CREATE TABLE variant
    (
      id serial NOT NULL,
      variant bytea
    );

    QVariant var = QDateTime::currentDateTime();
//    QVariant var = 1234567;
//    QVariant var = 1234.5678;
//    QVariant var = true;
    QByteArray arr;
    QDataStream out(&arr,QIODevice::WriteOnly);
    out << var;

    textEdit->append(QString("in = %1").arg(var.toString()));

    QSqlQuery query(db);
    query.prepare("INSERT INTO variant(variant) VALUES (:variant)");
    query.bindValue(":variant", arr);
    query.exec();
    int id = query.lastInsertId().toInt();

    // -------------

    query.prepare("select * from variant where id = :id");
    query.bindValue(":id", id);
    query.exec();

    query.next();
    QSqlRecord rec = query.record();

    QByteArray arr2 = query.value(rec.indexOf("variant")).toByteArray();
    QDataStream in(&arr2,QIODevice::ReadOnly);
    QVariant var2 = QVariant(in);

    textEdit->append(QString("out = %1").arg(var2.toString()));