create table emp1 as select * from emp
/
create or replace
  function one_row_only(
                        p_schema varchar2,
                        p_tab varchar2
                       )
    return varchar2
    is
    begin
        return 'rownum = 1';
end;
/
begin
    dbms_rls.add_policy(
                        object_schema => 'SCOTT',
                        object_name => 'EMP1',
                        policy_name => 'ONE_ROW_ONLY',
                        function_schema => 'SCOTT',
                        policy_function => 'ONE_ROW_ONLY',
                        statement_types => 'UPDATE'
                       );
end;
/
update emp1 set sal = sal
/
SQL> update emp1 set sal = sal
  2  /
update emp1 set sal = sal
       *
ERROR at line 1:
ORA-01732: data manipulation operation not legal on this view


SQL> 

update (select * 
        from   (select * from emp1) 
        where  rownum = 1
       )
  set sal = sal
/