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 (
    84911, 87981, 84687, 84709, 88027, 88024, 
    84708, 84749, 88042, 87974, 84271, 
    87991, 87966, 84270, 88033, 88007, 
    84269, 93150, 93154, 93156, 93789, 
    93795, 94448, 94804, 94805, 94806, 
    94809, 94832, 95753, 95786, 95788, 
    95885, 95886, 95887, 95888, 96117, 
    96118, 96176, 98036, 98037, 98038, 
    98229, 98233, 98325, 98882, 98883, 
    98884, 98885, 99136, 99145, 99153, 
    99154, 99158, 99167, 99181, 99185, 
    99187, 99202, 99205, 99212, 99220, 
    100386, 100387, 100434
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00118

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "73.86"
    },
    "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": 64,
            "rows_produced_per_join": 64,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (84911,87981,84687,84709,88027,88024,84708,84749,88042,87974,84271,87991,87966,84270,88033,88007,84269,93150,93154,93156,93789,93795,94448,94804,94805,94806,94809,94832,95753,95786,95788,95885,95886,95887,95888,96117,96118,96176,98036,98037,98038,98229,98233,98325,98882,98883,98884,98885,99136,99145,99153,99154,99158,99167,99181,99185,99187,99202,99205,99212,99220,100386,100387,100434)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "38.41",
              "eval_cost": "6.40",
              "prefix_cost": "44.81",
              "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": 64,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "16.00",
              "eval_cost": "6.40",
              "prefix_cost": "67.21",
              "data_read_per_join": "28K"
            },
            "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": 64,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "6.40",
              "prefix_cost": "73.86",
              "data_read_per_join": "1024"
            },
            "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)"
          }
        }
      ]
    }
  }
}