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 (
    89547, 89543, 89546, 85620, 89544, 85618, 
    89542, 85616, 85619, 85617, 85615, 
    93928, 93929, 93930, 93931, 93932, 
    93933, 93934, 93935, 93936, 94992, 
    94993, 94994, 94995, 94996, 94997, 
    94998, 94999, 95000, 95001, 95002, 
    95003, 95451, 95523, 95524, 95525, 
    95533, 95534, 95535, 95549, 95550, 
    95551, 96875, 96876, 96877, 96878, 
    96879, 96880, 96881, 96882, 96883, 
    96884, 96885, 96886, 98512, 98513, 
    98514, 100218, 100219, 100220, 100221, 
    100222, 100223, 100224, 100225, 100226, 
    100311, 100313, 100315, 100318, 100320, 
    101037
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00108

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "83.06"
    },
    "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": 72,
            "rows_produced_per_join": 72,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (89547,89543,89546,85620,89544,85618,89542,85616,85619,85617,85615,93928,93929,93930,93931,93932,93933,93934,93935,93936,94992,94993,94994,94995,94996,94997,94998,94999,95000,95001,95002,95003,95451,95523,95524,95525,95533,95534,95535,95549,95550,95551,96875,96876,96877,96878,96879,96880,96881,96882,96883,96884,96885,96886,98512,98513,98514,100218,100219,100220,100221,100222,100223,100224,100225,100226,100311,100313,100315,100318,100320,101037)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "43.21",
              "eval_cost": "7.20",
              "prefix_cost": "50.41",
              "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": 72,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "18.00",
              "eval_cost": "7.20",
              "prefix_cost": "75.61",
              "data_read_per_join": "31K"
            },
            "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": 72,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "7.20",
              "prefix_cost": "83.06",
              "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)"
          }
        }
      ]
    }
  }
}