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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 a lot like a function that you can define ad hoc, within the scope of one connection.

If you have an SQL statement that you're going to execute many times in quick succession, it may (but see below!) be more efficient to prepare it once and reuse it. This saves the database backend the effort of parsing the SQL and figuring out an efficient execution plan.

# Preparing a statement

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

The statement's name 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 &cx)
{
cx.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, call pqxx::transaction_base::exec() and pass a pqxx::prepped object instead of an SQL statement string. The pqxx::prepped type is just a wrapper that tells the library "this is not SQL text, it's the name of a prepared statement."

pqxx::result execute_my_statement(pqxx::transaction_base &t)
{
return t.exec(pqxx::prepped{"my_statement"});
}
pqxx::prepped
A string that is the name of a prepared statement.
Definition prepared_statement.hxx:70
pqxx::result
Result set containing data returned by a query or command.
Definition result.hxx:92

# Parameters

You can pass parameters to a prepared statemet, just like you can with a regular statement. The query text can contain $1, $2 etc. as placeholders for parameter values that you will provide when you invoke the prepared satement.

See Statement 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 &cx)
{
// Prepare a statement called "find" that looks for employees with a
// given name (parameter 1) whose salary exceeds a given number
// (parameter 2).
cx.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 &tx, std::string name, int min_salary)
{
return tx.exec(pqxx::prepped{"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.

So, as with any optimisation... measure where your real performance problems are before you start making changes, and then afterwards, measure whether your changes actually helped. Don't complicate your code unless it solves a real problem. Knuth's Law applies.

# Zero bytes

Warning
  Beware of zero ("nul") bytes!

Since libpqxx is a wrapper around libpq, the C-level client library, most strings you pass to the library should be compatible with C-style strings. So they must end with a single byte with value 0, and the text within them cannot contain any such zero bytes.

(The pqxx::zview type exists specifically to tell libpqxx: "this is a C-compatible string, containing no zero bytes but ending in a zero byte.")

One example is prepared statement names. But the same also goes for the parameters values. 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>.