I have a 'small but long' task. On a user request, the query to database should happen, this query can run for some noticeable time.
So this task is moved to a thread with a run() like this:

Qt Code: 
  1. void SubTask::run() {
  2.     const QString connName = QString::number((quintptr)QThread::currentThreadId());
  3.     QSqlDatabase db2 = QSqlDatabase::cloneDatabase(db, connName);    // db is SQLite database created in the main thread
  4.     if (!db2.open()) {
  5.         err = "Failed to open db connection" + connName;
  6.         qCritical() << err;
  7.     }
  8.     QSqlQuery qry(db2);
  9.     qry.exec( .... );
  10.     qry,clear();
  11.     db2.close();
  12.     QSqlDatabase::removeDatabase(connName);
  13. }
To copy to clipboard, switch view to plain text mode 

It works fine, query is executing each time user requests it.
But in the console I have a warning:
Qt Code: 
  1. QSqlDatabasePrivate::removeDatabase: connection '140551504512768' is still in use, all queries will cease to work.
To copy to clipboard, switch view to plain text mode 
One such warning for each execution.

What am I doing wrong? How the connection should be closed correctly?