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SELECT 
  companies.*, 
  AVG(
    cscart_discussion_rating.rating_value
  ) AS average_rating, 
  CONCAT(
    companies.company_id, 
    '_', 
    IF (
      cscart_discussion_rating.thread_id, 
      cscart_discussion_rating.thread_id, 
      '0'
    )
  ) AS company_thread_ids 
FROM 
  cscart_companies AS companies 
  LEFT JOIN cscart_discussion ON cscart_discussion.object_id = companies.company_id 
  AND cscart_discussion.object_type = 'M' 
  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 
WHERE 
  companies.company_id = 1

Query time 0.00025

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	companies	system	PRIMARY				1
1	SIMPLE	cscart_discussion	ref	object_id	object_id	6	const,const	1	Using where
1	SIMPLE	cscart_discussion_posts	ref	thread_id,thread_id_2	thread_id	3	cscart_dev_4_16.cscart_discussion.thread_id	2	Using where
1	SIMPLE	cscart_discussion_rating	eq_ref	PRIMARY,thread_id	PRIMARY	3	cscart_dev_4_16.cscart_discussion_posts.post_id	1	Using where

Result

company_id	status	company	lang_code	address	city	state	country	zipcode	email	phone	url	storefront	secure_storefront	entry_page	redirect_customer	countries_list	timestamp	shippings	logos	request_user_id	request_account_name	request_account_data	average_rating	company_thread_ids
1	A	CS-Cart	en	Boston street	Boston	MA	US	02125	cs-cart@example.com	16175556985	http://example.com	cs-cart.pixy.pro	cs-cart.pixy.pro	none	Y		1269610461	1		0				1_0