create table val_00 (
  id number(10),
  data varchar2(100),
  constraint pk_val_00 primary key (id) validate
) organization index;

create table val_01 (
  id number(10),
  data varchar2(100),
  constraint pk_val_01 primary key (id) validate
) organization index;

create or replace view val as
  select id, data from (
    select id, data from val_00
    union all
    select id, data from val_01
  );


create or replace trigger trg_val_ins
  instead of insert on val
  referencing new as new
  for each row
begin
  case mod(:new.id, 2) 
    when 0 then insert into val_00 (id, data) values (:new.id, :new.data);
    when 1 then insert into val_01 (id, data) values (:new.id, :new.data);
  end case;
end;
/

create or replace trigger trg_val_upd
  instead of update on val
  referencing old as old new as new
  for each row
begin
  if updating('id') then 
    raise_application_error(-20999, 'you can not update primary key in val');
  end if;
  case mod(:new.id, 2) 
    when 0 then update val_00 set data = :new.data where id = :old.id;
    when 1 then update val_01 set data = :new.data where id = :old.id;
  end case;
end;
/

create or replace trigger trg_val_del
  instead of delete on val
  referencing old as old
  for each row
begin
  case mod(:old.id, 2) 
    when 0 then delete from val_00 where id = :old.id;
    when 1 then delete from val_01 where id = :old.id;
  end case;
end;
/

insert into val values (1, 'one');
insert into val values (2, 'two');
insert into val values (3, 'three');
insert into val values (4, 'four');
insert into val values (5, 'five');
delete from val where id = 2;
insert into val values (2, 'twooo');
update val set data = 'two' where id = 2;
commit;

merge into val using 
( select 10 as id, 'ten' as data from dual ) newdata
on (val.id = newdata.id) 
when matched then 
update set val.data = newdata.data
when not matched then 
insert (id, data) values (newdata.id, newdata.data);

SQL> update val
  2     set id = id + 0
  3  /
update val
       *
ERROR at line 1:
ORA-20999: you can not update primary key in val
ORA-06512: at "SCOTT.TRG_VAL_UPD", line 3
ORA-04088: error during execution of trigger 'SCOTT.TRG_VAL_UPD'