Hi Loed,
You can do it in the query designer following this procedure:
1. Create a formula variable using replacement path on 0calday. Get the key for this formula variable.
2. Create a formula in the designer using NODIM(formula_variable_you_created).
3. Create a condition in the designer. Select first N (N=1) for the formula you have created. And Select transaction and calday for characteristic assignment.
Though you may still have performance problem. If you do, you can think of modelling changes such as adding a characteristic to your infoprovider (a check if it is the last transaction) and in each record you can check within the start routine. In this case you have to make sure that you both check the records in the infoprovider and in PSA.
Hope it gives an idea
Regards
Yasemin..