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 (
    4341, 4342, 6036, 4347, 6369, 4344, 4345, 
    5303, 6050, 7351, 5304, 4348, 4346, 
    6349, 6345, 4122, 2743, 3544, 3545, 
    3543, 3541, 3540, 4956, 5114, 2645, 
    7301, 7299, 7300, 2646, 2647, 2644, 
    1410, 5154, 5150, 5153, 2648, 5152, 
    5155, 5151, 2886, 6436, 2773, 6440, 
    7223, 7116, 7221, 7222, 7224
  ) 
  AND cscart_discussion.object_type = "P" 
  AND cscart_discussion.company_id = 1 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00091

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 (4341,4342,6036,4347,6369,4344,4345,5303,6050,7351,5304,4348,4346,6349,6345,4122,2743,3544,3545,3543,3541,3540,4956,5114,2645,7301,7299,7300,2646,2647,2644,1410,5154,5150,5153,2648,5152,5155,5151,2886,6436,2773,6440,7223,7116,7221,7222,7224)) 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)"
          }
        }
      ]
    }
  }
}