When it comes to optimizing the performance of SQL Server, one concept that often comes up is parameter sniffing. But what exactly is parameter sniffing and how does it affect memory usage in SQL Server?

Parameter sniffing is SQL Server’s ability to “sniff” the value for a given parameter and cache an optimal execution plan based on the row estimates for that parameter value. This means that when a stored procedure or parameterized query is executed for the first time, the execution plan is cached and re-used each time that stored procedure or query is called. However, this optimal plan may not be suitable for subsequent calls with different parameter values, leading to what is known as bad parameter sniffing.

So, how does this relate to memory usage in SQL Server? Every query submitted to SQL Server requires some memory, but certain queries that involve sort and/or hash operations require more memory to process these operations. This memory is allocated from the Buffer Pool, and memory grants can take up to 75% of the buffer pool.

When a query is submitted to SQL Server without a plan in cache that it can re-use, SQL Server relies on statistics to estimate how much memory the sort or hash operation requires. However, if the query has a plan in cache that it can re-use, the amount of memory granted will be the same as the amount granted for the first call to the procedure, regardless of whether it is enough or not. This is where parameter sniffing comes into play.

If the memory grant turns out to be insufficient, the operation will spill to TempDB, which is a much slower workspace than in-memory. This happens because once a query receives its memory grant, it cannot request more. This can lead to performance issues such as sort or hash warnings in SQL Profiler, decreased performance of stored procedures, IO waits in TempDB, and high internal object allocations in TempDB.

So, what can be done to address bad parameter sniffing? There are a few options that can be explored:

• Alter the stored procedure to include a RECOMPILE hint: This tells SQL Server not to cache a plan for the stored procedure.
• Alter the stored procedure to include an OPTIMIZE FOR hint: This tells SQL Server to cache a plan based on a specific parameter value.
• Trace flag 4136: This turns off parameter sniffing altogether.

However, it is important to thoroughly research and test these options before implementing them as a fix. It is crucial to understand the implications of bad parameter sniffing in terms of SQL Server granting too little or too much memory for sort and hash operations.

By understanding parameter sniffing and its impact on memory usage in SQL Server, you can optimize your queries and stored procedures to improve overall performance. Remember to carefully analyze and test any changes before implementing them in a production environment.

We hope this insight into parameter sniffing and memory usage in SQL Server has been helpful. Stay tuned for more articles on SQL Server optimization and best practices!