Parameter sniffing is a common performance issue in SQL Server databases, where the execution plan for a query is optimized for the parameter values used in the initial execution, but may not be optimal for subsequent invocations with different parameter values. This can lead to decreased query performance and resource contention.

Fortunately, SQL Server provides a powerful tool called the Query Store, starting from SQL Server 2016, to help identify and resolve parameter sniffing problems. The Query Store captures and retains query execution plans along with their related performance metrics, allowing database administrators to analyze and troubleshoot performance issues.

In this blog post, we will explore how to utilize the Query Store to identify and resolve parameter sniffing problems, step-by-step.

Table of Contents

1. Introduction to parameter sniffing
2. Introduction to the SQL Query Store
3. Enabling the Query Store on a database
4. Monitoring query performance using the Query Store
5. Identifying parameter sniffing problems
6. Resolving parameter sniffing issues
7. Best practices for using the Query Store

Step 1: Enabling the Query Store on a Database

To start using the Query Store, you need to enable it on the database you want to monitor. This can be done using either Transact-SQL or SQL Server Management Studio (SSMS).

-- Enabling the Query Store using Transact-SQL
ALTER DATABASE YourDatabase SET QUERY_STORE = ON;

-- Enabling the Query Store using SSMS
1. Right-click on the database in SSMS
2. Select "Properties" from the context menu
3. Navigate to the "Query Store" tab
4. Check the "Enable" checkbox and click "OK"

It is important to note that enabling the Query Store does have some overhead, so it’s recommended to monitor the impact on system performance before enabling it in a production environment.

Step 2: Monitoring Query Performance using the Query Store

Once the Query Store is enabled, it starts capturing and retaining query execution plans and performance statistics. You can use the Query Store Reports or execute queries against the Query Store views to monitor query performance.

-- Query to retrieve top resource-consuming queries from the Query Store
SELECT TOP 10 query_stats.query_id, query_stats.plan_id, query_stats.total_worker_time
FROM sys.query_store_query query_stats
ORDER BY query_stats.total_worker_time DESC;

Step 3: Identifying Parameter Sniffing Problems

To identify parameter sniffing problems in the Query Store, you can look for queries that have different execution plans for different parameter values. Examine the execution plans and check if there are significant differences in resource usage or query performance.

Step 4: Resolving Parameter Sniffing Issues

Once you have identified parameter sniffing problems, there are several techniques you can use to resolve them. One common approach is to use query hints or options such as OPTIMIZE FOR UNKNOWN, which can help create a more generic execution plan.

Another technique is to use plan guides to force a specific execution plan for a query, overriding the automatic plan selection. Plan guides are particularly useful when you want to apply specific optimizations for parameter values that were not well-handled by the automatic plan selection.

It’s important to thoroughly test and validate any changes made to address parameter sniffing problems, as they can have unintended consequences on query performance.

Conclusion

The SQL Query Store is a powerful tool that can help identify and resolve parameter sniffing problems in SQL Server databases. By enabling the Query Store, monitoring query performance, and analyzing execution plans, database administrators can proactively address parameter sniffing issues and optimize the performance of their queries.

Remember to always test and validate any changes made to the execution plans to ensure they have the desired effect and do not negatively impact overall system performance.

#hashtags: #SQLServer #ParameterSniffing