How can I get additional fields from Search API query?
Unfortunately, it looks like this is impossible.

Looking at \Drupal\search_api_db\Plugin\search_api\backend\Database::search:

  $result = $db_query-&gt;execute();

  foreach ($result as $row) {
    $item = $this-&gt;getFieldsHelper()-&gt;createItem($index, $row-&gt;item_id);
    $item-&gt;setScore($row-&gt;score / self::SCORE_MULTIPLIER);
    $results-&gt;addResultItem($item);
  }


The $item will receive only the item_id from the fetched row, so all added fields are lost :(