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 (
    323729, 323745, 324475, 324479, 324489, 
    326336, 326389, 327714, 329001, 329002, 
    329026, 330320, 330348, 331082, 331092, 
    331657, 332936, 332946, 332959, 333756, 
    334264, 334281, 334310, 335049
  ) 
  AND discussion.object_type = "P" 
GROUP BY 
  discussion.object_id

Query time 0.00108

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "62.40"
    },
    "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": 24,
            "rows_produced_per_join": 24,
            "filtered": "100.00",
            "index_condition": "((`goimagine`.`discussion`.`object_id` in (323729,323745,324475,324479,324489,326336,326389,327714,329001,329002,329026,330320,330348,331082,331092,331657,332936,332946,332959,333756,334264,334281,334310,335049)) and (`goimagine`.`discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "35.01",
              "eval_cost": "2.40",
              "prefix_cost": "37.41",
              "data_read_per_join": "576"
            },
            "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": 42,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "6.00",
              "eval_cost": "4.22",
              "prefix_cost": "47.64",
              "data_read_per_join": "18K"
            },
            "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": 42,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "10.55",
              "eval_cost": "4.22",
              "prefix_cost": "62.40",
              "data_read_per_join": "674"
            },
            "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)"
          }
        }
      ]
    }
  }
}