Today, we will explore a simple yet interesting question that was asked by one of my clients during a Comprehensive Database Performance Health Check. The question was about how to have multiple optimize hints for a single query in SQL Server. Let’s dive into this topic and learn more about it.

Before we proceed, it would be helpful to understand the concept of parameter sniffing. Parameter sniffing refers to the behavior of SQL Server where it generates an execution plan based on the first set of parameters passed to a query or stored procedure. This can sometimes lead to performance issues when subsequent executions of the same query or stored procedure use different parameter values.

In previous blog posts, we have discussed various techniques to overcome parameter sniffing, such as recompiling stored procedures, using local variables, leveraging the OPTIMIZE FOR UNKNOWN query hint, and utilizing the DATABASE SCOPED CONFIGURATION feature. These techniques can help improve query performance by addressing parameter sniffing related issues.

Now, let’s focus on the main topic of this blog post – optimizing queries with multiple optimize hints. To illustrate this, consider the following example:

SELECT *
FROM items
WHERE ProductName = @ProductName AND StockNumber = @StockNumber
OPTION ( OPTIMIZE FOR (@ProductName = 'Toy', @StockNumber UNKNOWN) );

In the above query, we have two different parameters – @ProductName and @StockNumber. To optimize the query for these parameters, we provide a hint using the OPTIMIZE FOR clause. Notice that the hint for each parameter is different, as they represent different statistics and data types.

By using multiple optimize hints, we can guide the SQL Server query optimizer to generate an execution plan that is specifically optimized for the given parameter values. This can greatly improve query performance and ensure that the query performs optimally for different combinations of parameter values.

It is important to note that the use of optimize hints should be done judiciously and only when necessary. In most cases, SQL Server’s query optimizer is capable of generating efficient execution plans without the need for explicit hints. However, in certain scenarios where parameter sniffing issues persist, using optimize hints can be a valuable tool in optimizing query performance.

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