Introduction
Intelligent Query Processing (IQP) is a suite of advanced features introduced in SQL Server 2017 and enhanced in subsequent versions, including SQL Server 2022. By upgrading to SQL Server 2022 and setting your database compatibility level to 160, you enable capabilities that dynamically optimize query performance. Understanding when these performance improvements take effect can help database administrators and developers plan and manage their optimization strategies effectively. The journey toward maximum performance improvement is gradual and influenced by factors such as workload characteristics and system configurations.
Initial Improvements (Within Days to Weeks)
After enabling compatibility level 160, you may notice immediate performance enhancements. Features like Memory Grant Feedback (MGF) and Parameter Sensitive Plan Optimization (PSPO) start to take effect almost instantly.
Memory Grant Feedback (MGF)
Memory Grant Feedback dynamically adjusts the memory allocated for query execution based on feedback from previous executions. Initially, a query might request too much or too little memory. After each execution, MGF learns from the actual memory usage and adjusts future memory grants accordingly, reducing wasted resources and improving performance. In SQL Server 2022, MGF extends its benefits to support row mode execution, enhancing a wider range of queries beyond the batch mode support in earlier versions.
Example: Consider a scenario where a report generates a large dataset and initially requests excessive memory, leading to resource contention. With MGF, subsequent executions receive a more accurate memory allocation, minimizing contention and speeding up query execution.
Parameter Sensitive Plan Optimization (PSPO)
Parameter Sensitive Plan Optimization addresses the common issue of parameter sniffing by allowing the optimizer to create and store multiple execution plans for a single parameterized query, each tailored to different parameter values. This means queries begin to execute with plans optimized for their specific parameters, enhancing performance without requiring code changes. PSPO is a new feature in SQL Server 2022, offering immediate benefits upon upgrading.
Example: In a sales database, queries filtering data based on different regions can benefit from PSPO. Each region might have vastly different data distributions, and with PSPO, execution plans are optimized for each region’s data, resulting in faster query responses compared to using a generic plan.
Deferred Compilation of Table Variables
Deferred Compilation of Table Variables was introduced in SQL Server 2019 (compatibility level 150). This feature improves how table variables are handled by deferring their compilation until actual row counts are known. This leads to more accurate cardinality estimations and better execution plans right from the first execution, especially benefiting workloads that heavily utilize table variables.
Example: A query using a table variable that initially holds a small number of rows but grows significantly during operations benefits from deferred compilation. The execution plan adapts to the actual data volume, enhancing performance from the outset.
Performance Stabilization (Within Weeks)
As your system continues to run under the new compatibility level, the optimizer collects more execution data, allowing other IQP features to refine their effectiveness.
Adaptive Joins
Adaptive Joins enable the query processor to choose the most efficient join strategy (nested loops or hash joins) during the execution of a query based on the actual number of rows processed. This decision happens in real-time within a single execution, not over several executions. Introduced in SQL Server 2017 (compatibility level 140), Adaptive Joins provide immediate adaptability to varying data volumes.
Example: A query joining a large table with a small one can automatically choose the most efficient join type during execution, optimizing performance.
Automatic Plan Correction
Automatic Plan Correction leverages the Query Store to automatically detect and correct execution plan regressions. If the optimizer identifies that a plan is consistently underperforming, it can force the previous, better-performing plan without manual intervention. Introduced in SQL Server 2017 and enhanced in later versions, this feature becomes more effective as more execution data is collected.
Example: If a frequently run query experiences degraded performance due to a new execution plan, Automatic Plan Correction reverts to the last known good plan, reducing execution time and resource usage without requiring manual troubleshooting.
Long-Term Optimization (1–3 Months)
Over time, as the system continues to process queries, the optimizer and IQP features further fine-tune performance.
Query Store Tuning
The Query Store collects historical query performance data. Over time, it builds a comprehensive performance baseline, allowing features like Automatic Plan Correction to make informed decisions about optimizing query plans. This process may take one to three months for complex workloads, as sufficient data needs to be accumulated to identify and correct performance issues effectively.
Example: Over several months, the Query Store identifies that certain queries consistently perform better with alternative execution plans and automatically applies these corrections, ensuring sustained performance improvements.
Managing Expectations: Patience Is Key
While it’s natural to anticipate immediate and dramatic performance improvements after enabling compatibility level 160 and IQP features, it’s important to maintain realistic expectations.
Avoiding Unrealistic Expectations
- Short-Term Observations: You might not witness massive improvements within the first hour, four hours, or even the first day of enabling IQP features. The optimizer needs time to analyze query patterns, collect execution data, and adjust plans accordingly.
- Gradual Benefits: Performance enhancements typically become more noticeable over the course of a week, two weeks, or even a month. As the system processes more queries, IQP features like MGF, PSPO, and Adaptive Joins become increasingly effective.
- Long-Term Gains: The most substantial and sustained performance improvements are often realized after one to three months. This period allows the Query Store to build a comprehensive performance baseline and for the optimizer to fully leverage all IQP features.
Example: A database experiencing varied query loads may not show immediate performance gains. However, over several weeks, as IQP features adapt to the workload, noticeable improvements in query execution times and resource utilization will emerge.
Monitoring and Continuous Improvement
To fully benefit from IQP features, it’s essential to monitor system performance continuously and allow adequate time for the optimizer to adjust. Implementing regular performance reviews and leveraging tools like Query Store can help track the progressive improvements and ensure that the system is evolving toward optimal performance.
Conclusion
While you may observe immediate performance gains after enabling compatibility level 160 and the associated IQP features, the full spectrum of improvements unfolds over time. Managing expectations is key; significant performance improvements may take weeks to months to materialize. By understanding the timeline of when query optimization features kick in and allowing the system time to adjust, you can fully leverage the capabilities of SQL Server 2022’s Intelligent Query Processing. This leads to sustained and meaningful performance improvements, allowing your SQL Server to handle workloads more efficiently and reliably.