Calculate Amount Credit using index hint in Ax 2012

static void CalculateAmountCredit(Args _args)
{
    CustTable   custTable;
    CustTrans   custTrans;

    while select sum(AmountCur),AccountNum from custTrans
        index hint voucherdateIdx
        //group by AccountNum
        join custTable where CustTable.AccountNum == CustTrans.AccountNum
    {
        info(strFmt("%1,",custTrans.AmountCur));
    }
}