C/C++ Postgres libpq execute query with params chatGpt example

PGresult *PQexecParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); give example

ChatGPT said:

Here’s a basic example demonstrating how to use PQexecParams in C++ to execute a parameterized query in PostgreSQL.

Example: Inserting Data Using PQexecParams

Let's assume we are inserting a row into a table called users with the following columns:

• id (integer)
• name (text)
• age (integer)

cpp
#include <iostream>
#include <libpq-fe.h>
#include <memory>
#include <string>

void checkResult(PGresult* res, PGconn* conn) {
    if (PQresultStatus(res) != PGRES_COMMAND_OK) {
        std::cerr << "Execution failed: " << PQerrorMessage(conn) << std::endl;
        PQclear(res);
        PQfinish(conn);
        exit(1);
    }
}

int main() {
    // Connect to the database
    PGconn* conn = PQconnectdb("dbname=yourdb user=youruser password=yourpassword");
    
    if (PQstatus(conn) != CONNECTION_OK) {
        std::cerr << "Connection to database failed: " << PQerrorMessage(conn) << std::endl;
        PQfinish(conn);
        return 1;
    }

    // Query to insert a new user
    const char* command = "INSERT INTO users (id, name, age) VALUES ($1::int4, $2::text, $3::int4)";

    // Parameters
    int nParams = 3;

    // Define the OID types for each parameter (integer and text types)
    Oid paramTypes[] = {23, 25, 23};  // 23 = Oid for integer, 25 = Oid for text

    // Parameter values
    std::string id = "101";
    std::string name = "Alice";
    std::string age = "25";

    const char* paramValues[] = { id.c_str(), name.c_str(), age.c_str() };

    // Specify the lengths of each parameter. If text-based parameters, set to 0.
    int paramLengths[] = { 0, 0, 0 };

    // Specify the format of each parameter (0 = text, 1 = binary)
    int paramFormats[] = { 0, 0, 0 };

    // Result format (0 = text, 1 = binary)
    int resultFormat = 0;

    // Execute the command
    PGresult* res = PQexecParams(
        conn,
        command,
        nParams,
        paramTypes,
        paramValues,
        paramLengths,
        paramFormats,
        resultFormat
    );

    // Check for a successful execution
    checkResult(res, conn);

    std::cout << "Insert successful!" << std::endl;

    // Clean up
    PQclear(res);
    PQfinish(conn);

    return 0;
}

Explanation:

• command: The SQL query uses placeholders $1, $2, and $3 to specify the positions for the parameters.
• nParams: The number of parameters passed to the query (in this case, 3).
• paramTypes[]: The OIDs of the parameter types. We use OIDs 23 for int4 (integers) and 25 for text.
• paramValues[]: The actual values for each placeholder in the query.
• paramLengths[]: Since all values are text-based, the lengths are set to 0 (indicating null-terminated C-strings).
• paramFormats[]: Specifies the format of the parameters (0 for text, 1 for binary). Here, all are text.
• resultFormat: The result format. If 0, results are returned in text format.

Notes:

1. This example assumes a table structure that matches the query.
2. The checkResult function checks for query execution errors and exits if necessary.
3. The PQclear(res) and PQfinish(conn) calls ensure proper cleanup of the PGresult and connection.

This C++ example allows for inserting data into a PostgreSQL database in a parameterized manner using PQexecParams.

You said:

also giv example using combination of binary n text format

ChatGPT said:

Using a combination of binary and text formats with PQexecParams involves specifying which parameters should be transmitted in binary format and which in text. Similarly, you can request the result in either binary or text format.

In this example, we will insert data into a table where:

• id (integer) will be passed in binary format.
• name (text) will be passed in text format.
• age (integer) will be passed in binary format.

Here's an example illustrating the use of a combination of binary and text formats:

Example: Using Mixed Binary and Text Formats

Assume you have a users table like before:

sql
CREATE TABLE users (
    id INT PRIMARY KEY,
    name TEXT,
    age INT
);

Now, the corresponding C++ code:

cpp
#include <iostream>
#include <libpq-fe.h>
#include <memory>
#include <cstring>
#include <arpa/inet.h>  // For htonl to convert to network byte order

void checkResult(PGresult* res, PGconn* conn) {
    if (PQresultStatus(res) != PGRES_COMMAND_OK) {
        std::cerr << "Execution failed: " << PQerrorMessage(conn) << std::endl;
        PQclear(res);
        PQfinish(conn);
        exit(1);
    }
}

int main() {
    // Connect to the database
    PGconn* conn = PQconnectdb("dbname=yourdb user=youruser password=yourpassword");
    
    if (PQstatus(conn) != CONNECTION_OK) {
        std::cerr << "Connection to database failed: " << PQerrorMessage(conn) << std::endl;
        PQfinish(conn);
        return 1;
    }

    // Query to insert a new user
    const char* command = "INSERT INTO users (id, name, age) VALUES ($1::int4, $2::text, $3::int4)";

    // Parameters
    int nParams = 3;

    // Define the OID types for each parameter (integer and text types)
    Oid paramTypes[] = {23, 25, 23};  // 23 = Oid for integer, 25 = Oid for text

    // Binary representation of `id` and `age`
    int id = 101;
    int age = 25;

    // We need to convert integer values to network byte order for binary transmission.
    int32_t binaryId = htonl(id);
    int32_t binaryAge = htonl(age);

    // Text parameter
    std::string name = "Alice";

    // Parameter values
    const char* paramValues[] = {
        reinterpret_cast<const char*>(&binaryId),  // Pass id as binary
        name.c_str(),                             // Pass name as text
        reinterpret_cast<const char*>(&binaryAge)  // Pass age as binary
    };

    // Specify the lengths of each parameter (binary parameters need their actual size, text = 0)
    int paramLengths[] = { sizeof(binaryId), 0, sizeof(binaryAge) };

    // Specify the format of each parameter (0 = text, 1 = binary)
    int paramFormats[] = { 1, 0, 1 };  // id and age in binary, name in text

    // Result format (0 = text, 1 = binary)
    int resultFormat = 0;  // Requesting result in text format

    // Execute the command
    PGresult* res = PQexecParams(
        conn,
        command,
        nParams,
        paramTypes,
        paramValues,
        paramLengths,
        paramFormats,
        resultFormat
    );

    // Check for a successful execution
    checkResult(res, conn);

    std::cout << "Insert successful!" << std::endl;

    // Clean up
    PQclear(res);
    PQfinish(conn);

    return 0;
}

Explanation:

1. paramTypes[]: We specify the OIDs of the types: 23 for int4 (integer) and 25 for text.

2. Binary Integer Conversion:

   • PostgreSQL expects integers in network byte order when transmitted as binary. The function htonl (Host to Network Long) is used to convert integers to network byte order.
   • We pass binaryId and binaryAge as binary parameters using reinterpret_cast<const char*> to treat them as raw byte data.

3. Text and Binary Formats:

   • paramFormats[]: This specifies the format of each parameter:
     • 1 indicates binary format for id and age.
     • 0 indicates text format for name.

4. paramLengths[]:

   • For binary parameters, we specify the size of the data in bytes (sizeof(int32_t)).
   • For text parameters, the length is set to 0 as the C string is null-terminated.

5. Result Format:

   • The result format is set to 0, meaning we request the result (if any) in text format.

Output:

• The program inserts the row (id = 101, name = Alice, age = 25) into the users table using a combination of binary and text formats.

Important Notes:

• For binary parameters (like id and age), we ensure they are in network byte order using htonl.
• Binary parameters are passed as raw byte arrays using reinterpret_cast<const char*>.
• The result format can be either text (0) or binary (1), depending on how you want to handle the response data.

This approach allows you to mix both text and binary formats in your queries, depending on what suits your data or performance needs.

----------------------------------------

PGresult *PQdescribePortal(PGconn *conn, const char *portalName); what is portal?

ChatGPT said:

In PostgreSQL, a portal represents a cursor or the internal name for a query result in a session. It is mainly used when executing queries that are intended to return a large amount of data, especially in scenarios where you want to fetch a portion of the result at a time.

Key Points About Portals:

1. Cursors: When you declare and use a cursor in PostgreSQL, it creates a portal. A cursor allows you to retrieve query results incrementally (i.e., a few rows at a time) instead of loading the entire result set into memory.

   For example:

   sql
   DECLARE my_cursor CURSOR FOR SELECT * FROM my_table;
   FETCH 10 FROM my_cursor;
   

2. Explicit and Implicit Portals:

   • Explicit Portals: Created when you explicitly declare a cursor using SQL's DECLARE CURSOR statement.
   • Implicit Portals: PostgreSQL also creates a portal for every SQL query executed by the server. However, these portals are usually unnamed and are not used directly in most typical SQL queries.

3. How Portals Work:

   • When a portal (cursor) is created, PostgreSQL prepares to execute a query but doesn't fetch all the data immediately. Instead, it retrieves rows in chunks when you use commands like FETCH or MOVE.
   • This behavior is especially useful for large result sets, as it helps conserve memory by not loading the entire dataset at once.

4. Portals in the Context of libpq (C Library):

   • In the PostgreSQL C client library (libpq), portals are relevant when you want to perform extended query protocol operations like fetching rows from a cursor (portal).
   • PQdescribePortal is a function used to retrieve information about the portal, such as the result types (columns) that it will produce when rows are fetched.

   Example in C:

   c
   PGconn *conn = PQconnectdb("dbname=mydb user=myuser password=mypass");
   PGresult *res = PQexec(conn, "DECLARE my_cursor CURSOR FOR SELECT * FROM my_table");
   
   // Describe the portal (cursor) named 'my_cursor'
   PGresult *descRes = PQdescribePortal(conn, "my_cursor");
   
   if (PQresultStatus(descRes) == PGRES_COMMAND_OK) {
       printf("Portal described successfully.\n");
   } else {
       printf("Error describing portal: %s\n", PQerrorMessage(conn));
   }
   
   PQclear(res);
   PQclear(descRes);
   PQfinish(conn);
   

5. Fetching Rows from a Portal: After a portal is created, you can fetch rows from it using the FETCH command (or equivalent libpq functions). This allows you to control how many rows you retrieve from the result set at a time.

6. Closing a Portal: Once you're done with a portal, you can explicitly close it using the CLOSE command or allow it to be closed implicitly when the session ends.

Use Case:

• Portals are commonly used when querying large tables where you want to retrieve results incrementally, either for performance reasons (memory management) or to allow for processing results in chunks without waiting for the entire result set to be returned.