Android Database operation in Asynctask blocks UI Thread

Question

I'm performing a Database operation in the doInBackground method inside an AsyncTask on Android.

For some reason, the UI get blocked during the 5-6 seconds that the operation takes.

It does not make any sense for me, the operacion inside the doInBackground should not be executed in the UI Thread, right?

Here is my code:

private class CountItems extends AsyncTask<String, Void, Integer> {
    private ProgressDialog dialog;

    @Override
    protected void onPreExecute() {
        dialog = new ProgressDialog(context);
        dialog.setCancelable(false);
        dialog.setMessage(getString(R.string.asynTask_loading));
        dialog.show();
    }

    @Override
    protected Integer doInBackground(String... params) {
        // This operation takes 5-6 seconds.
        return app.databaseSession().getMyObjectDao().count(selectedProject, filter, null, false);
    }

    @Override
    protected void onPostExecute(Integer result) {
        counterTextView.setText(String.valueOf(result));

        if (dialog.isShowing()) {
            dialog.dismiss();
        }
    }
}

I've made a test. If I put a Thread.sleep() inside the doInBackground method, it is executed in a different Thread without blocking the UI.

Like this:

 @Override
    protected Integer doInBackground(String... params) {
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return 0;
    }

Any ideas? Thanks in advance.