create table aaa (id number not null);

create unique index u_aaa$id on aaa(id);


insert into aaa values( 1 );
insert into aaa values( 8 );
insert into aaa values( 10 );
insert into aaa values( 11 );
insert into aaa values( 12 );
insert into aaa values( 15 );
commit;



 -- variant 1
 
select num from (
select rownum num from all_objects o1, all_objects o2 
where rownum < (select max(id) from aaa)
) n, aaa a
where n.num = a.id (+)
and a.id is null;


 -- variant 2
 
select num from (
select rownum num from all_objects o1, all_objects o2 
where rownum < (select max(id) from aaa)
) n
where not exists(select null from aaa where id=n.num);

create type numTable as table of number; 

create or replace function f
return numTable as
    l_doc_id_tbl	numTable	default numTable();
    l_seq_tbl		numTable	default numTable();
    l_miss_id_tbl	numTable	default numTable();
    l_cnt		number		default  0 ;
    l_max_id		number		default  0 ;
begin

    select id
    bulk collect
    into l_doc_id_tbl
    from aaa;

    select max(id) 
    into l_max_Id
    from aaa;

    l_seq_tbl.extend(l_max_id);

    for i in  1  .. l_max_id
    loop
        l_seq_tbl(i) := i;
    end loop;

    select num
    bulk collect into l_miss_id_tbl
    from (
    select column_value as num from TABLE ( cast(l_seq_tbl as numTable) )
    minus
    select column_value as num from TABLE ( cast(l_doc_id_tbl as numTable) )
    );

    l_seq_tbl.delete;
    l_doc_id_tbl.delete;

    return l_miss_id_tbl;

end;
/
show errors;

 -- show results
 
select * from TABLE(cast(f() as NumTable));

 select t.column_value
from table(cast(all_numbers( 1 , 10000 ))) as tb_numbers)) t
minus
select doc_id from doc_table
;