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 (
    8418, 8415, 8417, 8403, 8409, 8396, 8399, 
    8416, 8404, 8398, 8420, 8394, 8419, 
    8401, 8400, 8402, 8412, 8407, 8393, 
    8411, 8408, 8406, 8405
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00052

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "606.36"
    },
    "grouping_operation": {
      "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": 28,
            "rows_produced_per_join": 28,
            "filtered": "100.00",
            "index_condition": "((`primapearl_r_new`.`cscart_discussion`.`object_id` in (8418,8415,8417,8403,8409,8396,8399,8416,8404,8398,8420,8394,8419,8401,8400,8402,8412,8407,8393,8411,8408,8406,8405)) and (`primapearl_r_new`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "15.56",
              "eval_cost": "2.80",
              "prefix_cost": "18.36",
              "data_read_per_join": "672"
            },
            "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_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "ref": [
              "primapearl_r_new.cscart_discussion.thread_id",
              "const"
            ],
            "rows_examined_per_scan": 30,
            "rows_produced_per_join": 840,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "210.00",
              "eval_cost": "84.00",
              "prefix_cost": "312.36",
              "data_read_per_join": "367K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ]
          }
        },
        {
          "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": [
              "primapearl_r_new.cscart_discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 840,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "210.00",
              "eval_cost": "84.00",
              "prefix_cost": "606.36",
              "data_read_per_join": "13K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_rating), ((`primapearl_r_new`.`cscart_discussion_rating`.`thread_id` = `primapearl_r_new`.`cscart_discussion`.`thread_id`) and (`primapearl_r_new`.`cscart_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}