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 (
    83550, 83551, 83552, 83553, 86599, 86600, 
    86668, 86669, 89043, 85133, 85373, 
    86617, 86686, 83039, 90875, 90954, 
    85056, 85142, 85188, 85234, 84673, 
    84674, 85076, 85077, 85162, 85163, 
    85208, 85209, 85254, 85255, 84816, 
    84817, 86493, 86495, 86769, 86771, 
    91144, 83035, 83036, 83307, 83592, 
    83593, 85307, 85309, 85404, 85406, 
    85455, 85457
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00112

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 (83550,83551,83552,83553,86599,86600,86668,86669,89043,85133,85373,86617,86686,83039,90875,90954,85056,85142,85188,85234,84673,84674,85076,85077,85162,85163,85208,85209,85254,85255,84816,84817,86493,86495,86769,86771,91144,83035,83036,83307,83592,83593,85307,85309,85404,85406,85455,85457)) 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
83035 D 91073
83036 D 91074
83039 D 91077
83307 D 91345
83550 D 91588
83551 D 91589
83552 D 91590
83553 D 91591
83592 D 91630
83593 D 91631
84673 D 92711
84674 D 92712
84816 D 92854
84817 D 92855
85056 D 93094
85076 D 93114
85077 D 93115
85133 D 93171
85142 D 93180
85162 D 93200
85163 D 93201
85188 D 93226
85208 D 93246
85209 D 93247
85234 D 93272
85254 D 93292
85255 D 93293
85307 D 93345
85309 D 93347
85373 D 93411
85404 D 93442
85406 D 93444
85455 D 93493
85457 D 93495
86493 D 94531
86495 D 94533
86599 D 94637
86600 D 94638
86617 D 94655
86668 D 94706
86669 D 94707
86686 D 94724
86769 D 94807
86771 D 94809
89043 D 97081
90875 D 98948
90954 D 99027
91144 D 99217