create table t (
  x NUMBER   not null
, y NUMBER   not null ) ;

insert into t
select *
from (
	select rownum x, rownum y
	from   dual
	connect by level <= 5000000
)
order by dbms_random.value  
;

alter table t add
constraint t_pk primary key (x);

alter system flush buffer_cache;

set arraysize 5
set timing on

select /*+ index(t t_pk) */ x, y
from   t
where  x >  0
and    y <= 6 ;


select /*+ index(t t_pk) */ x, y
from   t
where  x >  0
and    y <= 1 ;


select /*+ index(t t_pk) */ * from t where x > 0 and mod(y, 50) = 0

select /*+ index(t t_pk) */ * from t where x > 0 and mod(y, 50000) = 0




