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 (
    8338, 8339, 8346, 8341, 8462, 8334, 8335, 
    8355, 8342, 8340, 8360, 8064, 8473, 
    8332, 8267, 8318, 8336, 8337, 8243, 
    8257, 8381, 8315, 8265, 8313, 8063, 
    8358, 8127
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00054

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "671.71"
    },
    "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": 31,
            "rows_produced_per_join": 31,
            "filtered": "100.00",
            "index_condition": "((`primapearl_r_new`.`cscart_discussion`.`object_id` in (8338,8339,8346,8341,8462,8334,8335,8355,8342,8340,8360,8064,8473,8332,8267,8318,8336,8337,8243,8257,8381,8315,8265,8313,8063,8358,8127)) and (`primapearl_r_new`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "17.61",
              "eval_cost": "3.10",
              "prefix_cost": "20.71",
              "data_read_per_join": "744"
            },
            "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": 930,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "232.50",
              "eval_cost": "93.00",
              "prefix_cost": "346.21",
              "data_read_per_join": "406K"
            },
            "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": 930,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "232.50",
              "eval_cost": "93.00",
              "prefix_cost": "671.71",
              "data_read_per_join": "14K"
            },
            "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)"
          }
        }
      ]
    }
  }
}