presto-main/src/main/java/io/prestosql/sql/planner/SortExpressionExtractor.java/SortExpressionExtractor/visitNode
class SortExpressionExtractor:        @Override
        protected Boolean visitNode(Node node, Void context)
        {
            for (Node child : node.getChildren()) {
                if (process(child, context)) {
                    return true;
                }
            }
            return false;
        }