SQL Server’s Plan Guides: Controlling Query Plans and Performance
Introduction
The performance of an SQL Server database is critical to the smooth operation of modern businesses. One key to achieving optimal performance is the management of query plans. Plan guides in SQL Server are an advanced feature designed to address plan-related performance issues without modifying the actual code. This comprehensive guide aims to unravel the complexities of SQL Server plan guides, providing insights into their creation, use, and best practices.
What are Plan Guides?
Plan guides in SQL Server allow database administrators and developers to optimize query performance by influencing the optimization process. They can be used to instruct the SQL Server query optimizer to use specific query hints or plan suggestions to correct issues such as suboptimal query plans, parameter sniffing problems, or to achieve consistency in performance across different environments.
Why Use Plan Guides?
Plan guides can be essential tools when direct query or procedure modification is impractical or impossible. This could be due to a range of reasons including:
- Lack of access to modify third-party application code
- The need to maintain base code for future updates
- Desire to test performance outcomes with different query execution plans without altering the existing code
Furthermore, their use is often crucial when the application relies on ad hoc SQL or when different environments lead to discrepancies in how queries are optimized and executed.
Types of Plan Guides
SQL Server supports three types of plan guides:
- OBJECT plan guides: Linked to a specific query in a stored procedure, function, or trigger
- SQL plan guides: Used for queries that execute in the context of a standalone batch or a batch within an application
- TEMPLATE plan guides: Influence batches that resemble a parameterized form of a batch, providing a way to template the optimization process
Understanding the Query Optimization Process
SQL Server uses a complex optimization process called the Query Optimizer to determine the best way to execute a query. The updated statistics, indexes, and database structures inform this process, influencing the choice of a query plan. However, even with a robust optimizer, SQL Server can sometimes choose less than optimal plans due to atypical circumstances.
When to Implement Plan Guides
Plan guides are not a universal remedy and should be used judiciously. Ideal scenarios for their use include:
- Consistently poor query performance despite updated statistics and appropriate indexes
- Inability to rewrite queries due to application constraints, such as with third-party software
- Parameter sniffing issues where the optimizer chooses a plan based on atypical parameters, negatively impacting performance for more common parameter values
Creating Plan Guides
Creating a plan guide involves identifying the problematic query, understanding why the plan chosen by SQL Server is suboptimal, and then using the sp_create_plan_guide system stored procedure to define the plan guide.
-- Syntax for creating an OBJECT plan guide
sp_create_plan_guide @name, @stmt, @type, @module_or_batch, @params = NULL, @hints;
-- Syntax for creating a SQL plan guide
sp_create_plan_guide @name, @stmt, @type, @batch_or_proc, @params, @hints;
-- Syntax for creating a TEMPLATE plan guide
sp_create_plan_guide @name, @stmt, @type, NULL, @params, @hints;
Consideration must be given to ensuring proper parameter definitions and using appropriate query hints or plan forcing to encourage the desired plan.
Testing Plan Guides
After implementation, constant monitoring and testing of plan guides are crucial to ensure they are providing the desired effects. DBAs can utilize tools such as Query Store, the plan cache, and execution statistics to gauge the effectiveness of the applied guides.
Best Practices and Considerations
While plan guides can be powerful, there are best practices and considerations to keep in mind:
- Keep track of plan guides and document their purpose thoroughly
- Review plan guides regularly, as database changes may render them obsolete or counterproductive
- Use plan guides as a temporary fix and strive for long-term solutions through query or index optimization
- Be cautious with the hints used, as they can backfire and end up degrading performance instead of improving it
- Always test plan guides in a non-production environment before implementing on a live system
- Keep in mind that plan guides are specific to the version and configuration of the SQL Server, and may need adjustments after major system changes or upgrades
Removing or Disabling Plan Guides
When plan guides are no longer necessary or need alteration, they can be disabled or removed altogether with the sp_control_plan_guide stored procedure.
-- Disable a plan guide
sp_control_plan_guide @operation = 'disable', @name = @my_plan_guide;
-- Remove a plan guide
sp_control_plan_guide @operation = 'drop', @name = @my_plan_guide;
It’s essential to review the impact of these changes on query performance to ensure that any performance regressions are addressed promptly.
Conclusion
Plan guides offer SQL Server users a sophisticated means of influencing query performance without altering application code. Their savvy use can mitigate performance problems derived from query optimization issues. Although potent, they should be applied with care, considering the impact they have on the system. With this guide, database professionals can better understand their purpose, implementation, and best utilization practices.
Remember, a stitch in time saves nine, and a well-implemented plan guide may just prevent a performance decline!
