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 (
    130681, 145323, 114897, 131992, 165403, 
    257589, 115093, 108241, 197488, 175093, 
    165284, 156319, 202170, 167561, 108245, 
    139394, 159546, 156799, 211705, 252518, 
    108234, 164905, 108385, 112103, 129512, 
    161993, 112102, 129527, 144425, 131847, 
    108248, 120905, 108242, 242304, 111949, 
    196622, 111951, 133995, 128503, 131987, 
    156326, 114827, 132626, 135126, 138125, 
    161899, 114282, 174366
  ) 
  AND discussion.object_type = "P" 
GROUP BY 
  discussion.object_id

Query time 0.01477

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "129.68"
    },
    "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 (130681,145323,114897,131992,165403,257589,115093,108241,197488,175093,165284,156319,202170,167561,108245,139394,159546,156799,211705,252518,108234,164905,108385,112103,129512,161993,112102,129527,144425,131847,108248,120905,108242,242304,111949,196622,111951,133995,128503,131987,156326,114827,132626,135126,138125,161899,114282,174366)) and (`goimagine`.`discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "74.90",
              "eval_cost": "4.80",
              "prefix_cost": "79.70",
              "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": "100.15",
              "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": "129.68",
              "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)"
          }
        }
      ]
    }
  }
}