create or replace function idxF(status IN char) return char deterministic is
retVal CHAR(1);
begin
  dbms_output.put_line('P');
  retVal:=CASE status
  when 'P' then 'P'
  when 'H' then 'H'
  else null
  end  ;
  return retVal;
end idxF;
  create index setIndexOnStatus on ORDER(idXF(STATUS));

  SQL Error: ORA-00904: "IDXF": invalid identifier

create index setIndexOnStatus on "ORDER"(idXF(STATUS));