Chaim,
Try any of these.
1. Use the customer table and set a relation to the invoice table, then navigate the customer table showing data from the invoice table.
2.
SELECT Customer.Name, Invoice.InvNo ;
FROM Customer JOIN Invoice ON Custoemr.CustId = Invoice.CustId ;
ORDER BY 1 ;
INTO CURSOR MyOrder
SET REALTION TO InvNo INTO Invoice
Now navigate the MyOrder Cursor and show the invoice data.
Either of these will give you the result you want without using an index that is keyed to a table other than the one that the index is on.
