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 (
    101704, 101705, 101706, 101707, 101708, 
    101709, 101710, 101711, 101712, 101713, 
    101714, 101715, 101716, 101717, 101718, 
    101719, 101720, 101721, 101722, 101723, 
    101724, 101725, 101726, 101727, 101728, 
    101729, 101730, 101731, 101732, 101733, 
    101734, 101735, 101736, 101737, 101738, 
    101739, 101744, 101745, 101764, 101765, 
    101766, 101767, 101768, 101769, 101770, 
    101771, 101772, 101773, 101774, 101775, 
    101776, 101777, 101778, 101779, 101780, 
    101781, 101782, 101783, 101784, 101785, 
    101786, 101787, 101788, 101789, 101790, 
    101791, 101792, 101793, 101952, 101953, 
    101954, 101955
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00187

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 (101704,101705,101706,101707,101708,101709,101710,101711,101712,101713,101714,101715,101716,101717,101718,101719,101720,101721,101722,101723,101724,101725,101726,101727,101728,101729,101730,101731,101732,101733,101734,101735,101736,101737,101738,101739,101744,101745,101764,101765,101766,101767,101768,101769,101770,101771,101772,101773,101774,101775,101776,101777,101778,101779,101780,101781,101782,101783,101784,101785,101786,101787,101788,101789,101790,101791,101792,101793,101952,101953,101954,101955)) 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)"
          }
        }
      ]
    }
  }
}