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 (
    421, 427, 408, 382, 418, 1930, 400, 424, 
    1931, 381, 1529, 401, 1465, 1932, 1933, 
    1935, 428, 415, 391, 406, 439, 390, 393, 
    394, 1929, 396, 397, 398, 399, 422, 434, 
    383, 412, 1927, 419, 430, 431, 432, 1928, 
    429, 384, 1934, 395, 420, 385, 405, 410, 
    413, 417, 1937, 414, 425, 407, 389, 402, 
    388, 426, 409, 404, 1936, 387, 386, 416, 
    441, 322
  ) 
  AND cscart_discussion.object_type = "P" 
  AND cscart_discussion.company_id = 1 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00156

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "312.01"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "range",
            "possible_keys": [
              "object_id",
              "company_id"
            ],
            "key": "object_id",
            "used_key_parts": [
              "object_id",
              "object_type",
              "company_id"
            ],
            "key_length": "10",
            "rows_examined_per_scan": 65,
            "rows_produced_per_join": 65,
            "filtered": "100.00",
            "index_condition": "((`eshopuga_upgr_4122`.`cscart_discussion`.`company_id` = 1) and (`eshopuga_upgr_4122`.`cscart_discussion`.`object_id` in (421,427,408,382,418,1930,400,424,1931,381,1529,401,1465,1932,1933,1935,428,415,391,406,439,390,393,394,1929,396,397,398,399,422,434,383,412,1927,419,430,431,432,1928,429,384,1934,395,420,385,405,410,413,417,1937,414,425,407,389,402,388,426,409,404,1936,387,386,416,441,322)) and (`eshopuga_upgr_4122`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "143.01",
              "eval_cost": "13.00",
              "prefix_cost": "156.01",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type",
              "company_id"
            ]
          }
        },
        {
          "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": [
              "eshopuga_upgr_4122.cscart_discussion.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 65,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "65.00",
              "eval_cost": "13.00",
              "prefix_cost": "234.01",
              "data_read_per_join": "28K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_posts), (`eshopuga_upgr_4122`.`cscart_discussion_posts`.`status` = 'A'), true)"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_rating",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "eshopuga_upgr_4122.cscart_discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 65,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "65.00",
              "eval_cost": "13.00",
              "prefix_cost": "312.01",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_rating), ((`eshopuga_upgr_4122`.`cscart_discussion_rating`.`thread_id` = `eshopuga_upgr_4122`.`cscart_discussion`.`thread_id`) and (`eshopuga_upgr_4122`.`cscart_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}