function hook_views_query_alter(ViewExecutable $view, QueryPluginBase $query) {

  // (Example assuming a view with an exposed filter on node title.)
  // If the input for the title filter is a positive integer, filter against
  // node ID instead of node title.
  if ($view
    ->id() == 'my_view' && is_numeric($view->exposed_raw_input['title']) && $view->exposed_raw_input['title'] > 0) {

    // Traverse through the 'where' part of the query.
    foreach ($query->where as &$condition_group) {
      foreach ($condition_group['conditions'] as &$condition) {

        // If this is the part of the query filtering on title, change the
        // condition to filter on node ID.
        if ($condition['field'] == 'node.title') {
          $condition = [
            'field' => 'node.nid',
            'value' => $view->exposed_raw_input['title'],
            'operator' => '=',
          ];
        }
      }
    }
  }
}