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 (
    88783, 88689, 88736, 88581, 85016, 85022, 
    85028, 85034, 89214, 89227, 89240, 
    88249, 89043, 89042, 88245, 89451, 
    89464, 89472, 89480, 88237, 88178, 
    90809, 85724, 85741, 85751, 85761, 
    89456, 87459, 88977, 88994, 87458, 
    87568, 88241, 91419, 91420, 85731, 
    88782, 88983, 89841, 89893, 91532, 
    85666, 85686, 85698, 85710, 88202, 
    88214, 85674, 87512, 87528, 88688, 
    88735, 87511, 87527, 88456, 88474, 
    88490, 89789, 89213, 89226, 89239, 
    87545, 87544, 89031, 89033, 89041, 
    89030, 89032, 89040, 89045, 89044, 
    88781, 88687, 88734, 88580, 88579
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00145

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "87.66"
    },
    "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": 76,
            "rows_produced_per_join": 76,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (88783,88689,88736,88581,85016,85022,85028,85034,89214,89227,89240,88249,89043,89042,88245,89451,89464,89472,89480,88237,88178,90809,85724,85741,85751,85761,89456,87459,88977,88994,87458,87568,88241,91419,91420,85731,88782,88983,89841,89893,91532,85666,85686,85698,85710,88202,88214,85674,87512,87528,88688,88735,87511,87527,88456,88474,88490,89789,89213,89226,89239,87545,87544,89031,89033,89041,89030,89032,89040,89045,89044,88781,88687,88734,88580,88579)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "45.61",
              "eval_cost": "7.60",
              "prefix_cost": "53.21",
              "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": 76,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "19.00",
              "eval_cost": "7.60",
              "prefix_cost": "79.81",
              "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": 76,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "7.60",
              "prefix_cost": "87.66",
              "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)"
          }
        }
      ]
    }
  }
}