Qt 6.2 QStandardItemModel::setRowCount(0) very slow

[Rory_1]

I have a QStandardItemModell that keeps track of image file information. Every time I select a new folder the datamodel is rebuilt. One of the resets is to call QStandardItemModel::setRowCount(0). The model has three views and a QSortFilterProxyModel.

Here is a performance comparison between Qt versions (time units are milliseconds):

                     1 row           13 rows         40 rows        50 rows     158 rows
Qt 6.2 MSVC2019          0                11            1048           1495        31767
Qt 6.2 Mac M1            0                 0               0              0            1
Qt 5.15 MSVC2019         0                 0               1              1            3

I've traced the slow preformance to qabstractitemmodel.cpp function QAbstractItemModelPrivate::rowsRemoved.

void QAbstractItemModelPrivate::rowsRemoved(const QModelIndex &parent,
                                            int first, int last)
{
    const QList<QPersistentModelIndexData *> persistent_moved = persistent.moved.pop();
    const int count = (last - first) + 1; // it is important to only use the delta, because the change could be nested
    for (auto *data : persistent_moved) {
        QModelIndex old = data->index;
        persistent.indexes.erase(persistent.indexes.constFind(old));
        data->index = q_func()->index(old.row() - count, old.column(), parent);
        if (data->index.isValid()) {
            persistent.insertMultiAtEnd(data->index, data);
        } else {
            qWarning() << "QAbstractItemModel::endRemoveRows:  Invalid index (" << old.row() - count << ',' << old.column() << ") in model" << q_func();
        }
    }
    const QList<QPersistentModelIndexData *> persistent_invalidated = persistent.invalidated.pop();

    // THIS LOOP IS SLOW IN QT 6.2 RUNNING MSVC2019
    for (auto *data : persistent_invalidated) {
        auto pit = persistent.indexes.constFind(data->index);
        if (pit != persistent.indexes.cend())
            persistent.indexes.erase(pit);
        data->index = QModelIndex();
    }
}

I built a simple minimum code to try to replicate the issue, to no avail. So, something in my program is making it difficult to update persistent indexes, but only in Qt 6+ and MSVC2019. Any clues on what could cause this would be very much appreciated.

[Christian Ehrlicher]

You should not use setRowCount() in this case but begin/endResetModel().

[Rory_1]

@Christian-Ehrlicher said in Qt 6.2 QStandardItemModel::setRowCount(0) very slow:

You should not use setRowCount() in this case but begin/endResetModel().

Thanks very much! That solved the slowdown.