CASE 1:


create table t

as
select decode(mod(rownum,2), 0, 'M', 'F' ) gender, all_objects.*
  from all_objects
/
create index t_idx on t(gender,object_id)
/


analyze table t compute statistics
for table
for all indexes
for all indexed columns
/


alter session set sql_trace=true;
select * from t t1 where object_id = 55;
alter session set sql_trace=false;

drop table t;


CASE 2: 


as
select chr(mod(rownum,256)) gender, all_objects.*
  from all_objects
/
create index t_idx on t(gender,object_id)
/

analyze table t compute statistics

for table
for all indexes
for all indexed columns
/

alter session set sql_trace=true;
select * from t t2 where object_id = 55;
alter session set sql_trace=false;