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SELECT 
  cscart_discussion.object_id AS product_id, 
  AVG(
    cscart_discussion_rating.rating_value
  ) AS average_rating, 
  cscart_discussion.type AS discussion_type, 
  cscart_discussion.thread_id AS discussion_thread_id 
FROM 
  cscart_discussion 
  LEFT JOIN cscart_discussion_posts ON cscart_discussion_posts.thread_id = cscart_discussion.thread_id 
  AND cscart_discussion_posts.status = "A" 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion.thread_id = cscart_discussion_rating.thread_id 
  AND cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
  AND cscart_discussion_rating.rating_value != 0 
WHERE 
  cscart_discussion.object_id IN (504, 658, 659) 
  AND cscart_discussion.object_type = "P" 
  AND cscart_discussion.company_id = 1 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00056

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_discussion	range	object_id,company_id	object_id	10		3	Using index condition
1	SIMPLE	cscart_discussion_posts	ref	thread_id,thread_id_2	thread_id	3	mahm3t_cs443.cscart_discussion.thread_id	4	Using where
1	SIMPLE	cscart_discussion_rating	eq_ref	PRIMARY,thread_id	PRIMARY	3	mahm3t_cs443.cscart_discussion_posts.post_id	1	Using where

Result

product_id	average_rating	discussion_type	discussion_thread_id
504	5.0000	B	2902
658		B	1039
659		B	2952