Why are there performance differences when a SQL function is called from .Net app vs when the same call is made in Management Studio

Question

We are having a problem in our test and dev environments with a function that runs quite slowly at times when called from an .Net Application. When we call this function di

Answer 1

A likely cause is out of date statistics and/or parameter sniffing causing a cached query plan to be re-used that is sub-optimal.

SSMS emits pre-amble statements that you don't see, that cause the submitted query to be re-compiled each time, thus eliminating the possibility of using an incorrect cached plan.

This will update all statistics and refresh views and stored procs (but be careful about running on a Production machine):

EXEC sp_updatestats

EXEC sp_refreshview 

EXEC sp_msForEachTable 'EXEC sp_recompile ''?'''