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 (
    84770, 84783, 84792, 87538, 88418, 89787, 
    85664, 85684, 85696, 85708, 84765, 
    85660, 85721, 85738, 85748, 85758, 
    87449, 88494, 88446, 88463, 88479, 
    89020, 89785, 85718, 88167, 88215, 
    88192, 88204, 88400, 84769, 84782, 
    84791, 85663, 85683, 85695, 85707, 
    87448, 88492, 84764, 85659, 89019, 
    89104, 89112, 87504, 87520, 88444, 
    88462, 88478, 90831, 88191, 88203, 
    87536, 88399, 88493, 84768, 84781, 
    84790, 87503, 87519, 88442, 88459, 
    88476, 88521, 89838, 89890, 89962, 
    88445, 84763, 89103, 89111, 90830, 
    89786, 89836, 89888, 89960, 87535, 
    88398, 88491, 88443, 90816, 89018, 
    89784, 87616, 87624, 90815, 89102, 
    89110, 90829, 89017, 88547, 87620, 
    87622, 88543, 89016
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00089

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "108.36"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "range",
            "possible_keys": [
              "object_id"
            ],
            "key": "object_id",
            "used_key_parts": [
              "object_id",
              "object_type"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 94,
            "rows_produced_per_join": 94,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (84770,84783,84792,87538,88418,89787,85664,85684,85696,85708,84765,85660,85721,85738,85748,85758,87449,88494,88446,88463,88479,89020,89785,85718,88167,88215,88192,88204,88400,84769,84782,84791,85663,85683,85695,85707,87448,88492,84764,85659,89019,89104,89112,87504,87520,88444,88462,88478,90831,88191,88203,87536,88399,88493,84768,84781,84790,87503,87519,88442,88459,88476,88521,89838,89890,89962,88445,84763,89103,89111,90830,89786,89836,89888,89960,87535,88398,88491,88443,90816,89018,89784,87616,87624,90815,89102,89110,90829,89017,88547,87620,87622,88543,89016)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "56.41",
              "eval_cost": "9.40",
              "prefix_cost": "65.81",
              "data_read_per_join": "2K"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "nuie_scalesta_net.cscart_discussion.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 94,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "23.50",
              "eval_cost": "9.40",
              "prefix_cost": "98.71",
              "data_read_per_join": "41K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_posts), (`nuie_scalesta_net`.`cscart_discussion_posts`.`status` = 'A'), true)"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_rating",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 94,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "9.40",
              "prefix_cost": "108.36",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_rating), ((`nuie_scalesta_net`.`cscart_discussion_rating`.`post_id` = `nuie_scalesta_net`.`cscart_discussion_posts`.`post_id`) and (`nuie_scalesta_net`.`cscart_discussion_rating`.`thread_id` = `nuie_scalesta_net`.`cscart_discussion`.`thread_id`) and (`nuie_scalesta_net`.`cscart_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}