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 
  LEFT JOIN cscart_cp_pr_reviews_reply ON cscart_cp_pr_reviews_reply.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 11913 
  AND cscart_discussion_posts.status = 'A' 
  AND cscart_discussion_posts.cp_pr_user_delete = 'N' 
GROUP BY 
  cscart_discussion_rating.rating_value

Query time 0.00069

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_discussion_rating.rating_value",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_discussion_posts",
              "access_type": "ref",
              "possible_keys": ["thread_id", "thread_id_2"],
              "key": "thread_id_2",
              "key_length": "6",
              "used_key_parts": ["thread_id", "status"],
              "ref": ["const", "const"],
              "rows": 1,
              "filtered": 100,
              "index_condition": "cscart_discussion_posts.`status` = 'A'",
              "attached_condition": "cscart_discussion_posts.cp_pr_user_delete = 'N'"
            }
          },
          {
            "table": {
              "table_name": "cscart_discussion_rating",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["post_id"],
              "ref": ["lillyshop_lillyshop.cscart_discussion_posts.post_id"],
              "rows": 1,
              "filtered": 100
            }
          }
        ]
      }
    }
  }
}