Is there any table instead of RSEG table ?

Linking the shipment cost document item to the purchase order to the AP document: ( My functional consultant suggested me like below ) :

The purchase order number and reference number is stored on the line item of the shipment cost document. So, I have to use the value in table-field VFKP-EBELN (purchase order number on the shipment cost document) to determine the purchase order number and table-field VFKP-EXTI1 to determine the reference number for that shipment cost item. I need to take that purchase order number and lookup in table-field RSEG-EBELN (purchase order number) and take the reference number for that shipment cost item and look up in table-field RSEG-XBLNR. Return table-field RSEG-BELNR (AP doc number) for that combination.

He said lile below :

RSEG is a huge table, and there certainly may be a better way to do it.

Could anybody suggest me, instead of RSEG table is there any other alternative table to do this better way ?