yii\elasticsearch\BatchQueryResult::fetchData PHP Method

    protected function fetchData()
    {
        if (null === $this->_lastScrollId) {
            //first query - do search
            $options = ['scroll' => $this->scrollWindow];
            if (!$this->query->orderBy) {
                $options['search_type'] = 'scan';
            }
            $result = $this->query->createCommand($this->db)->search($options);
            //if using "scan" mode, make another request immediately
            //(search request returned 0 results)
            if (!$this->query->orderBy) {
                $result = $this->query->createCommand($this->db)->scroll(['scroll_id' => $result['_scroll_id'], 'scroll' => $this->scrollWindow]);
            }
        } else {
            //subsequent queries - do scroll
            $result = $this->query->createCommand($this->db)->scroll(['scroll_id' => $this->_lastScrollId, 'scroll' => $this->scrollWindow]);
        }
        //get last scroll id
        $this->_lastScrollId = $result['_scroll_id'];
        //get data
        return $this->query->populate($result['hits']['hits']);
    }