如何在Postgresql中准备语句和绑定参数

Question

我是C++的新手，对pqxx库有一点了解。我要实现的是准备语句和绑定参数。在PHP中，我习惯以这样一种简洁的方式做到这一点:

$s = $db->prepare("SELECT id FROM mytable WHERE id = :id");
$s->bindParam(':id', $id);
$s->execute();

或使用令牌:

$data = array();
$data[] = 1;
$data[] = 2;
$s = $db->prepare("SELECT id FROM mytable WHERE id = ? or id = ?");
$s->execute($data);

我试图从pqxx文档中找出如何实现这一点，但对我来说，文档看起来一团糟，缺乏简短的示例(就像我上面提供的那样)。我希望在C++中处理Postgresql时，有人也能提供这样简单的例子(或类似的简单-不必编写一些庞然大物的代码)。

Answer 1

一个简单的例子。这只是打印id值为0的条目数。

#include<pqxx/pqxx>
#include<iostream>
int main()
{
    std::string name = "name";
    int id = 0;
    try {
        //established connection to data base
        pqxx::connection c("dbname=mydb user=keutoi");
        pqxx::work w(c);
        //statement template
        c.prepare("example", "SELECT id  FROM mytable WHERE id = $1");
        //invocation as in varible binding
        pqxx::result r = w.prepared("example")(id).exec();
        
        w.commit();
        //result handling for accessing arrays and conversions look at docs
        std::cout << r.size() << std::endl;
    }
    catch(const std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    }
    return 0;
}

函数w.p reed()有点复杂。它类似于haskell中的curry (curry)函数，因为它接受一个参数并返回另一个函数，该函数反过来接受另一个参数。诸如此类。

文档说:

如何传递这些参数?c++没有很好的方法让您向函数调用传递无限可变数量的参数，并且编译器不知道您将传递多少个参数。这里有一个技巧:您可以将从prepared返回的值视为一个函数，调用该函数来传递参数。你从那个调用中得到的结果还是一样的，所以你可以再次调用它来传递另一个参数，等等。

一旦以这种方式传递了所有参数，就可以通过在调用

时调用exec来调用带有参数的语句。

如果有更多的参数，则在prepare函数中使用$1 $2等等

c.prepare("SELECT id name FROM mytable WHERE id = $1 AND name = $2")

并给出变量

w.prepared("example")(dollar1_var)(dollar2_var).exec()

动态准备示例

#include<pqxx/pqxx>
#include<iostream>
#include<vector>
//Just give a vector of data you can change the template<int> to any data type
pqxx::prepare::invocation& prep_dynamic(std::vector<int> data, pqxx::prepare::invocation& inv)
{
    for(auto data_val : data)
        inv(data_val);
    return inv;
}
int main()
{
    std::string name = "name";
    //a data array to be used.
    std::vector<int> ids;
    ids.push_back(0);
    ids.push_back(1);
    try {
        pqxx::connection c("dbname=mydb user=keutoi");
        pqxx::work w(c);
        c.prepare("example", "SELECT id  FROM mytable WHERE id = $1 or id = $2");
        pqxx::prepare::invocation w_invocation = w.prepared("example");
        //dynamic array preparation
        prep_dynamic(ids, w_invocation);
        //executing prepared invocation.
        pqxx::result r = w_invocation.exec();
        w.commit();
        std::cout << r.size() << std::endl;
    }
    catch(const std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    }
    return 0;
}

如果你想处理其他数据类型，使用这个函数定义

template<class T> pqxx::prepare::invocation& prep_dynamic(std::vector<T> data, pqxx::prepare::invocation& inv)
{
    for(auto data_val : data)
        inv(data_val);
    return inv;
}

Answer 2

尽可能使用pqxx::prepare::调用，并在执行前绑定更多的值，因为它更稳定，更能防止错误，但还有一种更快的方法，我将在下面描述它。

。调用:

pqxx::nontransaction W(C);
std::string m_insertCommand = "INSERT INTO tableforperftest(column1, column2) VALUES";

unsigned  int m_nCurrentRow = 32767;
for (size_t i = 0; i < m_nCurrentRow; i++)
{
    unsigned int countOf$ = i * 2;
    for (unsigned int i = 0; i < 2; ++i)
    {
        if (i == 0)
        {
            m_insertCommand += "(";
        }
        else
        {
            m_insertCommand += ", ";
        }
        m_insertCommand += "$";
        std::stringstream ss;
        ss << countOf$ + i + 1;
        m_insertCommand += ss.str();
    }
   if(i < m_nCurrentRow - 1)
    m_insertCommand += ") ,";
}
m_insertCommand += ")";
C.prepare("insert_into_db", m_insertCommand);
pqxx::prepare::invocation inv = W.prepared("insert_into_db");
for (size_t i = 0; i < m_nCurrentRow; i++)
{
    inv(i)(i);
}
inv.exec();

二世。使用存储过程获取更多参数值:

CREATE OR REPLACE FUNCTION insertintoboosted(valuesforinsert TEXT) RETURNS VOID AS
$$ 
BEGIN
     EXECUTE 'INSERT INTO tableforperftestproof(column1, column2) VALUES (' || valuesforinsert || ')';
END;
$$
LANGUAGE plpgsql;

代码:

for (size_t i = 0; i < m_nCurrentRow; i++)
{
    if (i == 0)
        ss  << i << "," << i;
    else
        ss << "(" << i << "," << i;
    if (i < m_nCurrentRow - 1)
        ss << "),";
}
C.prepare("prep2", "select insertintoboosted($1::text)");
W.prepared("prep2")(ss).exec();

III。每次使用参数绑定和执行:

std::string m_insertCommand3 = "INSERT INTO tableforperftest(column1, column2) VALUES ($1, $2)";
C.prepare("insert_into_db3", m_insertCommand3);
for (size_t i = 0; i < m_nCurrentRow; i++)
{
    W.prepared("insert_into_db3")(i)(i).exec();
}

比较32767个insert的解决方案:

Invocation:                              --> Elapsed:    0.250292s
Stored Proc:                             --> Elapsed:    0.154507s 
Parameter binding + execution each time: --> Elapsed:    29.5566s