PostgreSQL compatibility

It would be a better idea to convert the query to use Drupal's database abstraction layer, with something like this:

$query = $database->select('search_api_stats')
  ->fields('search_api_stats', ['keywords'])
  ->addExpression('COUNT(*)', 'num')
  ->condition('s_name', $serverName)
  ->condition('i_name', $indexName)
  ->condition('keywords', '', '<>')
  ->groupBy('keywords')
  ->orderBy('num', 'DESC')
  ->range(0, $config['num_phrases']);

$stats = $query->execute()->fetchAll(\PDO::FETCH_ASSOC);