Is a WITH...SELECT query more efficient than a SELECT query ?

Question

Hi folks,
Is the WITH...SELECT just a convenience or is it really efficient than a simple SELECT with UNION ALL ? e.g. is the following:
with rset as (select dname,empno,ename,hiredate,sal from emp e,dept d where e.deptno=d.deptno)
select dname,empno,ename,hiredate,sal,
case
when trunc(hiredate) < to_date('19800101','yyyymmdd') then 'Hired before 1980'
when trunc(hiredate) between to_date('19800101','yyyymmdd') and to_date('19851231','yyyymmdd') then 'Hired between 1980 and 1985'
else 'Hired after 1985'
end as notes
from rset
union all
select dname,empno,ename,hiredate,sal,
case
when sal < 500 then 'Salary less than 500'
when sal between 501 and 1500 then 'Salary between 501 and 1500'
else 'Salary greater than 1500'
end as notes
from rset;
better than the following:
select dname,empno,ename,hiredate,sal,
case
when trunc(hiredate) < to_date('19800101','yyyymmdd') then 'Hired before 1980'
when trunc(hiredate) between to_date('19800101','yyyymmdd') and to_date('19851231','yyyymmdd') then 'Hired between 1980 and 1985'
else 'Hired after 1985'
end as notes
from emp e,dept d where e.deptno=d.deptno
union all
select dname,empno,ename,hiredate,sal,
case
when sal < 500 then 'Salary less than 500'
when sal between 501 and 1500 then 'Salary between 501 and 1500'
else 'Salary greater than 1500'
end as notes
from emp e,dept d where e.deptno=d.deptno;
I am a newbie at sql tuning. Apparently, the first query should be faster because it runs the actual query only once and then just works on the resultset obtained. Is this thinking correct ?
Thanks a lot!!
JP

Answer 1

Also I tried a test here with a ten million row emp table queried five times, and explain plan showed the optimizer would read emp five times and not once.
Re: Intresting question
Apparently, the first query should be faster because it runs the actual query only once
and then just works on the resultset obtained.But my test combined with Jonathan's article made me question whether materializing ten million rows somewhere would be faster than querying them five times. Somehow I doubt it.