Creating Index to a master table

Hello All,

I have a database table with the following fields

CLIENT

MATNR

KORDX

KCATV

VARIANT_NBR

With the initial four fields making the primary key. ( CLIENT, MATNR, KORDX, KCATV).

I would like to put an index to the table with the fields (CLIENT, VARIANT_NBR), because there are many reads in the program for the field VARIANT_NBR. Could you please put across on views on this. What all factors should I consider for creating an index in performance perspective.

Thanks in advance

Sudha