Returning NULL on NULL Input In UDFs

Jonathan Kehayias shows us a performance improvement you can get if your user-defined function is expected to return NULL if you pass in NULLs for inputs:

I was really curious about the RETURNS NULL ON NULL INPUT function option so I decided to do some testing. I was very surprised to find out that it’s actually a form of scalar UDF optimization that has been in the product since at least SQL Server 2008 R2.
It turns out that if you know that a scalar UDF will always return a NULL result when a NULL input is provided then the UDF should ALWAYS be created with the RETURNS NULL ON NULL INPUT option, because then SQL Server doesn’t even run the function definition at all for any rows where the input is NULL – short-circuiting it in effect and avoiding the wasted execution of the function body.

The more often you pass in NULL to that function, the better your performance will be relative to the default case.