perf(folder): avoid on-disk temp table when sorting folders by group count

[solracsf]

Summary

Found while working on nextcloud/server#60932

The admin list, when sorted by the Groups column, calls FolderManager::getAllFoldersWithSize($offset, $limit, 'groups', $order). That code path ordered folders by their applicable-group count with a LEFT JOIN group_folders_groups + GROUP BY f.folder_id:

$query
    ->leftJoin('f', 'group_folders_groups', 'g', $query->expr()->eq('f.folder_id', 'g.folder_id'))
    ->groupBy('f.folder_id')
    ->orderBy($query->func()->count('g.applicable_id'), $order);

The GROUP BY forces the database to materialize an internal temporary table. Because selectWithFileCache() selects the options column, a LONGTEXT, that temporary table cannot use the in-memory MEMORY engine (it cannot hold TEXT/BLOB), so MariaDB/MySQL always spill it to an on-disk (Aria) temporary table, regardless of tmp_table_size, on every request that uses this sort.

Validation (MariaDB 10.11, 3 group folders, 60k oc_filecache rows)

EXPLAIN, driving table f:

	Extra
before	Using where; Using temporary; Using filesort
after	Using where; Using filesort

SHOW STATUS around the query (tmp_table_size = max_heap_table_size = 64M):

	Created_tmp_disk_tables
before	1
after	0

Result-equivalence: the returned folder order is identical for both ASC and DESC, verified by building the query through IQueryBuilder against the real database.

Tests

Adds FolderManagerTest::testGetAllFoldersWithSizeOrderedByGroups, which creates folders with 1/2/3 applicable groups and asserts the 'groups' sort returns them ordered by group count (both ascending and descending).

[icewind1991]

It's probably best to just drop the ordering altogether

[solracsf]

Cc @provokateurin since this has been introduced by 38dfb30