presto-main/src/main/java/io/prestosql/sql/planner/SubqueryPlanner.java/ColumnReferencesExtractor/visitIdentifier
class ColumnReferencesExtractor:        @Override
        protected Void visitIdentifier(Identifier node, ImmutableSet.Builder<NodeRef<Expression>> builder)
        {
            builder.add(NodeRef.of(node));
            return null;
        }