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 (
    296840, 296925, 296815, 296844, 296874, 
    296922, 296924, 296843, 296902, 296891, 
    296863, 296923, 296850, 296916, 296813, 
    296856, 296854, 296901, 296878, 296871, 
    296867, 296879, 296918, 296870, 296855, 
    296866, 296896, 296812, 296865, 296898, 
    296877, 296920, 296889, 296921, 296926, 
    296849, 296814, 296859, 296873, 296839, 
    296893, 296841, 296919, 296881, 296861, 
    296883, 296838, 296853
  ) 
  AND discussion.object_type = "P" 
GROUP BY 
  discussion.object_id

Query time 0.00139

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "122.28"
    },
    "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 (296840,296925,296815,296844,296874,296922,296924,296843,296902,296891,296863,296923,296850,296916,296813,296856,296854,296901,296878,296871,296867,296879,296918,296870,296855,296866,296896,296812,296865,296898,296877,296920,296889,296921,296926,296849,296814,296859,296873,296839,296893,296841,296919,296881,296861,296883,296838,296853)) and (`goimagine`.`discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "67.51",
              "eval_cost": "4.80",
              "prefix_cost": "72.31",
              "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": "92.75",
              "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": "122.29",
              "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)"
          }
        }
      ]
    }
  }
}