Degree of Parallelism Feedback in SQL Server 2025

Degree of Parallelism (DOP) feedback is a feature in SQL Server 2025 that automatically tunes the number of threads used for query execution, optimizing performance and resource usage. This feature is now generally available and enabled by default in SQL Server 2025 as well as in Azure SQL Database.

How Degree of Parallelism (DOP) Feedback Works -

Dynamic Adjustment: DOP feedback dynamically adjusts the degree of parallelism for queries based on actual runtime performance metrics such as CPU time and elapsed time.

Self-Tuning: If a query consistently underperforms due to excessive parallelism (e.g., too many threads causing overhead), DOP feedback will reduce the DOP for future executions of that specific query.

No Manual Tuning Required: This process is automatic, eliminating the need for DBAs to manually adjust parallelism settings for recurring queries.

Persistent Feedback: The feedback and adjustments are stored in the Query Store, ensuring that optimizations persist across query executions and server restarts (except on readable secondaries, where persistence is not yet available).

To enable DOP feedback, enable the DOP_FEEDBACK database scoped configuration in a database. For example, in the user database:

ALTER DATABASE SCOPED CONFIGURATION SET DOP_FEEDBACK = ON;

To disable DOP feedback at the database level, use the DOP_FEEDBACK database scoped configuration. For example, in the user database:

ALTER DATABASE SCOPED CONFIGURATION SET DOP_FEEDBACK = OFF;

Key Benefits -

Improved Scalability: By preventing excessive parallelism, SQL Server 2025 can handle more concurrent workloads efficiently.

Reduced Resource Contention: DOP feedback helps avoid scenarios where too many threads compete for CPU, leading to better overall system performance.

Automatic Tuning: The system continuously learns and adapts to workload patterns, providing optimal parallelism settings without manual intervention.

Example Scenario -

If a recurring query is assigned a high degree of parallelism but consistently experiences performance bottlenecks (e.g., high CPU waits), DOP feedback will automatically lower the DOP for that query in future runs, improving efficiency and reducing unnecessary thread usage.

Additional Enhancements in SQL Server 2025

Intelligent Thread Allocation: SQL Server 2025 further improves parallelism by dynamically allocating threads based on real-time system resource availability, maximizing throughput and minimizing contention.

Integration with Other IQP Features: DOP feedback works alongside other intelligent query processing features like memory grant feedback and cardinality estimation feedback for holistic query optimization.




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