presto-main/src/main/java/io/prestosql/sql/planner/RelationPlan.java/RelationPlan/getSymbol
class RelationPlan:    public Symbol getSymbol(int fieldIndex)
    {
        checkArgument(fieldIndex >= 0 && fieldIndex < fieldMappings.size(), "No field->symbol mapping for field %s", fieldIndex);
        return fieldMappings.get(fieldIndex);
    }