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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 = 1701 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00033

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_2	6	const,const	1	Using index condition; 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
1623	1701	Yvan Liège, Belgique	1585905180	0	959ae5ac	A	Conforme aux attentes	5