Secondary index problem for the custom table

Hello Friends,

I am having one custom table where Purchase order number and other 5 fields are the primary keys. Where as Vendor is in a secondary index. Now I am using one select query where I have to pass PO no and Vendor number.

When I am seeing in ST05 it is showing the query is taking only the secondary index into account and the execution time is also too much high.

So my question is how can I use primary index here. Or shall I create another secondary index with PO and Vendor to improve the performance?