presto-parser/src/main/java/io/prestosql/sql/tree/DefaultTraversalVisitor.java/DefaultTraversalVisitor/visitOrderBy
class DefaultTraversalVisitor:    @Override
    protected R visitOrderBy(OrderBy node, C context)
    {
        for (SortItem sortItem : node.getSortItems()) {
            process(sortItem, context);
        }
        return null;
    }