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 (
    3656, 1610, 1606, 1608, 491, 1609, 1676, 
    1675, 1607, 1677, 3582, 1737, 1736, 
    1620, 1520, 1519, 858, 2692, 2988, 3580, 
    3581, 3587, 3639, 3579, 3642, 3627, 
    3178, 5225, 3453, 3452, 5220, 5223, 
    3577, 3451, 3450, 3449, 5226, 5227, 
    5219, 5217, 5221, 3179, 3138, 3448, 
    5224, 5218, 100, 95
  ) 
  AND cscart_discussion.object_type = "P" 
  AND cscart_discussion.company_id = 1 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "67.21"
    },
    "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": 48,
            "rows_produced_per_join": 48,
            "filtered": "100.00",
            "index_condition": "((`dbggbern`.`cscart_discussion`.`company_id` = 1) and (`dbggbern`.`cscart_discussion`.`object_id` in (3656,1610,1606,1608,491,1609,1676,1675,1607,1677,3582,1737,1736,1620,1520,1519,858,2692,2988,3580,3581,3587,3639,3579,3642,3627,3178,5225,3453,3452,5220,5223,3577,3451,3450,3449,5226,5227,5219,5217,5221,3179,3138,3448,5224,5218,100,95)) and (`dbggbern`.`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",
              "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": [
              "dbggbern.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), (`dbggbern`.`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": [
              "dbggbern.cscart_discussion_posts.post_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": "67.21",
              "data_read_per_join": "768"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_rating), ((`dbggbern`.`cscart_discussion_rating`.`thread_id` = `dbggbern`.`cscart_discussion`.`thread_id`) and (`dbggbern`.`cscart_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}

Result

product_id average_rating discussion_type discussion_thread_id
95 D 128
100 D 133
491 D 587
858 D 972
1519 D 1737
1520 D 1738
1606 D 1855
1607 D 1856
1608 D 1857
1609 D 1858
1610 D 1859
1620 D 1870
1675 D 1931
1676 D 1932
1677 D 1933
1736 D 1992
1737 D 1993
2692 D 2832
2988 D 3158
3138 D 3329
3178 D 3372
3179 D 3373
3448 D 3669
3449 D 3670
3450 D 3671
3451 D 3672
3452 D 3673
3453 D 3674
3577 D 3816
3579 D 3818
3580 D 3819
3581 D 3820
3582 D 3821
3587 D 3826
3627 D 3867
3639 D 3879
3642 D 3882
3656 D 3896
5217 D 5601
5218 D 5602
5219 D 5603
5220 D 5604
5221 D 5605
5223 D 5607
5224 D 5608
5225 D 5609
5226 D 5610
5227 D 5611