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 = 2

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "cscart_discussion_posts",
      "access_type": "system",
      "possible_keys": ["thread_id", "thread_id_2"],
      "rows": 0,
      "filtered": 0,
      "const_row_not_found": true
    },
    "table": {
      "table_name": "cscart_discussion_rating",
      "access_type": "system",
      "possible_keys": ["PRIMARY", "thread_id"],
      "rows": 0,
      "filtered": 0,
      "const_row_not_found": true
    },
    "table": {
      "table_name": "companies",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["company_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_discussion",
      "access_type": "ref",
      "possible_keys": ["object_id"],
      "key": "object_id",
      "key_length": "6",
      "used_key_parts": ["object_id", "object_type"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_discussion.object_type = 'M')"
    }
  }
}