Question about CDS limitation

Hello SAP Developers,

In HANA studio, customer made a CDS view for ABAP code pushdown, but happened the
syntax is error.

1. Could you check if this function below(SUM() + SUM() ) is not possible to work in CDS
   view???
2. If you are possible, please give some workaround to us in CDS view.
3. How is it the best way to
   create CDS view using SUM() + SUM() instead of OpenSQL for Code pushdown??
4. How can I find the documentation about CDS limitation or syntax?

For
your information. Customer has Netweaver 7.4 SP8 for ERP on HANA. They upgrade
Netweaver version for using HANA code pushdown recently.