Battling Inefficient SQL Queries: A Deep Dive into Optimization ( Part 4 of 16 )

Battling Inefficient SQL Queries: A Deep Dive into Optimization

Summary

Inefficient queries can significantly impact SQL Server's performance, often overloading the disk subsystem. Even in systems with sufficient memory to cache data in the buffer pool, these queries can burn CPU, increase blocking, and ultimately degrade the customer experience.

SQL Server tracks execution metrics for each cached plan and makes them available through the `sys.dm_exec_query_stats` view. Additionally, you can retrieve execution statistics for stored procedures, triggers, and user-defined scalar functions using the `sys.dm_exec_procedure_stats`, `sys.dm_exec_trigger_stats`, and `sys.dm_exec_function_stats` views, respectively. However, it’s important to note that plan cache-based execution statistics do not track runtime execution metrics for plans not cached. This should be factored into your analysis and query-tuning process.

You can capture inefficient queries in real-time using Extended Events and SQL Traces. While both approaches can introduce overhead, especially in busy systems, they provide raw data that requires processing and aggregation for further analysis. In SQL Server 2016 and later, the Query Store is a valuable tool for pinpointing plan regressions without relying on the plan cache. Though it adds some overhead, this is often acceptable and manageable with monitoring. Lastly, third-party monitoring tools can also be used to find inefficient queries; however, it’s essential to research these tools thoroughly to understand their limitations and potential overhead.

Troubleshooting Checklist

1. Get the List of Inefficient Queries :

- Use the `sys.dm_exec_query_stats` view to identify inefficient queries.

2. Sort Data According to Your Troubleshooting Strategy :

- Prioritize your analysis based on CPU, I/O, or other relevant metrics.

3. Detect the Most Expensive Stored Procedures

- Utilize the `sys.dm_exec_procedure_stats` view to find resource-intensive stored procedures.

4. Consider Enabling the Query Store :

- If feasible, enable the Query Store to collect and analyze data. This is particularly useful if you are not already using external monitoring tools.

5. Enable Trace Flags T7745 and 7752 :

- These flags can improve SQL Server shutdown and startup performance when using the Query Store.

6. Analyze Data from Third-Party Monitoring Tools :

- Cross-check the data with SQL Server metrics to ensure accuracy and comprehensiveness.

7. Assess the Overhead of Inefficient Queries :

- Determine the resource consumption of these queries and their impact on system performance.

8. Correlate Resource Consumption with Wait Statistics and Server Load :

- Analyze how the queries affect overall system load and wait times.

9. Optimize Queries as Needed :

- Based on your analysis, optimize the inefficient queries to improve performance.

By following this troubleshooting checklist, you can systematically identify and address inefficient queries, enhancing SQL Server performance and reducing system overload. Stay tuned for more insights as we explore the art of detecting and optimizing queries in the next chapter. Happy optimizing!