presto-main/src/main/java/io/prestosql/sql/planner/SymbolsExtractor.java/SymbolsExtractor/visitDereferenceExpression
class SymbolsExtractor:        @Override
        protected Void visitDereferenceExpression(DereferenceExpression node, ImmutableSet.Builder<QualifiedName> builder)
        {
            if (columnReferences.contains(NodeRef.<Expression>of(node))) {
                builder.add(DereferenceExpression.getQualifiedName(node));
            }
            else {
                process(node.getBase(), builder);
            }
            return null;
        }