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 (
    92556, 86235, 86238, 86229, 89593, 89594, 
    86231, 86228, 86223, 89004, 91961, 
    91962, 91963, 91964, 91965, 91966, 
    91967, 86227, 89591, 89592, 86226, 
    86237, 86230, 91960, 91953, 91954, 
    91955, 91956, 91957, 91958, 91959, 
    86236, 86222, 86224, 82333, 86225, 
    82335, 82401, 82409, 82419, 86234, 
    82384, 82345, 91952, 82400, 82408, 
    82418, 82344
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00101

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 (92556,86235,86238,86229,89593,89594,86231,86228,86223,89004,91961,91962,91963,91964,91965,91966,91967,86227,89591,89592,86226,86237,86230,91960,91953,91954,91955,91956,91957,91958,91959,86236,86222,86224,82333,86225,82335,82401,82409,82419,86234,82384,82345,91952,82400,82408,82418,82344)) 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
82333 D 90370
82335 D 90372
82344 D 90381
82345 D 90382
82384 D 90422
82400 D 90438
82401 D 90439
82408 D 90446
82409 D 90447
82418 D 90456
82419 D 90457
86222 D 94260
86223 D 94261
86224 D 94262
86225 D 94263
86226 D 94264
86227 D 94265
86228 D 94266
86229 D 94267
86230 D 94268
86231 D 94269
86234 D 94272
86235 D 94273
86236 D 94274
86237 D 94275
86238 D 94276
89004 D 97042
89591 D 97629
89592 D 97630
89593 D 97631
89594 D 97632
91952 D 100030
91953 D 100031
91954 D 100032
91955 D 100033
91956 D 100034
91957 D 100035
91958 D 100036
91959 D 100037
91960 D 100038
91961 D 100039
91962 D 100040
91963 D 100041
91964 D 100042
91965 D 100043
91966 D 100044
91967 D 100045
92556 B 100696