getAggregationInfo example

    $handler_type = $type = $form_state->get('type');
    if (!empty($types[$type]['type'])) {
      $handler_type = $types[$type]['type'];
    }

    $override = NULL;
    $executable = $view->getExecutable();
    if ($executable->display_handler->useGroupBy() && !empty($item['group_type'])) {
      if (empty($executable->query)) {
        $executable->initQuery();
      }
      $aggregate = $executable->query->getAggregationInfo();
      if (!empty($aggregate[$item['group_type']]['handler'][$type])) {
        $override = $aggregate[$item['group_type']]['handler'][$type];
      }
    }

    // Create a new handler and unpack the options from the form onto it. We     // can use that for storage.     $handler = Views::handlerManager($handler_type)->getHandler($item, $override);
    $handler->init($executable, $executable->display_handler, $item);

    // Add the incoming options to existing options because items using

$string .= $field['table'] . '.';
      }
      $string .= $field['field'];
      $fieldname = (!empty($field['alias']) ? $field['alias'] : $string);

      if (!empty($field['count'])) {
        // Retained for compatibility.         $field['function'] = 'count';
      }

      if (!empty($field['function'])) {
        $info = $this->getAggregationInfo();
        if (!empty($info[$field['function']]['method']) && is_callable([$this, $info[$field['function']]['method']])) {
          $string = $this::{$info[$field['function']]['method']}($field['function'], $string);
          $placeholders = !empty($field['placeholders']) ? $field['placeholders'] : [];
          $query->addExpression($string, $fieldname, $placeholders);
        }

        $this->hasAggregate = TRUE;
      }
      // This is a formula, using no tables.       elseif (empty($field['table'])) {
        $placeholders = !empty($field['placeholders']) ? $field['placeholders'] : [];
        

$info['id'] = $id;
        }

        // If aggregation is on, the group type might override the actual         // handler that is in use. This piece of code checks that and,         // if necessary, sets the override handler.         $override = NULL;
        if ($this->useGroupBy() && !empty($info['group_type'])) {
          if (empty($this->view->query)) {
            $this->view->initQuery();
          }
          $aggregate = $this->view->query->getAggregationInfo();
          if (!empty($aggregate[$info['group_type']]['handler'][$type])) {
            $override = $aggregate[$info['group_type']]['handler'][$type];
          }
        }

        if (!empty($types[$type]['type'])) {
          $handler_type = $types[$type]['type'];
        }
        else {
          $handler_type = $type;
        }

        

$field = $this->getFormula();
      }
      else {
        $field = $this->tableAlias . '.' . $this->realField;
      }
    }

    // If grouping, check to see if the aggregation method needs to modify the field.     if ($this->view->display_handler->useGroupBy()) {
      $this->view->initQuery();
      if ($this->query) {
        $info = $this->query->getAggregationInfo();
        if (!empty($info[$this->options['group_type']]['method'])) {
          $method = $info[$this->options['group_type']]['method'];
          if (method_exists($this->query, $method)) {
            return $this->query->$method($this->options['group_type'], $field);
          }
        }
      }
    }

    return $field;
  }