/**
     * Used to indicate "natural" sorting order, usually according
     * FID (hopefully based on Key attribtues).
     * <p>
     * This is the order that is most likely to be available in optimzied
     * form, if an Attribute is marked as "key" an optimized ordering should
     * be considered avaialble.
     * </p>
     * <p>
     * Non optimized orderings are will at the very least require as pass
     * through the data to bring it into memory, you can assume somekind
     * of TreeSet would be used. Where the nodes in the tree would indicate
     * a list of FeatureIds assoicated with the node, in the order encountered.
     * </p>
     * <p>
     * This is a "NullObject".
     * </p>
     */
    SortBy NATURAL_ORDER = new NullSortBy(SortOrder.ASCENDING);