Fine-Tuning SQL Server’s Query Store for Maximum Performance Gains
SQL Server is a highly complex database management system, offering a multitude of features designed to store, retrieve, and manage data efficiently. One of the essential tools provided by SQL Server for optimizing query performance is the Query Store. Since its introduction in SQL Server 2016, the Query Store has provided database administrators and developers an easier way to track query performance, identify issues, and make the necessary adjustments to optimize SQL queries. This blog entry aims to provide a comprehensive analysis of how the Query Store can be fine-tuned to achieve maximum performance gains.
Understanding the Query Store
The Query Store functions by capturing a history of queries, plans, and runtime statistics. It is essentially a ‘flight data recorder’ for your SQL Server, storing data that can be used to understand query performance trends, find regressions, and ensure the server runs optimally. The tool enables SQL administrators to fix issues quickly by forcing the use of more efficient query plans or recognizing when performance anomalies occur.
Benefits of Using Query Store
- Historical data collection to identify performance trends.
- Query performance troubleshooting is easier and faster.
- Plan regression mitigation by forcing optimal query plans.
- Performance overload prevention with automatic query capture.
- Improved insight with integrated reporting capabilities.
Setting Up the Query Store
Enabled at the database level, the Query Store must be configured with specific options to start collecting data. The process is straightforward:
-- Enable Query Store with default options
ALTER DATABASE [YourDatabaseName] SET QUERY_STORE = ON;
-- Query Store Settings example
ALTER DATABASE [YourDatabaseName] SET QUERY_STORE (OPERATION_MODE = READ_WRITE, CLEANUP_POLICY = (STALE_QUERY_THRESHOLD_DAYS = 14), DATA_FLUSH_INTERVAL_SECONDS = 900, INTERVAL_LENGTH_MINUTES = 60, MAX_STORAGE_SIZE_MB = 1024, QUERY_CAPTURE_MODE = AUTO, MAX_PLANS_PER_QUERY = 200);
Each setting affects how the Query Store captures and retains data, and these must be carefully managed to optimize the Query Store’s efficiency, minimize performance impacts, and ensure valuable data is not lost.
Fine-Tuning Query Store Settings
Once the Query Store is enabled, fine-tuning various settings can drastically enhance its effectiveness and server performance.
OPERATION_MODE
This option can be set to READ_ONLY or READ_WRITE. Most often, the Query Store should be set to READ_WRITE to continuously collect and utilize performance data. However, setting it to READ_ONLY could be useful during performance reviews of historical data without capturing new data.
DATA_FLUSH_INTERVAL_SECONDS
This setting dictates the frequency at which Query Store data is written to disk. Lower settings reduce the risk of data loss on a crash but can increase I/O write operations. Adjusting this requires balancing immediacy of persisted data against potential performance overhead.
CLEANUP_POLICY
The Query Store retains data based on the duration defined in this setting. It’s crucial to set this high enough to capture relevant performance trends but not so high that it bloats the storage footprint unnecessarily. A typically recommended starting point is 30 days.
MAX_STORAGE_SIZE_MB
This setting limits the size of the Query Store data. Once this limit is reached, the Query Store will cease capturing new data unless OPERATION_MODE is set to AUTO, which transitions to read-only mode when the size limit is reached.
QUERY_CAPTURE_MODE
With options of NONE, AUTO, or ALL, this setting controls the volume of query execution data collected. AUTO mode intelligently captures only significant plan changes and queries that differ in performance, whereas ALL can lead to quicker storage size saturation with the increased volume of captured queries.
STALE_QUERY_THRESHOLD_DAYS
Queries that have not been executed within the time specified will be considered stale, and their statistics and plans removed from the Query Store. Adjusting this value should reflect the typical lifecycle of queries in your application, avoiding deletion of rarely executed but still relevant queries.
SIZING QUERY STORE
Determining the accurate initial size for the Query Store can eliminate the need for frequent adjustments and potential performance impacts. Monitoring can help forecast the appropriate size by observing Query Store’s storage consumption initially set on conservative estimates.
Identifying Performance Issues with Query Store
Identifying and resolving performance issues is an integral aspect of Query Store use. Routine monitoring of reports provided by the Query Store can bring attention to problematic queries.
Top Resource Consuming Queries
Identifying queries with the highest resource usage is the first step in troubleshooting. This includes queries that use the most CPU, I/O, or run for the longest times. Focusing on these can yield the most significant performance improvements.
Queries with Regressed Plans
SQL Server can sometimes choose a suboptimal plan for a query. Using the Query Store, you can spot such regression by comparing past and present query plans and revert to the optimal plan, if necessary.
Query Runtime Statistics
Query execution time and other runtime statistics such as execution count can signal when performance degrades. Such metrics should be regularly reviewed to maintain an overview of system health.
Query Store Best Practices and Performance Tuning Strategies
Having an actionable strategy is essential for realizing the Query Store’s full potential.
Maintenance Plans
Query Store performance can degrade over time, necessitating a maintenance plan to defragment and optimize the internal tables that store the query information. This should be done regularly, with considerations for off-peak hours to minimize impact.
Set Appropriate Retention Settings
The duration and exactness of historical data required play into these settings. For systems with more frequent releases, shorter retention periods may be acceptable, while long-term trend analysis will require longer retention.
Use Forced Plans Judiciously
Forcing query plans can lead to immediate performance improvements, but be aware that as data distributions change, the same plan may not always be optimal. Regular review of forced plans is needed to avoid future regressions.
Monitoring and Alerts
Proactive monitoring of Query Store metrics can provide early warning to potential issues. Custom alerts can be set up within SQL Server to notify of critical performance changes, leading to faster remediations.
Query Store as a Tuning Assistant
The Query Store captures substantial amounts of data that can be overwhelming, but when used as an assistant to point you to performance inefficiencies, its value is significant. Approach the data with specific performance questions you aim to answer.
Conclusion
SQL Server’s Query Store provides a powerful platform for achieving optimized database performance. By understanding and effectively configuring various Query Store settings, database professionals can take control of query execution plans, historical data analysis, and proactive performance monitoring to achieve significant performance gains. The aforementioned practices and strategies serve as a guide for SQL Server users to tailor the Query Store to their specific system needs, ensuring that their databases perform efficiently and reliably.
Remember, while Query Store offers excellent insights and controls, it also requires attention and frequent adjustments as workload patterns change and your SQL Server environment evolves. Staying diligent will help in leveraging the maximum performance benefits from the Query Store, thus maintaining the health and productivity of your databases.
