SELECT 
  AVG (dr.rating_value) 
FROM 
  cscart_discussion_rating AS dr 
  LEFT JOIN cscart_discussion_posts AS dp ON dp.post_id = dr.post_id 
  LEFT JOIN cscart_discussion AS d ON d.thread_id = dp.thread_id 
WHERE 
  d.object_id = 120 
  AND d.object_type = 'M' 
  AND type IN ('B', 'R') 
  AND dp.status = 'A' 
  AND dr.rating_value > 0

Query time 0.00239

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "d",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "object_id"],
          "key": "object_id",
          "key_length": "6",
          "used_key_parts": ["object_id", "object_type"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "dp",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "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": "dp.`status` = 'A'"
        }
      },
      {
        "table": {
          "table_name": "dr",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "ref": ["live_clonned.dp.post_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "dr.rating_value > 0"
        }
      }
    ]
  }
}