ColumnString avoid memory copy

[1261385937]

void ColumnString::Append(std::string_view str) {
    if (blocks_.size() == 0 || blocks_.back().GetAvailble() < str.length())
    {
        blocks_.emplace_back(std::max(DEFAULT_BLOCK_SIZE, str.size()));
    }

    items_.emplace_back(blocks_.back().AppendUnsafe(str));
}

Always cpoy the str into the Block, this is not necessary. We can move the data into std::queue<std::string> if the str is not useful any more.
I remeber a previous version use the std::vector<std::string> to hold the str, but also use the cpoy. The code like :

void ColumnString::Append(const std::string& str) {
       data_.push_back(str)
}

some advantage that i missed ?

[Enmk]

Hi! Interesting proposition, please submit a PR.

[Enmk]

IMO we can add ColumnString::Append(std::string && str).

That would steal ownership of data and put it into some internal container, like std::list<std::string>, and add reference to ColumnString::items_.
We don't need std::deque here, since operator[] should use ColumnString::items_ for random access;

That way we can have both fast de-serealization (thanks to amortized memory allocations with ColumnString::Block) and more efficient Appending.

Alternatively, if string data lifetime is managed elsewhere and guaranteed to outlive the Block sent to the server, we can introduce ColumnString::AppendNoCopy(std::string_view), that would just update ColumnString::items_.

[Enmk]

Closed in #202