get the remaining qty in PO

Hi experts,

I use the following code to get the remaining qty in PO:

SELECT ERFMG from MSEG into wa_qty where  EBELN = PO_NUMBER AND EBELP = PO_ITEM AND MATNR = ARTICLENO .
      QUANTITY = QUANTITY - wa_qty.
ENDSELECT.

But it's very slow.How to optimize it?

thanks.