Leveraging SQL Server’s Memory Grant Feedback to Improve Query Performance
Introduction
Performance optimization is critical for any application or system relying on databases, and SQL Server plays a pivotal role in managing relational data for countless enterprises. As data grows and query complexity increases, performance issues often emerge, and database administrators continuously seek ways to enhance efficiency. In recent years, SQL Server introduced a compelling feature named Memory Grant Feedback, which helps optimize query performance dynamically. This article provides a detailed guide on understanding and leveraging Memory Grant Feedback to achieve a more performant SQL Server experience.
Understanding SQL Server Memory Grante
Before delving into Memory Grant Feedback, it’s essential to comprehend the concept of memory grants in SQL Server. A memory grant is a portion of memory allocated to a query for processing sort, hash, and other memory-intensive operations. Insufficient memory grants can lead to a condition known as spillage, where SQL Server needs to use tempdb, a system database, to perform these operations, resulting in a potential performance bottleneck. On the contrary, an excessive memory grant can waste valuable SQL Server resources and reduce overall concurrency by tying up memory that other queries could use more efficiently.
The ideal scenario is where SQL Server allocates just the right amount of memory for the workload at hand. Traditionally, this has been determined by the SQL Server Query Optimizer through a mechanism that estimates the memory required, based on the statistics of the involved tables and indexes. However, these estimations can occasionally be inaccurate, leading to either undersized or oversized grants, adversely affecting query performance.
What is SQL Server Memory Grant Feedback?
Introduced in SQL Server 2017 (starting with version 1400 compatibility level) under the Automatic Tuning feature, Memory Grant Feedback is a process where SQL Server adjusts the memory grant size for subsequent executions of a cached query plan based on the actual memory used by previous executions. This innovation is part of SQL Server’s broader push towards intelligent performance features that automate tuning tasks and lighten the load on database administrators. Memory Grant Feedback is available both in on-premises SQL Server installations and in Azure SQL Database.
How does Memory Grant Feedback work? When a query is executed, SQL Server tracks the actual memory consumed. If the memory grant was too low (causing spillage) or too high (wasting resources), SQL Server will adjust the memory grant size for the next execution of that query. This adjustment seeks to prevent spillage by increasing memory allocation or improving system concurrency by decreasing allocation, depending on the situation. SQL Server will continue to adjust the memory grants dynamically over time as the characteristics of the query executions change, which can be due to updates in underlying data or index structures.
This dynamic adjustment works differently for batch and row mode executions. With SQL Server 2019 (starting with version 1500 compatibility level), Memory Grant Feedback became available for both batch and row mode queries. Before this, the feature only supported batch mode queries.
How to Enable Memory Grant Feedback
Enabling Memory Grant Feedback is relatively straightforward. It’s part of the Automatic Tuning options that you can configure at the database level. Automatic Tuning needs to be enabled on the SQL Server where the database resides. Here’s how to turn on the Memory Grant feedback for a database:
ALTER DATABASE SET AUTOMATIC_TUNING ( FORCE_LAST_GOOD_PLAN = ON );
After the above statement is executed, SQL Server will automatically enable Memory Grant Feedback if the appropriate conditions arise for a query and it’s running on SQL Server 2017 or newer with the applicable compatibility level.
Note: Automatic Tuning options can also be enabled and managed through SQL Server Management Studio (SSMS) or Azure portal for Azure SQL Database users by navigating to the Database settings and locating the Automatic tuning section.
Monitoring Memory Grant Feedback Activity
To monitor the Memory Grant Feedback, SQL Server provides several dynamic management views (DMVs). One of the fundamental DMVs for observing the effects of Memory Grant Feedback is sys.dm_exec_query_memory_grants. This DMV offers real-time insights into the memory grants for currently executing queries.
SELECT
session_id,
plan_id,
query_id,
memory_granted_kb,
ideal_memory_kb,
query_plan_hash
FROM sys.dm_exec_query_memory_grants;Another powerful tool is the Query Store, which retains a history of query execution details, including memory grant information. The Query Store collects and displays data on how Memory Grant Feedback has adjusted memory grants for individual queries over time. This can be highly valuable when troubleshooting performance issues or confirming whether the Memory Grant Feedback is causing a tangible impact on your queries.
When analyzing the impact of Memory Grant Feedback on the query performance, it’s vital to compare the memory grants before and after the feature’s interventions. A reduced frequency of spillage in tempdb and an increased system concurrency without running out of memory are indicators that Memory Grant Feedback is making positive adjustments.
Best Practices and Considerations
While Memory Grant Feedback is a powerful feature for improving query performance in SQL Server, there are best practices and some considerations to keep in mind:
Use the latest available update of SQL Server to benefit from ongoing improvements to the Automatic Tuning feature and Memory Grant Feedback.
Ensure that the compatibility level of your database aligns with the feature requirements (1400 for SQL Server 2017 for batch mode and 1500 for SQL Server 2019 for both batch and row mode execution).
Maintain updated statistics for your tables and indexes. This helps the Query Optimizer to make better initial estimations for memory grants.
Monitor your system regularly to confirm that Memory Grant Feedback is having a positive impact and not leading to any adverse effects due to incorrect adjustments.
Understand that Memory Grant Feedback is just one of multiple performance optimization features within SQL Server. It should be combined with other best practices such as index optimization, query refactoring, and configuration management for best results.
Keep in mind that certain scenarios like cold starts or ad-hoc queries that don’t benefit from plan caching might not gain the same level of benefit from this feature.
Conclusion
Memory Grant Feedback in SQL Server represents a step forward in the journey towards performance optimization through intelligent automation. By dynamically adjusting memory grants based on actual runtime data, this feature unlocks better resource utilization and improved query performance. While using Memory Grant Feedback, database administrators should monitor its outcomes, maintain the supporting environment soundly, and stay informed on the latest SQL Server developments to maximize its potential fully. With proper implementation and management, leveraging Memory Grant Feedback can contribute significantly to smoother and faster data operations, ultimately leading to a more reliable and efficient database environment.
