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 (
    284648, 282838, 282836, 278049, 275782, 
    272636, 269248, 266156, 265783, 266092, 
    265784, 258086, 252518, 257589, 256790, 
    256001, 252277, 242304, 239493, 237409, 
    235920, 231091, 227946, 224968, 226497, 
    225207, 225304, 225273, 204337, 201932, 
    197488, 196621, 196622, 201889, 192454, 
    193258, 192450, 108385, 144425, 175093, 
    167561, 167552, 161993, 167239, 166202, 
    165403, 165290, 164900, 164913, 161865, 
    164905, 162133, 164899, 160627, 156319, 
    132617, 139306, 120811, 139394, 132610, 
    139524, 131992, 127555, 126324, 126318, 
    126321, 108204, 114844, 120905, 108233, 
    120730, 108232, 120456, 108246, 108180
  ) 
  AND discussion.object_type = "P" 
GROUP BY 
  discussion.object_id

Query time 0.00946

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "196.51"
    },
    "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": 75,
            "rows_produced_per_join": 75,
            "filtered": "100.00",
            "index_condition": "((`goimagine`.`discussion`.`object_id` in (284648,282838,282836,278049,275782,272636,269248,266156,265783,266092,265784,258086,252518,257589,256790,256001,252277,242304,239493,237409,235920,231091,227946,224968,226497,225207,225304,225273,204337,201932,197488,196621,196622,201889,192454,193258,192450,108385,144425,175093,167561,167552,161993,167239,166202,165403,165290,164900,164913,161865,164905,162133,164899,160627,156319,132617,139306,120811,139394,132610,139524,131992,127555,126324,126318,126321,108204,114844,120905,108233,120730,108232,120456,108246,108180)) and (`goimagine`.`discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "110.93",
              "eval_cost": "7.50",
              "prefix_cost": "118.43",
              "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": 131,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "18.77",
              "eval_cost": "13.18",
              "prefix_cost": "150.37",
              "data_read_per_join": "57K"
            },
            "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": 131,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "32.96",
              "eval_cost": "13.18",
              "prefix_cost": "196.52",
              "data_read_per_join": "2K"
            },
            "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)"
          }
        }
      ]
    }
  }
}