Related functions
If your user-defined function compares or acts as a filter for two instances of the same data type and returns a Boolean result, you can specify related functions to optimize the execution of the function when it is called in the WHERE clause of a SELECT statement.
Important: Related functions must exist before you can
choose them. You can create them before you create the function they
are related to, or you can update the original routine to add related
routines after you create them.