public function ensureGetIndex(array $beforeSort = [], array $afterSort = [])
{
//using general rule: equality, sort, range or more equality tests in that order for index
$completeFields = ['running' => 1];
self::verifySort($beforeSort, 'beforeSort', $completeFields);
$completeFields['priority'] = 1;
$completeFields['created'] = 1;
self::verifySort($afterSort, 'afterSort', $completeFields);
$completeFields['earliestGet'] = 1;
//for the main query in get()
$this->ensureIndex($completeFields);
//for the stuck messages query in get()
$this->ensureIndex(['running' => 1, 'resetTimestamp' => 1]);
}