Statement parameters

In an SQL statement (including a prepared statemen), you may write special placeholders in the query text. They look like $1, $2, and so on.

When executing the query later, you pass parameter values. The call will respectively substitute the first parameter value where it finds $1 in the query, the second where it finds $2, et cetera.

For example, let's say you have a transaction called tx. Here's how you execute a plain statement:

pqxx::result r = tx.exec("SELECT name FROM employee where id=101");

Inserting the 101 in there is awkward and even dangerous. We'll get to that in a moment. Here's how you do it better, using parameters:

pqxx::result r = tx.exec("SELECT name FROM employee WHERE id=$1", {101});

That second argument to exec(), the {101}, constructs a pqxx::params object. The exec() call will fill this value in where the query says $1.

Doing this saves you work. If you don't use statement parameters, you'll need to quote and escape your values (see connection::quote() and friends) as you insert them into your query as literal values.

Or if you forget to do that, you leave yourself open to horrible SQL injection attacks. Trust me, I was born in a town whose name started with an apostrophe!

With parameters you can pass your values as they are, and they will go across the wire to the database in a safe format.

In some cases it may even be faster! When a parameter represents binary data (as in the SQL BYTEA type), libpqxx will send it directly as binary, which is a bit more efficient than the standard textual format in which the data normally gets sent to the database. If you insert the binary data directly in your query text, your CPU will have some extra work to do, converting the data into a text format, escaping it, and adding quotes; and the data will take up more bytes, which take time to transmit.

Multiple parameters

The pqxx::params class is quite flexible. It can contain any number of parameter values, of many different types.

You can pass them in while constructing the params object:

pqxx::params{23, "acceptance", 3.14159}

Or you can add them one by one:

pqxx::params p;
p.append(23);
p.append("acceptance");
p.append(3.14159);

You can also combine the two, passing some values int the constructor and adding the rest later. You can even insert a params into a params:

pqxx::params p{23};
p.append(params{"acceptance", 3.14159});

Each of these examples will produce the same list of parameters.

Encodings and complex parameters

For some parameter types, the params code will need to know about the text encoding. This happens especially with composite types. You may see errors about the encoding not being known.

To prevent that, pass as your first parameter the pqxx::encoding_group for the connection's current client encoding. You can get this by calling the connection's get_encoding_group() member function, but as a shortcut, you may just pass in a reference to your connection or your transaction instead. Or if you already had a pqxx::conversion_context, you can pass that instead.

In those cases, that special first argument will not become a parameter to the SQL statement. It will only be used to obtain the necessary text encoding information.

Generating placeholders

If your code gets particularly complex, it may sometimes happen that it becomes hard to track which parameter value belongs with which placeholder. Did you intend to pass this numeric value as $7, or as $8? The answer may depend on an if that happened earlier in a different function.

(Generally if things get that complex, it's a good idea to look for simpler solutions. But especially when performance matters, sometimes you can't avoid complexity like that.)

There's a little helper class called placeholders. You can use it as a counter which produces those placeholder strings, $1, $2, $3, et cetera. When you start generating a complex statement, you can create both a params and a placeholders:

pqxx::params values;
pqxx::placeholders name;

Let's say you've got some complex code to generate the conditions for an SQL "WHERE" clause. You'll generally want to do these things close together in your, so that you don't accidentally update one part and forget another:

if (extra_clause)
{
  // Extend the query text, using the current placeholder.
  query += " AND x = " + name.get();
  // Add the parameter value.
  values.append(my_x);
  // Move on to the next placeholder value.
  name.next();
}

Depending on the starting value of name, this might add to query a fragment like " `AND x = $3` " or " `AND x = $5` ".