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 (
    83397, 83407, 86140, 86163, 86648, 86717, 
    83448, 83453, 89760, 89761, 89811, 
    89863, 89935, 91501, 85299, 85396, 
    85447, 85498, 85328, 85329, 85425, 
    85426, 85476, 85477, 85527, 85528, 
    85090, 85176, 85222, 85268, 85345, 
    83066, 83067, 83619, 83620, 89775, 
    90902, 90903, 90981, 90982, 82611, 
    85374, 85375, 82638, 83535, 85130, 
    89814, 89866
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00138

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "55.46"
    },
    "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": 48,
            "rows_produced_per_join": 48,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (83397,83407,86140,86163,86648,86717,83448,83453,89760,89761,89811,89863,89935,91501,85299,85396,85447,85498,85328,85329,85425,85426,85476,85477,85527,85528,85090,85176,85222,85268,85345,83066,83067,83619,83620,89775,90902,90903,90981,90982,82611,85374,85375,82638,83535,85130,89814,89866)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "28.81",
              "eval_cost": "4.80",
              "prefix_cost": "33.61",
              "data_read_per_join": "1K"
            },
            "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": 48,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "12.00",
              "eval_cost": "4.80",
              "prefix_cost": "50.41",
              "data_read_per_join": "21K"
            },
            "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": 48,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "4.80",
              "prefix_cost": "55.46",
              "data_read_per_join": "768"
            },
            "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)"
          }
        }
      ]
    }
  }
}

Result

product_id average_rating discussion_type discussion_thread_id
82611 D 90649
82638 D 90676
83066 D 91104
83067 D 91105
83397 D 91435
83407 D 91445
83448 D 91486
83453 D 91491
83535 D 91573
83619 D 91657
83620 D 91658
85090 D 93128
85130 D 93168
85176 D 93214
85222 D 93260
85268 D 93306
85299 D 93337
85328 D 93366
85329 D 93367
85345 D 93383
85374 D 93412
85375 D 93413
85396 D 93434
85425 D 93463
85426 D 93464
85447 D 93485
85476 D 93514
85477 D 93515
85498 D 93536
85527 D 93565
85528 D 93566
86140 D 94178
86163 D 94201
86648 D 94686
86717 D 94755
89760 D 97798
89761 D 97799
89775 D 97813
89811 D 97849
89814 D 97852
89863 D 97901
89866 D 97904
89935 D 97973
90902 D 98975
90903 D 98976
90981 D 99054
90982 D 99055
91501 D 99574