libpqxx: Prepared statements

libpqxx 4.0.1

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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 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.

You create a prepared statement by preparing it on the connection, passing an identifier and its SQL text. The identifier is the name by which the prepared statement will be known; it should consist of letters, digits, and underscores only and start with a letter. The name is case-sensitive.

void prepare_my_statement(pqxx::connection_base &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. You start an invocation by looking up your statement using a member function called `"prepared"`. (The definition used a different member function, called `"prepare"` ).

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

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.

void prepare_find(pqxx::connection_base &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");
}

How do you pass those parameters? C++ has no good way to let you pass an unlimited, variable number of arguments to a function call, and the compiler does not know how many you are going to pass. There's a trick for that: you can treat the value you get back from `prepared` as a function, which you call to pass a parameter. What you get back from that call is the same again, so you can call it again to pass another parameter and so on.

Once you've passed all parameters in this way, you invoke the statement with the parameters by calling `exec` on the invocation.

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.prepared("find")(name)(min_salary).exec();
}

Warning
  There are cases where prepared statements are actually slower than plain SQL. Sometimes the backend can produce a better execution plan when it knows the 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 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 can be optimized based on table statistics, partial indexes, etc.