SELECT 
  tmall_discussion_posts.*, 
  tmall_discussion_messages.message, 
  tmall_discussion_rating.rating_value 
FROM 
  tmall_discussion_posts 
  LEFT JOIN tmall_discussion_messages ON tmall_discussion_messages.post_id = tmall_discussion_posts.post_id 
  LEFT JOIN tmall_discussion_rating ON tmall_discussion_rating.post_id = tmall_discussion_posts.post_id 
WHERE 
  tmall_discussion_posts.thread_id = 20 
  AND tmall_discussion_posts.status = 'A' 
ORDER BY 
  tmall_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00798

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.005892063,
    "nested_loop": [
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "tmall_discussion_posts.`timestamp` desc",
            "table": {
              "table_name": "tmall_discussion_posts",
              "access_type": "ref",
              "possible_keys": ["thread_id", "thread_id_2"],
              "key": "thread_id",
              "key_length": "3",
              "used_key_parts": ["thread_id"],
              "ref": ["const"],
              "loops": 1,
              "rows": 1,
              "cost": 0.002024411,
              "filtered": 100,
              "attached_condition": "tmall_discussion_posts.thread_id <=> 20 and tmall_discussion_posts.`status` = 'A'"
            }
          }
        }
      },
      {
        "table": {
          "table_name": "tmall_discussion_messages",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["tmall.tmall_discussion_posts.post_id"],
          "loops": 1,
          "rows": 1,
          "cost": 0.001933826,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "tmall_discussion_rating",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["tmall.tmall_discussion_posts.post_id"],
          "loops": 1,
          "rows": 1,
          "cost": 0.001933826,
          "filtered": 100
        }
      }
    ]
  }
}