QCombobox works very slow with QSqlQueryModel with large model

Question

I have few comboboxes with very dig data sets within ~ 100K rows and more. I tried it with QStandardItemModel - works fast enough if model is preloaded, also mo

Answer 1

Found the solution. 1st thought was to find what model will work faster, for example QStringListModel to replace QStandardItemModel or QSqlQueryModel. However seems that they work almost same speed. 2nd I found in Qt doc that combobox by default uses QStandardItemModel to store the items and a QListView subclass displays the popuplist. You can access the model and view directly (with model() and view()). That was strange for me as I know QTreeView works just fine with even bigger amount of data, and simpler QListView also inherited from QAbstractItemView should do this as well. I start to digg into QListView and found properties in it which had solved the problem: now combobox opens immediately on large amount of data. Static function was written to tweak all of such combos (comments with explanation are from Qt doc):

void ComboboxTools::tweak(QComboBox *combo)
{
  //  For performance reasons use this policy on large models
  // or AdjustToMinimumContentsLengthWithIcon
  combo->setSizeAdjustPolicy(QComboBox::AdjustToMinimumContentsLength);

  QListView *view = (QListView *)combo->view();
  // Improving Performance:  It is possible to give the view hints
  // about the data it is handling in order to improve its performance
  // when displaying large numbers of items. One approach that can be taken
  // for views that are intended to display items with equal sizes
  // is to set the uniformItemSizes property to true.
  view->setUniformItemSizes(true);
  // This property holds the layout mode for the items. When the mode is Batched,
  // the items are laid out in batches of batchSize items, while processing events.
  // This makes it possible to instantly view and interact with the visible items
  // while the rest are being laid out.
  view->setLayoutMode(QListView::Batched);
  // batchSize : int
  // This property holds the number of items laid out in each batch
  // if layoutMode is set to Batched. The default value is 100.
}