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 (
    91627, 83923, 91905, 91906, 91907, 91908, 
    91909, 91910, 91911, 92556, 92557, 
    84268, 89735, 82422, 89379, 89394, 
    89396, 89343, 89385, 89375, 89376, 
    89381, 89289, 89390, 89392, 89344, 
    89382, 89388, 89380, 89395, 89397, 
    89410, 89411, 89412, 82423, 89341, 
    89290, 91945, 91946, 91947, 91948, 
    91949, 91950, 91951, 91904, 82420, 
    89360, 89286
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00119

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 (91627,83923,91905,91906,91907,91908,91909,91910,91911,92556,92557,84268,89735,82422,89379,89394,89396,89343,89385,89375,89376,89381,89289,89390,89392,89344,89382,89388,89380,89395,89397,89410,89411,89412,82423,89341,89290,91945,91946,91947,91948,91949,91950,91951,91904,82420,89360,89286)) 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
82420 D 90458
82422 D 90460
82423 D 90461
83923 D 91961
84268 D 92306
89286 D 97324
89289 D 97327
89290 D 97328
89341 D 97379
89343 D 97381
89344 D 97382
89360 D 97398
89375 D 97413
89376 D 97414
89379 D 97417
89380 D 97418
89381 D 97419
89382 D 97420
89385 D 97423
89388 D 97426
89390 D 97428
89392 D 97430
89394 D 97432
89395 D 97433
89396 D 97434
89397 D 97435
89410 D 97448
89411 D 97449
89412 D 97450
89735 D 97773
91627 D 99700
91904 D 99982
91905 D 99983
91906 D 99984
91907 D 99985
91908 D 99986
91909 D 99987
91910 D 99988
91911 D 99989
91945 D 100023
91946 D 100024
91947 D 100025
91948 D 100026
91949 D 100027
91950 D 100028
91951 D 100029
92556 B 100696
92557 B 100697