`files:scan-app-data` database optimization

I found something that really speed up files:scan-app-data cron

function getUsersToScan in ./lib/BackgroundJob/ScanFiles.php generarate this SQL:

SELECT MAX(`user_id`) FROM `oc_filecache` `f` INNER JOIN `oc_mounts` `m`  ON `storage_id` = `storage` WHERE `size` < 0 GROUP BY `storage_id`;

but it use fs_size index on oc_filecache, and I think is wrong

Better will work to use fs_storege_size index

For my instance it took about 10 minutes on fs_size index, but on fs_storage_size it takes 2-3 seconds!

So my suggestion is change function to:

private function getUsersToScan(): array {
                $query = $this->connection->getQueryBuilder();
                $query->select($query->func()->max('user_id'))
                        ->from('filecache FORCE INDEX(`fs_storage_size`)', 'f')
                        ->innerJoin('f', 'mounts', 'm', $query->expr()->eq('storage_id', 'storage'))
                        ->where($query->expr()->lt('size', $query->createNamedParameter(0, IQueryBuilder::PARAM_INT)))
                        ->groupBy('storage_id')
                        ->setMaxResults(self::USERS_PER_SESSION);

                return $query->execute()->fetchAll(\PDO::FETCH_COLUMN);
        }

Start in lib/BackgroundJob/ScanFiles.php at getUsersToScan() and inspect the query builder call and the fs_size and fs_storage_size indexes mentioned in the issue. Run the files:scan-app-data cron against a comparable database and verify that the generated query uses the intended index and improves scan time without changing the returned users.