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 (
    4193, 4194, 4195, 4199, 4200, 4201, 4202, 
    4367, 4020, 4021, 4022, 4023, 3934, 
    3932, 3970, 4024, 4014, 4065, 4113, 
    4086, 4089, 4092, 4093, 4097, 4018, 
    4028, 3929, 4052, 4010, 3955
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00052

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": 30,
      "filtered": 100,
      "index_condition": "cscart_discussion.object_id in (4193,4194,4195,4199,4200,4201,4202,4367,4020,4021,4022,4023,3934,3932,3970,4024,4014,4065,4113,4086,4089,4092,4093,4097,4018,4028,3929,4052,4010,3955) 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": ["ymonline_livedata.cscart_discussion.thread_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_discussion_posts.`status` = 'A')"
    },
    "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": ["ymonline_livedata.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))"
    }
  }
}