leftQuery example

$def['table'] = $this->definition['base'];
    $def['field'] = $base_field;
    $def['left_table'] = $this->tableAlias;
    $def['left_field'] = $this->field;
    $def['adjusted'] = TRUE;
    if (!empty($this->options['required'])) {
      $def['type'] = 'INNER';
    }

    if ($this->options['subquery_regenerate']) {
      // For testing only, regenerate the subquery each time.       $def['left_query'] = $this->leftQuery($this->options);
    }
    else {
      // Get the stored subquery SQL string.       $cid = 'views_relationship_groupwise_max:' . $this->view->storage->id() . ':' . $this->view->current_display . ':' . $this->options['id'];
      $cache = \Drupal::cache('data')->get($cid);
      if (isset($cache->data)) {
        $def['left_query'] = $cache->data;
      }
      else {
        $def['left_query'] = $this->leftQuery($this->options);
        \Drupal::cache('data')->set($cid, $def['left_query']);
      }