Map 序列化 按Key排序

在序列化Map之前，按照key进行排序，构建数组,arr[i]记录应该在这个位置上的item在offsets上的偏移.

auto cmp = [](const StringRef& lhs, const StringRef& rhs){
    return (strcmp(lhs.data, rhs.data) < 0);
};
std::vector<unsigned long> sorted_row_indexes(next_offset-offset, 0);
std::map<StringRef, size_t, decltype(cmp)> _map(cmp);

auto resort_columns_index_by_key = [&](){
    for (size_t i = offset; i < next_offset; i++){
        _map.insert(std::make_pair(nested_tuple.getColumn(0).getDataAt(i), i));
    }
    size_t idx = 0;
    for (auto const & it: _map){
        sorted_row_indexes[idx++] = it.second;
    }
    _map.clear();
};
resort_columns_index_by_key();

// --------------------------- 上面代码是增加逻辑

writeChar('{', ostr);

for (size_t i = offset; i < next_offset; ++i)
{
    if (i != offset)
        writeChar(',', ostr);
    
    //key_writer(ostr, key, nested_tuple.getColumn(0), i);
　　 
    key_writer(ostr, key, nested_tuple.getColumn(0), sorted_row_indexes[i-offset]); // sorted_row_indexes: 是修正后的offset
    writeChar(':', ostr);
    value_writer(ostr, value, nested_tuple.getColumn(1), sorted_row_indexes[i-offset]);
}
writeChar('}', ostr);