presto-main/src/main/java/io/prestosql/sql/planner/ExpressionInterpreter.java/ExpressionInterpreter/visitIdentifier
class ExpressionInterpreter:        @Override
        protected Object visitIdentifier(Identifier node, Object context)
        {
            // Identifier only exists before planning.
            // ExpressionInterpreter should only be invoked after planning.
            // As a result, this method should be unreachable.
            // However, RelationPlanner.visitUnnest and visitValues invokes evaluateConstantExpression.
            return ((SymbolResolver) context).getValue(new Symbol(node.getValue()));
        }