public function column($field, $db = null)
    {
        if ($this->emulateExecution) {
            return [];
        }
        $command = $this->createCommand($db);
        $command->queryParts['_source'] = [$field];
        $result = $command->search();
        if (empty($result['hits']['hits'])) {
            return [];
        }
        $column = [];
        foreach ($result['hits']['hits'] as $row) {
            if (isset($row['fields'][$field])) {
                $column[] = $row['fields'][$field];
            } elseif (isset($row['_source'][$field])) {
                $column[] = $row['_source'][$field];
            } else {
                $column[] = null;
            }
        }
        return $column;
    }