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SELECT 
  cscart_discussion_posts.*, 
  cscart_discussion_messages.message, 
  cscart_discussion_rating.rating_value 
FROM 
  cscart_discussion_posts 
  LEFT JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 3360 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00045

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_discussion_posts	ref	thread_id,thread_id_2	thread_id	3	const	2	Using where; Using filesort
1	SIMPLE	cscart_discussion_messages	eq_ref	PRIMARY	PRIMARY	3	mahm3t_cs443.cscart_discussion_posts.post_id	1
1	SIMPLE	cscart_discussion_rating	eq_ref	PRIMARY	PRIMARY	3	mahm3t_cs443.cscart_discussion_posts.post_id	1

Result

post_id	thread_id	name	timestamp	user_id	ip_address	status	message	rating_value
2076	3360	Simone aus Meißen	1634658900	0	5f5aad17	A	Wir haben eine Scherentreppe bestellt und verbaute. Von Lieferung bis zum verbauen hat alles bestens geklappt, 5 Sterne	5
1362	3360	Marcel aus Kamsdorf	1537857300	0	c25ecebb	A	Bei meinen zwei Bestellungen hat alles super geklappt. Die Ware ist top, der Kundenservice ist sehr freundlich und hilfsbereit und die Lieferung kommt immer pünktlich. Vielen Dank und bis zur nächsten Bestellung!	5