Why does xmlsitemap_generate_chunk ORDER BY language and loc

Thanks for your reply earnie. My question is not really a bug report, I am trying to find out, whether some performance optimisations might break anything, it is difficult to notice with a large site and sitemaps.

Re: the keys
Yes those keys are present and they are being used. My problem is that I am seeing a "using filesort". (for explanation see http://s.petrunia.net/blog/?p=24)

This occurs even when using a minimal query on a large sitemap table:
SELECT x.loc, x.lastmod, x.changefreq, x.changecount, x.priority, x.language FROM `prefix_xmlsitemap` x WHERE x.access = 1 AND x.status = 1 ORDER BY x.language, x.loc LIMIT 0,30

The EXPLAIN output is as follows:

+----+-------------+-------+------+-------------------+-------------------+---------+-------------+--------+-----------------------------+
| id | select_type | table | type | possible_keys     | key               | key_len | ref         | rows   | Extra                       |
+----+-------------+-------+------+-------------------+-------------------+---------+-------------+--------+-----------------------------+
|  1 | SIMPLE      | x     | ref  | access_status_loc | access_status_loc | 2       | const,const | 520852 | Using where; Using filesort |
+----+-------------+-------+------+-------------------+-------------------+---------+-------------+--------+-----------------------------+

Now, this could maybe be solved by further increasing mysql caches etc. but I guess I would run into other performance problems sooner or later as I am using other modules similar to domain_xmlsitemap that implement hook_query_xmlsitemap_generate_alter. These modules typically add a JOIN on the id and joining on one column an sorting on another is often a performance problem in mysql from my experience or at least a pain in the *** to get the keys right. I played around a bit an noticed, that ordering by id seems to solve my performance problem even when joining other tables.

I am not using sitemaps by language, so I am not familiar how it is implemented by this module, but I do not really see the need for special sorting, as the the languages can be taken care of in the WHERE conditions, running one context at a time.

added related issue