presto-main/src/main/java/io/prestosql/sql/planner/SubqueryPlanner.java/SubqueryPlanner/ColumnReferencesExtractor
class SubqueryPlanner:        private ColumnReferencesExtractor(Set<NodeRef<Expression>> columnReferences)
        {
            this.columnReferences = requireNonNull(columnReferences, "columnReferences is null");
        }