public GroupingExpression getOrderBy(int i) {
        return orderBy.get(i);
    }
Returns the group-by expression at the given index.

@param i the index of the expression to return
@return the expression at the given index
@throws IndexOutOfBoundsException if the index is out of range
container-search/src/main/java/com/yahoo/search/grouping/request/GroupingOperation.java