SQL & PL/SQL

Is there a way to avoid a union query (which means two full table scans) for the following:

Here is the set up:
create table x
(id number(2),
leftid number(5),
rightid number(5)
);

insert into x values(1,11111,11111);
insert into x values(2,11111,22222);
insert into x values(3,22222,33333);

And here is the Query:
select id,leftid,'LEFT' side from x
union
select id,rightid,'RIGHT' side from x;


The tables I have are huge and to gain performance, I'd rather not have the table read twice, but still get result set as the union query above... THANKS!
ID LEFTID SIDE
---------- ---------- -----
1 11111 LEFT
1 11111 RIGHT
2 11111 LEFT
2 22222 RIGHT
3 22222 LEFT
3 33333 RIGHT