fast way to update 10M record in Mysql database using QT

[AmrCoder]

I have a 500MB file which contains a data that I need to use it to update a table that contains 10M rows so I use this code in using QT SQL library

QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
// my database info 
db.transaction(); // start the transaction
    QSqlQuery q;
    QString sql = ""; // to append many sql statment to this string

    QFile file(dataFile);
    file.open(QIODevice::Text | QIODevice::ReadOnly);
    QTextStream stream(&file);

    int counter = 0;
    clock_t tStart = clock();
    while (!stream.atEnd()) {
        counter++;
        QString str = stream.readLine();
        QStringList list = str.split(';');
        QString id = list[0];
        QString streetName = list[1];
        QString HouseName = list[2];
        QString HouseNumber = list[3];
        QString city = list[4];
        QString postalCode = list[5];

        QString temp_sql = "UPDATE temp_table_copy SET street_name='!1', house_number='!2',postal_code='!3',address_city='!4', house='!5' where id=" + id + ";";
        temp_sql = temp_sql.replace("!1", streetName);
        temp_sql = temp_sql.replace("!2", HouseNumber);
        temp_sql = temp_sql.replace("!3", postalCode);
        temp_sql = temp_sql.replace("!4", city);
        temp_sql = temp_sql.replace("!5", HouseName);

        sql += temp_sql;
        if (sql.size() >= 100000) {
            if (!q.exec(sql)) {
                 qDebug() << "Excute Error: " << q.lastError().text();
            }
            sql = "";
            printf("Finished: %d sql statment\n", counter);
        }

        if (counter == Number)
            break;
    }
    if (!sql.isEmpty()) {
        if (!q.exec(sql)) {
            qDebug() << "Excute Error: " << q.lastError().text();
        }
        printf("Finished: %d sql statment\n", counter);
    }
    db.commit(); // commit and


    printf("Time taken: %.2fs\n", (double)(clock() - tStart)/CLOCKS_PER_SEC);
    db.close();

the idea is I append the big SQL string with the SQL statement and when it reaches the 100,000 in size I execute it and empty the string to avoid bad allocation when the string is too much size in memory, and also I read that MySQL database has a certain size to the SQL statement to be executed and I get its limit using

SHOW VARIABLES LIKE 'max_allowed_packet';

the problem here is that not all the data are updated some ids have a value in the text file and not updated inside the MySQL database table so what can be the problem? those columns are null before the update so when I try to count how many not update I found 7M only which means some ids are not updated and also I did not get any error from the query, any help or suggestion to know why this happened, or if there is another way to it.
Thanks in advance.

[Christian Ehrlicher]

Use a prepared statement + db transaction. Your current approach is a big invitation for SQL injection.

First read the data into a QVector<struct> and then add it to the database:

QVector <MyStruct> data;
while (!stream.atEnd()) {
  data += ...
}

QSqlDatabase db = ...
db.beginTransaction()
QSqlQuery q(db);
if (q.prepare("UPDATE temp_table_copy SET street_name=:sn, house_number=:hn, postal_code=: pc, address_city=:ac, house=:house where id= :id"))
  return false;
for (const auto &line : data)
{
  q.bindValue(":sn", line.street_name);
  q.bindValue(":hn", line.house_number);
 ...
  q.exec();
}

db.commit();

[AmrCoder]

is this the faster thing we can get, is it fine to use exec on each loop? I still running the command and I will write to you how much it take to finish

[saulos]

Performance can be better by having an index on the WHERE field (ID) another think is use a Store Procedure and pass the data to the SP this way the SQL don't spend time to recompile the SQL statement every time you call it

[Christian Ehrlicher]

@saulos said in fast way to update 10M record in Mysql database using QT:

SQL don't spend time to recompile the SQL statement every time you call it

As you can see the statement is a prepared statement in my example - it's recompiled exactly once

[JonB]

@AmrCoder
One observation: you are doing 500MB's-worth of data read update statements inside a single transaction. I don't know what sort of behind-the-scenes usage that puts on your database. Do you want all of these updates to either succeed or fail as a single transaction? There are possibilities like taking a table lock for a large number of updates. Or there are "batch" operations, include QSqlQuery::execBatch(). You might like to investigate these possibilities.

[saulos]

@Christian-Ehrlicher
I was not referring to your code, it was just a generic suggestion :)