Parameter sniffing is a common issue that can affect the performance of your SQL Server queries. It occurs when the execution plan generated for a query is optimized based on the first set of parameters passed to the query, which may not be representative of the overall data distribution.

To help identify and resolve parameter sniffing issues, SQL Server introduced the Query Store feature. The Query Store captures query execution plans and performance metrics, allowing you to analyze and troubleshoot query performance problems.

In this blog post, we will explore some techniques for troubleshooting and resolving parameter sniffing issues using the Query Store.

Table of Contents

1. What is parameter sniffing?
2. Enabling the Query Store
3. Identifying parameter sniffing issues
4. Forcing query plan recompilation
5. Using query hints to resolve parameter sniffing
6. Updating statistics
7. Conclusion

What is parameter sniffing?

Parameter sniffing is a behavior in SQL Server where the initial execution plan for a query is optimized based on the first set of parameters passed to the query. This can be problematic if the data distribution or specific parameter values change over time, leading to suboptimal query performance.

Enabling the Query Store

Before you can use the Query Store feature to troubleshoot parameter sniffing issues, you need to ensure that it is enabled for your database. The Query Store can be enabled at the database level using the following command:

ALTER DATABASE [YourDatabaseName]
SET QUERY_STORE = ON;

Identifying parameter sniffing issues

Once the Query Store is enabled, you can start identifying parameter sniffing issues by analyzing the query execution plans and performance metrics captured in the Query Store.

You can use the following SQL query to identify queries with parameter sniffing issues:

SELECT
    rs.plan_id,
    qt.query_sql_text,
    qp.plan_compile_time,
    qp.total_worker_time,
    qp.count_executions
FROM
    sys.query_store_query q 
    INNER JOIN sys.query_store_plan qp ON q.plan_id = qp.plan_id
    INNER JOIN sys.query_store_runtime_stats rs ON qp.plan_id = rs.plan_id
CROSS APPLY 
    sys.dm_exec_sql_text(qt.query_sql_text_id) qt
WHERE
    qp.is_forced_plan = 0
    AND qp.plan_id IN (SELECT plan_id FROM sys.query_store_runtime_stats WHERE rs.count_executions > 10)
ORDER BY
    rs.count_executions DESC

Forcing query plan recompilation

One approach to resolving parameter sniffing issues is to force SQL Server to recompile the query plan every time it is executed. You can do this by adding the OPTION(RECOMPILE) query hint to your SQL statement.

For example:

SELECT *
FROM YourTable
WHERE YourColumn = @Parameter
OPTION(RECOMPILE)

By forcing plan recompilation, SQL Server will generate a new execution plan for each query execution based on the current parameter values. This can help mitigate parameter sniffing issues caused by a suboptimal execution plan.

Using query hints to resolve parameter sniffing

Another approach to resolving parameter sniffing issues is to use query hints to influence the execution plan chosen by SQL Server.

For example, you can use the OPTIMIZE FOR query hint to specify a specific parameter value that will be used during plan compilation:

SELECT *
FROM YourTable
WHERE YourColumn = @Parameter
OPTION(OPTIMIZE FOR (@Parameter = 'SpecificValue'))

By specifying a specific value with OPTIMIZE FOR, you can enforce a more suitable execution plan for that specific parameter value, potentially improving overall query performance.

Updating statistics

Outdated statistics can also contribute to parameter sniffing issues. SQL Server uses statistics to estimate the number of rows in a table and make informed decisions about query execution plans. If the statistics are no longer accurate, it can lead to suboptimal plans.

To resolve this, you can update statistics for the tables involved in the query using the UPDATE STATISTICS command:

UPDATE STATISTICS YourTable

By updating statistics, you provide SQL Server with accurate information about the data distribution, allowing it to generate more optimal query plans.

Conclusion

The Query Store feature in SQL Server provides valuable insights into query performance and can help identify and troubleshoot parameter sniffing issues. By enabling the Query Store, you can leverage its capabilities to analyze execution plans, identify problematic queries, and apply appropriate techniques like recompiling query plans, using query hints, and updating statistics to resolve parameter sniffing issues.

By taking these steps, you can improve the performance of your SQL Server queries and ensure optimal execution plans are used, reducing the impact of parameter sniffing.

#hashtags: #QueryStore #ParameterSniffing