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 = 20 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00759

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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": 1,
              "filtered": 100,
              "index_condition": "cscart_discussion_posts.`status` = 'A'",
              "attached_condition": "cscart_discussion_posts.thread_id <=> 20 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.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.cscart_discussion_posts.post_id"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}