SELECT 
  cscart_discussion_posts.*, 
  cscart_discussion_messages.message, 
  cscart_discussion_rating.rating_value 
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 IN (36990) 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00075

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_discussion_posts.`timestamp` desc",
        "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": 7,
          "filtered": 100,
          "index_condition": "cscart_discussion_posts.`status` = 'A'",
          "attached_condition": "cscart_discussion_posts.thread_id <=> 36990 and cscart_discussion_posts.`status` <=> 'A'"
        }
      }
    },
    "table": {
      "table_name": "cscart_discussion_messages",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["post_id"],
      "ref": ["cscart_migrate.cscart_discussion_posts.post_id"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_discussion_rating",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["post_id"],
      "ref": ["cscart_migrate.cscart_discussion_posts.post_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result

post_id thread_id name timestamp user_id ip_address status email message rating_value
122365 36990 Roxanne 1507836060 0 4cb0943b A I love these Sconces. I bought 2 for each side of my bed since my bedside lamp wasn't giving me the look and lighting I wanted. These are well-made, look amazing and the customer service I received was top notch. Thanks!! 5
120351 36990 Jane 1473504900 0 100.36.23.13 A We ordered 2 of these lights to go along side of the newly painted front door of our "farmhouse" style house. We don't know why we waited so long to get these - they look great! We even were able to install them ourselves. These are quality lights with a beautiful look. 5
118970 36990 Kristi 1460988300 0 162.157.61.133 A I couldn't possibly love these lights any more than I do ... LOVE the rustic galvanized metal finish and the vintage style was the perfect match for our farmhouse style bathroom! Production and shipping were pretty quick and the quality is incredible! Nothing but amazing things to say about these lights and the company that makes them! 5
118225 36990 Jeannie 1455850680 0 66.87.121.246 A couldn't say enough great things about my light or the company! Absolutely fabulous 5
17117 36990 Dr. Linda 1399725420 0 137.118.138.231 A Finally got the electricians to put up the sconce in a guest bedroom on either side of a pull out sleeper. We ordered in black and the feature wall is a color called "Blonde". It looks wonderful, but have not turned on as we do not have electrical to the house yet. But get lots of comments on it! 5
7045 36990 Cathe Lisk 1374687060 0 64.35.193.38 A I love this bantam laramie, its the perfect size and looks great. Well packaged, arrived on time and is beautiful. Thank you! 5
7044 36990 M. Butler 1368395820 0 107.3.31.194 A I am so happy with this light - it was the perfect piece for my bathroom! 5