// gets pointer to function via pg_operator
SelectSortFunction(Oid sortOperator,
                   bool nulls_first,
                   Oid *sortFunction,
                   int *sortFlags);

// returns negative, zero, positive
ApplySortFunction(FmgrInfo *sortFunction,
                  int sortFlags,
                  Datum datum1, bool isNull1,
                  Datum datum2, bool isNull2);