Analytics Software

Hello
I saw sth strange in my report. I assume that it could happens very often.
I have report with few columns which two of them ar most complicated (many joins subqueries aggreagations on joined values etc.) These two columns (i.e C3,C4) should be multiplied (C3*C4).
When i do pure report without multiplying only columns C1,C2,C3,C4 everything is ok - duration about 15 sec. but... when I put next column on report which multiply these columns (in Answers C5=C3*C4)
I wait 3-4 minutes and my database hungs :(. After investigation I saw that in first case to databese goes pure "SELECT" statement it means:

"Select ... as C1, ... as C2, max(...) as C3, sum(xxx)... C4 from yyy,sss,ttt WHERE aaa"

but in second case BI uses WITH clause it means:

WITH SAWITH0 AS
( Select ... as C1, ... as C2, max(...) as C3, sum(xxx)... C4 from yyy,sss,ttt WHERE aaa )
SELECT SAWITH0.C1 as C1,
SAWITH0.C2 as C2,
SAWITH0.C3 as C3,
SAWITH0.C4 as C4,
SAWITH0.C3*SAWITH0.C4 as C5 FROM SSS

and this statement is long runninq query and kills my database :(.
I checked that SQL like this:

Select ... as C1, ... as C2, max(...) as C3, sum(xxx)... C4, max(...)*sum(xxx)... As C5 from yyy,sss,ttt WHERE aaa" -
runs few times faster than that above

I know that I can do this multiply in business model layer but sometimes users can multiply(or other operations) on columns in reports without my knowledge and it kills my db :(. Where is bug? Why SQLs with WITH clause takes so much db time?

Thank you for each kind of help