When you execute a prepared statement (see Prepared statements), or a parameterised statement (using functions like pqxx::connection::exec_params), you may write special placeholders in the query text. They look like $1, $2, and so on.

If you execute the query and pass parameter values, the call will respectively substitute the first where it finds $1, the second where it finds $2, et cetera.

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!

Statement parameters save you this work. With these parameters you can pass your values as-is, 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. 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.

Dynamic parameter lists

In rare cases you may just not know how many parameters you'll pass into your statement when you call it.

For these situations, have a look at params. It lets you compose your parameters list on the fly, even add whole ranges of parameters at a time.

You can pass a params into your statement as a normal parameter. It will fill in all the parameter values it contains into that position of the statement's overall parameter list.

So if you call your statement passing a regular parameter a, a params containing just a parameter b, and another regular parameter c, then your call will pass parameters a, b, and c. Or if the params object is empty, it will pass just a and c. If the params object contains x and y, your call will pass a, x, y, c.

You can mix static and dynamic parameters freely. Don't go overboard though: complexity is where bugs happen!

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` ".