SELECT 
  cscart_discussion_rating.rating_value, 
  COUNT(
    cscart_discussion_rating.post_id
  ) as qty 
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 = 1 
  AND cscart_discussion_posts.status = 'A' 
GROUP BY 
  cscart_discussion_rating.rating_value

Query time 0.00051

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": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_discussion_rating",
      "access_type": "system",
      "possible_keys": ["PRIMARY"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result

rating_value qty
5 1