such as OPTIMIZE FOR or USE PLAN. This ensures that the same plan is used consistently, bypassing the query optimizer’s plan selection process.
Another technique is to update statistics on the relevant tables. Outdated statistics can lead to suboptimal query plans, so keeping the statistics up to date can improve plan stability. You can use the UPDATE STATISTICS command to accomplish this.
Furthermore, consider enabling the QUERY_OPTIMIZER_HOTFIXES database scoped configuration option. Hotfixes provided by Microsoft can address specific query plan stability issues, and enabling this option ensures that your database benefits from these fixes.
6. Conclusion
Query plan stability issues can significantly impact the performance of your SQL Server database. By effectively utilizing the Query Store, you can identify and resolve these issues, ensuring consistent and optimal query performance. The techniques discussed in this blog post provide a solid foundation for troubleshooting and resolving query plan stability problems, ultimately improving the overall performance of your database.