I think your problem is that you get a large synthetic key with your "Add 1 column with calculation" load.
You could do it with a presceding load:
//Add some columns to the base_Table
ordersValue*isZFlag as myTestField;
if (Segm2_Meng <> 0,
if(isEuroFlag = 1,
((Netto2*Kurs/Basis)/Segm2_Meng)* orderquantity), NULL())
//delete first table
DROP TABLE relFB_Base;
When you did your:
//Add 1 column with calculation
LOAD *, ordersValue * isZFlag as myTestField
you created a new internal table with the same data as relFB_Temp1 plus your new column (myTestField). This makes a synthetic key between all columns which are in both tables. What you could have done is name that load (re1FB_Temp2) and dropped reFB_Temp1 last.
DROP TABLE relFB_Base, reFBTemp1;
Presceding load is just a feature used for purposes like this. When you need to calculate something on a field in a load.