Performance issue in a custom table

Hi All,

I have a ztable used in a program wherin I have a doubt of performance issue in selection.Its like :

SELECT ship_no invoice_no

INTO TABLE it_ship_no_hist

FROM zco_cust_hist

FOR ALL ENTRIES IN it_freight

WHERE ship_no = it_freight-tknum.

there are 7 key fields in this table out of which one ( tknum ) is used in a where condition.The table is without any index.

For performance purpose should I create an index with the very field 'tknum' in the index..can I do that or index should be created only along with non key fields.