由于ORDER BY不使用索引，因此SQL查询速度较慢

I've got this query:

我有这个问题:

SELECT cl.title, cl.URL, cl.ID AS ad_id, cl.cat_id, cl.price, cs.name AS cat_name, pix.file_name, area.area_name 
FROM classifieds cl 
FORCE INDEX (advertiser_id) 
INNER JOIN classifieds_pix pix ON cl.ID = pix.classified_id 
INNER JOIN cat_names_sub cs ON cl.cat_id = cs.ID 
INNER JOIN zip_codes zip ON cl.zip_id = zip.zip_id 
INNER JOIN area_names area ON zip.area_id = area.id 
WHERE cl.confirmed = 1 AND cl.price != '' AND cl.country = 'de' 
GROUP BY cl.advertiser_id
ORDER BY cl.timestamp DESC 
LIMIT 5
SELECT cl.titl