SELECT 
  cscart_discussion.object_id AS product_id, 
  AVG(
    cscart_discussion_rating.rating_value
  ) AS average_rating, 
  cscart_discussion.type AS discussion_type, 
  cscart_discussion.thread_id AS discussion_thread_id 
FROM 
  cscart_discussion 
  LEFT JOIN cscart_discussion_posts ON cscart_discussion_posts.thread_id = cscart_discussion.thread_id 
  AND cscart_discussion_posts.status = "A" 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion.thread_id = cscart_discussion_rating.thread_id 
  AND cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
  AND cscart_discussion_rating.rating_value != 0 
WHERE 
  cscart_discussion.object_id IN (
    64941, 62258, 60732, 60733, 60737, 64621, 
    61385, 64942, 60751, 60735, 64943, 
    64339, 63128, 60738, 62671, 64340, 
    61386, 64620, 64815, 65519, 65568, 
    65851, 65870, 65875
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00109

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_discussion",
      "access_type": "range",
      "possible_keys": ["object_id"],
      "key": "object_id",
      "key_length": "6",
      "used_key_parts": ["object_id", "object_type"],
      "rows": 24,
      "filtered": 100,
      "index_condition": "cscart_discussion.object_id in (64941,62258,60732,60733,60737,64621,61385,64942,60751,60735,64943,64339,63128,60738,62671,64340,61386,64620,64815,65519,65568,65851,65870,65875) and cscart_discussion.object_type = 'P'"
    },
    "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": ["cscart_migrate.cscart_discussion.thread_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_discussion_posts.`status` = 'A')",
      "using_index": true
    },
    "table": {
      "table_name": "cscart_discussion_rating",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "thread_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["post_id"],
      "ref": ["cscart_migrate.cscart_discussion_posts.post_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_discussion_rating.thread_id = cscart_discussion.thread_id and cscart_discussion_rating.rating_value <> 0 and trigcond(cscart_discussion_posts.post_id is not null))"
    }
  }
}

Result