SELECT 
  cscart_discussion.object_id AS product_id, 
  AVG(
    cscart_discussion_rating.rating_value
  ) AS average_rating, 
  cscart_discussion.type AS discussion_type, 
  cscart_discussion.thread_id AS discussion_thread_id 
FROM 
  cscart_discussion 
  LEFT JOIN cscart_discussion_posts ON cscart_discussion_posts.thread_id = cscart_discussion.thread_id 
  AND cscart_discussion_posts.status = "A" 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion.thread_id = cscart_discussion_rating.thread_id 
  AND cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
  AND cscart_discussion_rating.rating_value != 0 
WHERE 
  cscart_discussion.object_id IN (
    84716, 84738, 92171, 84726, 84742, 84935, 
    84858, 92175, 84740, 84942, 84934, 
    92167, 84737, 84739, 84725, 87980, 
    87983, 84741, 87984, 87970, 88023, 
    84947, 88032, 87985, 84736, 84688, 
    87979, 84960, 87978, 84850, 84743, 
    84849, 84951, 84747, 84761, 84717, 
    87969, 84846, 85274, 88022, 88017, 
    93158, 93159, 93797, 94450, 94796, 
    94797, 94802, 94803, 94807, 94808, 
    94826, 94842, 95555, 95738, 95739, 
    95740, 95741, 95742, 95754, 95819, 
    96529, 98416, 99139, 99140, 99148, 
    99149, 99150, 99156, 99157, 99159, 
    99160, 99161, 99196, 99201, 99210, 
    99211
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00076

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "88.81"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_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": 77,
            "rows_produced_per_join": 77,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (84716,84738,92171,84726,84742,84935,84858,92175,84740,84942,84934,92167,84737,84739,84725,87980,87983,84741,87984,87970,88023,84947,88032,87985,84736,84688,87979,84960,87978,84850,84743,84849,84951,84747,84761,84717,87969,84846,85274,88022,88017,93158,93159,93797,94450,94796,94797,94802,94803,94807,94808,94826,94842,95555,95738,95739,95740,95741,95742,95754,95819,96529,98416,99139,99140,99148,99149,99150,99156,99157,99159,99160,99161,99196,99201,99210,99211)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "46.21",
              "eval_cost": "7.70",
              "prefix_cost": "53.91",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type",
              "type"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "nuie_scalesta_net.cscart_discussion.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 77,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "19.25",
              "eval_cost": "7.70",
              "prefix_cost": "80.86",
              "data_read_per_join": "33K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_posts), (`nuie_scalesta_net`.`cscart_discussion_posts`.`status` = 'A'), true)"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_rating",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 77,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "7.70",
              "prefix_cost": "88.81",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ],
            "attached_condition": "<if>(is_not_null_compl(cscart_discussion_rating), ((`nuie_scalesta_net`.`cscart_discussion_rating`.`post_id` = `nuie_scalesta_net`.`cscart_discussion_posts`.`post_id`) and (`nuie_scalesta_net`.`cscart_discussion_rating`.`thread_id` = `nuie_scalesta_net`.`cscart_discussion`.`thread_id`) and (`nuie_scalesta_net`.`cscart_discussion_rating`.`rating_value` <> 0)), true)"
          }
        }
      ]
    }
  }
}