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 = 0 
  AND d.object_type = 'E' 
  AND type IN ('B', 'R') 
  AND d.company_id = 1 
  AND dp.status = 'A' 
  AND dr.rating_value > 0

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "dr",
      "access_type": "system",
      "possible_keys": ["PRIMARY"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "dp",
      "access_type": "system",
      "possible_keys": ["PRIMARY", "thread_id", "thread_id_2"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "d",
      "access_type": "const",
      "possible_keys": ["PRIMARY", "object_id", "idx_company_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["thread_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    }
  }
}