Turning Off Parameter Sniffing in SQL Server

Hello Dev! Welcome to this article about turning off parameter sniffing in SQL Server. If you’ve been struggling with performance issues in your SQL Server, then you’ve probably heard about parameter sniffing. This is a process that allows SQL Server to optimize queries based on specific parameter values. However, sometimes this optimization can lead to poor performance, and that’s when you need to consider turning it off.

What is Parameter Sniffing?

Parameter sniffing is a feature in SQL Server that allows the optimizer to generate an optimized query plan based on the values of a query’s parameters. It uses the first set of parameter values used to generate the plan, and assumes they will be similar for future executions. This can result in faster query performance in some cases. However, when the parameter values change significantly over time, this can lead to poor query performance.

How Does Parameter Sniffing Affect Performance?

When the optimizer generates a plan based on a specific set of parameter values, it may not be the most efficient plan for other parameter values. This can result in slower query performance, especially when the optimizer has chosen a suboptimal plan.

For example, if you have a stored procedure that gets called with different parameter values, and the optimizer chooses a plan based on the first set of values used, this plan may not be optimal for other parameter values. If subsequent calls to the stored procedure use different parameter values that require a different execution plan, SQL Server may have to recompile the stored procedure and generate a new execution plan. This can result in slower query performance, as well as increased CPU and memory usage.

In some cases, parameter sniffing can even cause excessive blocking and deadlocks. This can occur when multiple sessions are executing the same query, but with different parameter values. If the optimizer has chosen a plan that requires exclusive locks on certain tables, this can cause blocking and deadlocks when other sessions try to access those same tables.

How to Turn Off Parameter Sniffing

Now that you understand how parameter sniffing can affect performance, let’s look at how to turn it off. There are a few different methods you can use, depending on your specific situation.

Method 1: Use Local Variables Instead of Parameters

One way to turn off parameter sniffing is to use local variables instead of parameters. This can be done by declaring input parameter values as local variables within the procedure or query, and then using the local variables in the query instead of the input parameters.

Input Parameter	Local Variable
@customerId int	DECLARE @localCustomerId int = @customerId
@startDate datetime	DECLARE @localStartDate datetime = @startDate

By using local variables, you prevent SQL Server from using the parameter sniffing process to optimize the query plan. Instead, SQL Server generates a new plan for each execution of the procedure, based on the values of the local variables.

Method 2: Use Query Hints

Another way to turn off parameter sniffing is to use query hints. Query hints are special instructions that you can add to a query to control how the optimizer generates the query plan.

One query hint that can be used to turn off parameter sniffing is OPTION (RECOMPILE). This hint tells SQL Server to recompile the query each time it is executed, rather than using a cached plan.

For example:

SELECT *FROM ordersWHERE orderDate >= @startDateOPTION (RECOMPILE)

This will cause SQL Server to generate a new plan each time the query is executed, based on the current value of @startDate. This can prevent parameter sniffing from affecting query performance.

Method 3: Use Plan Guides

A third way to turn off parameter sniffing is to use plan guides. Plan guides are a set of instructions that you can use to force SQL Server to use a specific query plan for a particular query.

You can create a plan guide that specifies a specific plan for a query, based on the current parameter values. This can prevent SQL Server from using parameter sniffing to optimize the plan.

For example:

EXEC sp_create_plan_guide@name = N'OrdersQueryGuide',@stmt = N'SELECT *FROM ordersWHERE orderDate >= @startDate',@type = N'SQL',@module_or_batch = NULL,@params = N'@startDate datetime',@hints = N'OPTION (USE PLAN N'OrdersPlan')'GO

This plan guide specifies a plan (@Hints) named OrdersPlan to be used for the specified query (@stmt) when the startDate parameter is specified (@params). This can help prevent the optimizer from using parameter sniffing to generate a suboptimal plan.

Frequently Asked Questions

Q1: What are the symptoms of parameter sniffing?

A1: Some symptoms of parameter sniffing include slow query performance, excessive CPU and memory usage, and blocking and deadlocks.

Q2: Can parameter sniffing be turned off globally for the entire SQL Server instance?

A2: No, parameter sniffing cannot be turned off globally for the entire SQL Server instance. However, you can use the query hint OPTION (RECOMPILE) to turn off parameter sniffing for individual queries.

Q3: Will turning off parameter sniffing always improve query performance?

A3: No, turning off parameter sniffing will not always improve query performance. In some cases, parameter sniffing may be beneficial for query performance. You should test the performance of your queries with and without parameter sniffing to determine which approach is best.

Q4: Are there any downsides to turning off parameter sniffing?

A4: Yes, there are downsides to turning off parameter sniffing. When you turn off parameter sniffing, SQL Server generates a new plan for each execution of the query, which can increase CPU and memory usage. It can also cause excessive plan cache fragmentation, which can slow down query performance.

Q5: Can I use local variables instead of parameters in all cases to avoid parameter sniffing?

A5: No, using local variables instead of parameters may not always be feasible or desirable. Local variables can only be used within the scope of the procedure or query, and they may not be visible to other parts of the application. In addition, using local variables can sometimes make the query harder to read and maintain.

Thank you for reading this article, Dev. We hope you found it informative and helpful. If you have any questions or feedback, please let us know in the comments below.