SQL Server : Query Store

The Query Store feature in SQL Server is a powerful tool for monitoring and managing database performance. It was introduced in SQL Server 2016 and has continued to be enhanced in subsequent versions, including SQL Server 2022. Query Store captures a history of queries, plans, and runtime statistics, providing deep insights into the performance of your workloads. It helps in identifying performance regressions, understanding query performance patterns, and providing a mechanism to force plan choices when necessary.

Key Features of Query Store

1. Query Performance Insights:

   - Query Store captures and retains the query text, query plans, and runtime statistics for all queries executed on the database. This historical data helps in understanding how queries are performing over time.

   - It stores different versions of query plans for the same query, allowing analysis of plan changes and their impact on performance.

2. Automatic Plan Forcing:

   - If a query's performance degrades due to a change in the execution plan, Query Store can force the use of a previously good plan, thereby stabilizing performance.

   - Plan forcing can be manual or automatic, based on performance regression detection.

3. Configuration Flexibility:

   - Administrators can configure Query Store to control the amount of data it captures and retains. This includes setting retention policies, specifying which queries to capture, and defining the frequency of data collection.

4. Reports and Visualization:

   - SQL Server Management Studio (SSMS) provides built-in reports that visualize Query Store data, making it easier to identify problematic queries, analyze plan changes, and understand workload patterns.

Enabling and Configuring Query Store

To enable Query Store on a database, use the following SQL command:

ALTER DATABASE [YourDatabaseName] 

SET QUERY_STORE = ON;

You can also configure various settings for Query Store, such as the data collection mode, data retention period, and capture mode. Here's an example configuration:

ALTER DATABASE [YourDatabaseName]

SET QUERY_STORE (OPERATION_MODE = READ_WRITE, -- Enable read-write mode

                 CLEANUP_POLICY = (STALE_QUERY_THRESHOLD_DAYS = 30), -- Retain data for 30 days

                 DATA_FLUSH_INTERVAL_SECONDS = 900, -- Flush data every 15 minutes

                 MAX_STORAGE_SIZE_MB = 1024, -- Limit storage to 1 GB

                 QUERY_CAPTURE_MODE = AUTO); -- Capture queries automatically

Analyzing Query Store Data

Query Store stores its data in system views. Here are some useful views for analyzing query performance:

1. sys.query_store_query: Contains information about the queries captured by Query Store.

2. sys.query_store_plan: Contains execution plan details for each query.

3. sys.query_store_runtime_stats: Contains runtime statistics such as execution count, duration, CPU time, and logical reads.

Example Queries

Finding Top 10 Longest Running Queries:

SELECT TOP 10 

    qsqt.query_text_id,

    qsqt.query_sql_text,

    MAX(qsrs.avg_duration) AS MaxAvgDuration

FROM 

    sys.query_store_query_text AS qsqt

JOIN 

    sys.query_store_query AS qsq

    ON qsqt.query_text_id = qsq.query_text_id

JOIN 

    sys.query_store_plan AS qsp

    ON qsq.query_id = qsp.query_id

JOIN 

    sys.query_store_runtime_stats AS qsrs

    ON qsp.plan_id = qsrs.plan_id

GROUP BY 

    qsqt.query_text_id, qsqt.query_sql_text

ORDER BY 

    MaxAvgDuration DESC;

Forcing a Specific Plan for a Query:

To force a particular plan for a query, use the sp_query_store_force_plan stored procedure:

EXEC sp_query_store_force_plan 

    @query_id = 1, -- Replace with your actual query ID

    @plan_id = 2; -- Replace with the plan ID you want to force

Removing a Forced Plan:

To remove a forced plan and let SQL Server choose the optimal plan, use the following command:

EXEC sp_query_store_unforce_plan 

    @query_id = 1; -- Replace with your actual query ID

Best Practices

1. Monitor Storage Usage:

   - Regularly monitor the storage used by Query Store to ensure it doesn't exceed the allocated space.

2. Use Appropriate Capture Mode:

   - For OLTP systems with a large number of queries, consider using the "AUTO" capture mode to limit data collection to significant queries.

3. Regular Cleanup:

   - Set appropriate cleanup policies to manage the retention of historical data and prevent excessive storage usage.

4. Leverage Built-in Reports:

   - Use the built-in reports in SSMS for a visual analysis of Query Store data. This helps in quickly identifying performance issues and trends.

Conclusion

Query Store is a valuable feature for maintaining and optimizing the performance of SQL Server databases. It provides the necessary insights and tools to diagnose and resolve performance issues, ensuring a stable and efficient database environment.