To `JSONEachRow`
{
    
    
    if (prev_positions.size() > key_index
        && prev_positions[key_index]
        && name == prev_positions[key_index]->getKey())
    {
        return prev_positions[key_index]->getMapped();
    }
    else
    {
        auto * it = name_map.find(name);
        if (it)
        {
            if (key_index < prev_positions.size())
                prev_positions[key_index] = it;
            return it->getMapped();
        }
        else
            return UNKNOWN_FIELD;
    }
}
/** Read the field name and convert it to column name
  *  (taking into account the current nested name prefix)
  * Resulting StringRef is valid only before next read from buf.
  */
StringRef JSONEachRowRowInputFormat::readColumnName(ReadBuffer & buf)