ZeroTierOne/ext/libpqxx-7.7.3/include/pqxx/doc/accessing-results.md

Accessing results and result rows

When you execute a query using one of the transaction exec functions, you normally get a result object back. A result is a container of rows.

(There are exceptions. The exec1 functions expect exactly one row of data, so they return just a row, not a full result.)

Result objects are an all-or-nothing affair. The exec function waits until it's received all the result data, and then gives it to you in the form of the result. (There is a faster, easier way of executing simple queries, so see "streaming rows" below as well.)

For example, your code might do:

    pqxx::result r = tx.exec("SELECT * FROM mytable");

Now, how do you access the data inside r?

Result sets act as standard C++ containers of rows. Rows act as standard C++ containers of fields. So the easiest way to go through them is:

    for (auto const &row: r)
    {
       for (auto const &field: row) std::cout << field.c_str() << '\t';
       std::cout << '\n';
    }

But results and rows also support other kinds of access. Array-style indexing, for instance, such as r[rownum]:

    std::size_t const num_rows = std::size(r);
    for (std::size_t rownum=0u; rownum < num_rows; ++rownum)
    {
      pqxx::row const row = r[rownum];
      std::size_t const num_cols = std::size(row);
      for (std::size_t colnum=0u; colnum < num_cols; ++colnum)
      {
        pqxx::field const field = row[colnum];
        std::cout << field.c_str() << '\t';
      }

      std::cout << '\n';
    }

Every row in the result has the same number of columns, so you don't need to look up the number of fields again for each one:

    std::size_t const num_rows = std::size(r);
    std::size_t const num_cols = r.columns();
    for (std::size_t rownum=0u; rownum < num_rows; ++rownum)
    {
      pqxx::row const row = r[rownum];
      for (std::size_t colnum=0u; colnum < num_cols; ++colnum)
      {
        pqxx::field const field = row[colnum];
        std::cout << field.c_str() << '\t';
      }

      std::cout << '\n';
    }

You can even address a field by indexing the row using the field's name:

    std::cout << row["salary"] << '\n';

But try not to do that if speed matters, because looking up the column by name takes time. At least you'd want to look up the column index before your loop and then use numerical indexes inside the loop.

For C++23 or better, there's also a two-dimensional array access operator:

    for (std::size_t rownum=0u; rownum < num_rows; ++rownum)
    {
        for (std::size_t colnum=0u; colnum < num_cols; ++colnum)
	    std::cout result[rownum, colnum].c_str() << '\t';
	std::cout << '\n';
    }

And of course you can use classic "begin/end" loops:

    for (auto row = std::begin(r); row != std::end(r); row++)
    {
      for (auto field = std::begin(row); field != std::end(row); field++)
        std::cout << field->c_str() << '\t';
      std::cout << '\n';
    }

Result sets are immutable, so all iterators on results and rows are actually const_iterators. There are also const_reverse_iterator types, which iterate backwards from rbegin() to rend() exclusive.

All these iterator types provide one extra bit of convenience that you won't normally find in C++ iterators: referential transparency. You don't need to dereference them to get to the row or field they refer to. That is, instead of row->end() you can also choose to say row.end(). Similarly, you may prefer field.c_str() over field->c_str().

This becomes really helpful with the array-indexing operator. With regular C++ iterators you would need ugly expressions like (*row)[0] or row->operator[](0). With the iterator types defined by the result and row classes you can simply say row[0].

Streaming rows

There's another way to go through the rows coming out of a query. It's usually easier and faster, but there are drawbacks.

One, you start getting rows before all the data has come in from the database. That speeds things up, but what happens if you lose your network connection while transferring the data? Your application may already have processed some of the data before finding out that the rest isn't coming. If that is a problem for your application, streaming may not be the right choice.

Two, streaming only works for some types of query. The stream() function wraps your query in a PostgreSQL COPY command, and COPY only supports a few commands: SELECT, VALUES, or an INSERT, UPDATE, or DELETEwith aRETURNINGclause. See theCOPY` documentation here: https://www.postgresql.org/docs/current/sql-copy.html

Three, when you convert a field to a "view" type (such as std::string_view or std::basic_string_view<std::byte>), the view points to underlying data which only stays valid until you iterate to the next row or exit the loop. So if you want to use that data for longer than a single iteration of the streaming loop, you'll have to store it somewhere yourself.

Now for the good news. Streaming does make it very easy to query data and loop over it:

    for (auto [id, name, x, y] :
        tx.stream<int, std::string_view, float, float>(
            "SELECT id, name, x, y FROM point"))
      process(id + 1, "point-" + name, x * 10.0, y * 10.0);

The conversion to C++ types (here int, std::string_view, and two floats) is built into the function. You never even see row objects, field objects, iterators, or conversion methods. You just put in your query and you receive your data.