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