prepared statement

Prepared statements

Prepared statements are SQL queries that you define once and then invoke as many times as you like, typically with varying parameters. It’s basically a function that you can define ad hoc.

If you have an SQL statement that you’re going to execute many times in quick succession, it may be more efficient to prepare it once and reuse it. This saves the database backend the effort of parsing complex SQL and figuring out an efficient execution plan. Another nice side effect is that you don’t need to worry about escaping parameters. Some corporate coding standards require all SQL parameters to be passed in this way, to reduce the risk of programmer mistakes leaving room for SQL injections.

Preparing a statement

You create a prepared statement by preparing it on the connection (using the pqxx::connection::prepare functions), passing an identifier and its SQL text.

The identifier is the name by which the prepared statement will be known; it should consist of ASCII letters, digits, and underscores only, and start with an ASCII letter. The name is case-sensitive.

    void prepare_my_statement(pqxx::connection &c)
    {
      c.prepare(
          "my_statement",
          "SELECT * FROM Employee WHERE name = 'Xavier'");
    }

Once you’ve done this, you’ll be able to call my_statement from any transaction you execute on the same connection. For this, use the pqxx::transaction_base::exec_prepared functions.

    pqxx::result execute_my_statement(pqxx::transaction_base &t)
    {
      return t.exec_prepared("my_statement");
    }

Parameters

Did I mention that prepared statements can have parameters? The query text can contain $1, $2 etc. as placeholders for parameter values that you will provide when you invoke the prepared satement.

See @ref parameters for more about this. And here’s a simple example of preparing a statement and invoking it with parameters:

    void prepare_find(pqxx::connection &c)
    {
      // Prepare a statement called "find" that looks for employees with a
      // given name (parameter 1) whose salary exceeds a given number
      // (parameter 2).
      c.prepare(
        "find",
        "SELECT * FROM Employee WHERE name = $1 AND salary > $2");
    }

This example looks up the prepared statement “find,” passes name and min_salary as parameters, and invokes the statement with those values:

    pqxx::result execute_find(
      pqxx::transaction_base &t, std::string name, int min_salary)
    {
      return t.exec_prepared("find", name, min_salary);
    }

A special prepared statement

There is one special case: the nameless prepared statement. You may prepare a statement without a name, i.e. whose name is an empty string. The unnamed statement can be redefined at any time, without un-preparing it first.

Performance note

Don’t assume that using prepared statements will speed up your application. There are cases where prepared statements are actually slower than plain SQL.

The reason is that the backend can often produce a better execution plan when it knows the statement’s actual parameter values.

For example, say you’ve got a web application and you’re querying for users with status “inactive” who have email addresses in a given domain name X. If X is a very popular provider, the best way for the database engine to plan the query may be to list the inactive users first and then filter for the email addresses you’re looking for. But in other cases, it may be much faster to find matching email addresses first and then see which of their owners are “inactive.” A prepared statement must be planned to fit either case, but a direct query will be optimised based on table statistics, partial indexes, etc.

Zero bytes

@warning Beware of “nul” bytes!

Any string you pass as a parameter will end at the first char with value zero. If you pass a string that contains a zero byte, the last byte in the value will be the one just before the zero.

So, if you need a zero byte in a string, consider that it’s really a binary string, which is not the same thing as a text string. SQL represents binary data as the BYTEA type, or in binary large objects (“blobs”).

In libpqxx, you represent binary data as a range of std::byte. They must be contiguous in memory, so that libpqxx can pass pointers to the underlying C library. So you might use pqxx::bytes, or pqxx::bytes_view, or std::vector<std::byte>.