Handle Missing Columns in C++ Client

Send rows with missing or optional columns to QuestDB using the C++ client.

Problem

In Python, you can handle missing columns easily with dictionaries:

{"price1": 10.0, "price2": 10.1}

And if price2 is not available:

{"price1": 10.0, "price2": None}

Which is equivalent to:

{"price1": 10.0}

You can pass the dict as the columns argument to sender.rows and it transparently sends the rows, with or without missing columns, to the server.

In C++, the buffer API requires explicit method calls:

buffer
    .table("trades")
    .symbol("symbol", "ETH-USD")
    .symbol("side", "sell")
    .column("price", 2615.54)
    .column("amount", 0.00044)
    .at(questdb::ingress::timestamp_nanos::now());

sender.flush(buffer);

How do you handle "ragged" rows with missing columns in C++?

Solution

You need to call at at the end of the buffer so the data gets queued to be sent, but you can call symbol and column as many times as needed for each row, and you can do this conditionally.

The example below builds a vector with three rows, one of them with an empty column, then it iterates over the vector and checks if the optional price column is null. If it is, it skips invoking column for the buffer on that column.

#include <questdb/ingress/line_sender.hpp>
#include <iostream>
#include <chrono>
#include <vector>
#include <optional>
#include <string>

int main()
{
    try
    {
        auto sender = questdb::ingress::line_sender::from_conf(
            "http::addr=localhost:9000;username=admin;password=quest;retry_timeout=20000;");

        auto now = std::chrono::system_clock::now();
        auto duration = now.time_since_epoch();
        auto nanos = std::chrono::duration_cast<std::chrono::nanoseconds>(duration).count();

        struct Row {
            std::string symbol;
            std::string side;
            std::optional<double> price;
            double amount;
        };

        std::vector<Row> rows = {
            {"ETH-USD", "sell", 2615.54, 0.00044},
            {"BTC-USD", "sell", 39269.98, 0.001},
            {"SOL-USD", "sell", std::nullopt, 5.5} // Missing price
        };

        questdb::ingress::line_sender_buffer buffer;

        for (const auto& row : rows) {
            buffer.table("trades")
                .symbol("symbol", row.symbol)
                .symbol("side", row.side);

            if (row.price.has_value()) {
                buffer.column("price", row.price.value());
            }

            buffer.column("amount", row.amount)
                .at(questdb::ingress::timestamp_nanos(nanos));
        }

        sender.flush(buffer);
        sender.close();

        std::cout << "Data successfully sent!" << std::endl;
        return 0;
    }
    catch (const questdb::ingress::line_sender_error& err)
    {
        std::cerr << "Error running example: " << err.what() << std::endl;
        return 1;
    }
}

Problem​

Solution​

Problem

Solution