Certainly! Let’s delve into a comprehensive and detailed exploration of Query Store Forced Plan in SQL Server, focusing on its functionality, benefits, limitations, and best practices. This analysis aims to provide a thorough understanding, catering to both novice and experienced database administrators and developers.
1. Introduction to Query Store Forced Plan
1.1 What is Query Store?
Query Store is a feature introduced in SQL Server 2016 that provides a mechanism for tracking query performance over time. It captures a history of queries, plans, and runtime statistics, allowing for easier identification of performance issues and plan regressions.
1.2 What is Plan Forcing?
Plan Forcing is a feature within Query Store that allows database administrators to force a specific execution plan for a query. This ensures that the query uses a known good plan, even if the query optimizer might choose a different plan due to changes in data distribution or statistics.
1.3 Importance of Forced Plans
Forcing a plan can be crucial in scenarios where:(Grant Fritchey)
- Performance Degradation: A previously optimal plan is no longer performing well.
- Plan Regressions: Automatic plan changes lead to performance issues.
- Stability: Ensuring consistent performance for critical queries.(Deepthi Goguri’s SQL Server Blog)
2. How Plan Forcing Works
2.1 Capturing the Plan
When a query is executed, SQL Server generates an execution plan. If Query Store is enabled, these plans are captured and stored. To force a plan, you need to identify the specific plan that you want to enforce.
2.2 Forcing the Plan
Once the desired plan is identified, you can force it using the following system stored procedure:
EXEC sp_query_store_force_plan @query_id = <QueryID>, @plan_id = <PlanID>;
This command forces the specified plan for the given query.
2.3 Monitoring Forced Plans
After forcing a plan, it’s essential to monitor its effectiveness. You can query the sys.query_store_plan view to check the status of forced plans:(Redgate Software)
SELECT query_id, plan_id, is_forced_plan, force_failure_count, last_force_failure_reason_desc
FROM sys.query_store_plan
WHERE is_forced_plan = 1;
This will provide information about the forced plans, including any failures and reasons.
3. Benefits of Plan Forcing
3.1 Performance Stability
By forcing a specific plan, you can ensure that a query consistently performs well, even if the optimizer might choose a suboptimal plan due to changes in data distribution or statistics.
3.2 Troubleshooting Tool
Plan forcing serves as a valuable tool for troubleshooting performance issues. If a query’s performance degrades, forcing a known good plan can help restore optimal performance.
3.3 Long-Term Solution
In some cases, forcing a plan can serve as a long-term solution while underlying issues are addressed, such as updating statistics or optimizing indexes.
4. Limitations and Considerations
4.1 Plan Stability
Forced plans are tied to specific query identifiers. If the query’s structure changes, such as modifications in parameters or the introduction of table variables, the query ID may change, rendering the forced plan ineffective.(milossql.com)
4.2 Dependency on Indexes
If a forced plan relies on a specific index, and that index is dropped or becomes unavailable, the forced plan may fail. It’s crucial to ensure that all dependencies for the forced plan are maintained.
4.3 Potential Side Effects
Forcing a plan can have unintended side effects, especially if the forced plan is not optimal for all scenarios. It’s essential to monitor the performance of the forced plan and be prepared to revert if necessary.
5. Best Practices for Plan Forcing
5.1 Identify the Root Cause
Before forcing a plan, investigate the root cause of performance issues. Forcing a plan should be a temporary measure while underlying issues are addressed.
5.2 Test in Development
Always test plan forcing in a development or staging environment before applying it to production. This helps identify potential issues and ensures that the forced plan improves performance.
5.3 Monitor Performance
After forcing a plan, continuously monitor the query’s performance. Use tools like Query Store reports and DMVs to assess the effectiveness of the forced plan.(Redgate Software)
5.4 Document Changes
Document any forced plans, including the reasons for forcing them and any dependencies. This documentation can be valuable for future troubleshooting and audits.
6. Advanced Topics
6.1 Plan Forcing with Table Variables
Be cautious when forcing plans for queries that use table variables. The query ID for such queries can be unstable, leading to forced plans being ineffective. (milossql.com)
6.2 Plan Forcing and Schema Changes
Schema changes, such as dropping or altering tables and indexes, can affect the stability of forced plans. Ensure that all dependencies are maintained to prevent forced plan failures.
6.3 Automation of Plan Forcing
Automating the identification and forcing of plans can be beneficial, especially in large environments. However, automation should be approached with caution and thorough testing.
7. Troubleshooting Forced Plans
7.1 Common Issues
- Plan Not Used: The forced plan is not being used. This can occur if the query ID has changed or if the plan is no longer optimal.(milossql.com)
- Performance Degradation: The forced plan causes performance issues. This can happen if the forced plan is not suitable for all scenarios.
7.2 Diagnostic Steps
- Check Query ID: Verify that the query ID has not changed.
- Check Plan Availability: Ensure that the forced plan is still available in the plan cache.(MSSQLTips.com)
- Check Dependencies: