DatabaseBackend tag invalidation performance

I just noticed that the tag invalidation logic for the DatabaseBackend is slightly flawed (if I am not reading it incorrectly). Currently, we invoke ->invalidateTags() from common.inc:

/**
 * Invalidates the items associated with given list of tags.
 *
 * Many sites have more than one active cache backend, and each backend my use
 * a different strategy for storing tags against cache items, and invalidating
 * cache items associated with a given tag.
 *
 * When invalidating a given list of tags, we iterate over each cache backend,
 * and call invalidate on each.
 *
 * @param array $tags
 *   The list of tags to invalidate cache items for.
 */
function cache_invalidate(array $tags) {
  foreach (CacheFactory::getBackends() as $bin => $class) {
    cache($bin)->invalidateTags($tags);
  }
}

That means that for every cache bin we execute that method once. So if I have 10 bins that use the DatabaseBackend (and we already got 10+ in core) we run this 10 times:

  /**
   * Implements Drupal\Core\Cache\CacheBackendInterface::invalidateTags().
   */
  public function invalidateTags(array $tags) {
    foreach ($this->flattenTags($tags) as $tag) {
      unset(self::$tagCache[$tag]);
      Database::getConnection()->merge('cache_tags')
        ->key(array('tag' => $tag))
        ->fields(array('invalidations' => 1))
        ->expression('invalidations', 'invalidations + 1')
        ->execute();
    }
  }

So, the problem with this is that ->flattenTags() may return an array with quite a few elements depending on the amount of tags and the number of key/value pairs in those tags.

Let's assume the most simple case: We want to clear 10 tags (no key/value pairs... just 10 simple tags). That means that ->flattenTags would return an array of 10 items. As previously mentioned we got about 10 bins, so that means that we run ->merge on the database 100 times (and since it's merge that means we basically run 200 queries).

I feel that I am missing something here or mis-read the code... Or simply didn't understand it fully. Otherwise it's a major issue :P. So... I apologize if this issue is nonsense!