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 (145723) 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00043

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": 2,
          "filtered": 100,
          "index_condition": "cscart_discussion_posts.`status` = 'A'",
          "attached_condition": "cscart_discussion_posts.thread_id <=> 145723 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
123796 145723 tim morrissey 1468594380 0 98.19.14.61 A I got the lights I ordered when they said I would. Lights look great, only problem I had was screwing the heads onto the pipe, did not seem to want to go on straight. Finally got them to go on and everything went good after that. 4
123795 145723 Robert 1407453480 0 207.195.86.15 A Our experience with your company has been a very pleasant one. We found you easily on the internet. Decided which lights we liked and did some measuring from your spec sheet to figure out which arm would work for our application. Then I phoned Derek who walked me through the entire order and helped me figure out that we could have a custom color painted on our lights. He said the order would come right. And it did!!! That was soooo exciting! Not having to deal with disappointments and returns. We have finally installed them and they are awesome. Thanks so much barn light. You've made our project extra special! 5