create table emp as select * from scott.emp;

create index emp_deptno_idx on emp(deptno);

create index emp_job_idx on emp(job);

alter session set "_b_tree_bitmap_plans" = false;

select * from emp
where  job = 'CLERK' or deptno = 20;

select * from emp
where  job = 'CLERK' 
union all
select * from emp
where  deptno = 20
and    LNNVL(job='CLERK');

select /*+ USE_CONCAT */ * from emp
where  job = 'CLERK' or deptno = 20;

select /*+ NO_EXPAND */ * from emp
where  job = 'CLERK' or deptno = 20;

