presto-main/src/main/java/io/prestosql/sql/relational/SqlToRowExpressionTranslator.java/SqlToRowExpressionTranslator/visitSymbolReference
class SqlToRowExpressionTranslator:        @Override
        protected RowExpression visitSymbolReference(SymbolReference node, Void context)
        {
            Integer field = layout.get(Symbol.from(node));
            if (field != null) {
                return field(field, getType(node));
            }

            return new VariableReferenceExpression(node.getName(), getType(node));
        }