Oracle SQL Plan operation INDEX FULL SCAN

drop table tq84_tab;
create table tq84_tab (
   col_1 varchar2(10),
   col_2 varchar2(10),
   col_3 varchar2(10),
   col_4 varchar2(10)
);

create index tq84_tab_ix on tq84_tab (col_3, col_4);

explain plan for
   select
      col_3,
      col_4
   from
      tq84_tab;

select * from table(dbms_xplan.display(format=>'basic'));
--
-- --------------------------------------
-- | Id  | Operation         | Name     |
-- --------------------------------------
-- |   0 | SELECT STATEMENT  |          |
-- |   1 |  TABLE ACCESS FULL| TQ84_TAB |
-- --------------------------------------


explain plan for
   select
      col_3,
      col_4
   from
      tq84_tab
   where
      col_3 is not null;

select * from table(dbms_xplan.display(format=>'basic'));
-- 
-- ----------------------------------------
-- | Id  | Operation        | Name        |
-- ----------------------------------------
-- |   0 | SELECT STATEMENT |             |
-- |   1 |  INDEX FULL SCAN | TQ84_TAB_IX |
-- ----------------------------------------

alter table tq84_tab modify col_3 not null;

explain plan for
   select
      col_3,
      col_4
   from
      tq84_tab;

select * from table(dbms_xplan.display(format=>'basic'));
--
-- ----------------------------------------
-- | Id  | Operation        | Name        |
-- ----------------------------------------
-- |   0 | SELECT STATEMENT |             |
-- |   1 |  INDEX FULL SCAN | TQ84_TAB_IX |
-- ----------------------------------------