SELECT 
  discussion.object_id AS product_id, 
  AVG(discussion_rating.rating_value) AS average_rating, 
  discussion.type AS discussion_type, 
  discussion.thread_id AS discussion_thread_id 
FROM 
  discussion 
  LEFT JOIN discussion_posts ON discussion_posts.thread_id = discussion.thread_id 
  AND discussion_posts.status = "A" 
  LEFT JOIN discussion_rating ON discussion.thread_id = discussion_rating.thread_id 
  AND discussion_rating.post_id = discussion_posts.post_id 
  AND discussion_rating.rating_value != 0 
WHERE 
  discussion.object_id IN (
    212190, 213341, 204738, 213340, 204740, 
    311173, 315133, 323613, 410174, 327013, 
    468818, 317773, 370573, 423934, 439778, 
    382453, 407534, 328333, 363213, 426014, 
    374533, 392254, 338893, 409414, 439218, 
    367173, 462218, 360013, 368493, 474098, 
    369813, 414134, 474858, 390374, 410734, 
    421294, 488058, 324373, 373213, 451098, 
    414694, 429974, 495218, 442418, 357373, 
    443738, 311733, 312493
  ) 
  AND discussion.object_type = "P" 
GROUP BY 
  discussion.object_id

Query time 0.00563

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "115.73"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "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": 48,
            "rows_produced_per_join": 48,
            "filtered": "100.00",
            "index_condition": "((`goimagine`.`discussion`.`object_id` in (212190,213341,204738,213340,204740,311173,315133,323613,410174,327013,468818,317773,370573,423934,439778,382453,407534,328333,363213,426014,374533,392254,338893,409414,439218,367173,462218,360013,368493,474098,369813,414134,474858,390374,410734,421294,488058,324373,373213,451098,414694,429974,495218,442418,357373,443738,311733,312493)) and (`goimagine`.`discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "60.95",
              "eval_cost": "4.80",
              "prefix_cost": "65.75",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "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": [
              "goimagine.discussion.thread_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 84,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "12.01",
              "eval_cost": "8.44",
              "prefix_cost": "86.20",
              "data_read_per_join": "36K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ]
          }
        },
        {
          "table": {
            "table_name": "discussion_rating",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "goimagine.discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 84,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "21.09",
              "eval_cost": "8.44",
              "prefix_cost": "115.73",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(discussion_rating), ((`goimagine`.`discussion_rating`.`thread_id` = `goimagine`.`discussion`.`thread_id`) and (`goimagine`.`discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}