QPixmap image;
QSqlQuery query;
PGresult* res = PQexec(conn, "begin");
Oid objID = query.value(0).toInt();
int descriptor = lo_open(conn, objID, INV_READ);

Now my problem is conn parameter. What must i use?
I connected to database like this.

QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");
db.setHostName(HOST);
db.setDatabaseName(DBNAME);
db.setUserName(USER);
db.setPassword(PASSWORD);