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 (
    100716, 100717, 100718, 100719, 100720, 
    100721, 100722, 100723, 100724, 100725, 
    100726, 100727, 100728, 100729, 100730, 
    100731, 100732, 100733, 100734, 100735, 
    100736, 100737, 100738, 100739, 100740, 
    100741, 100742, 100743, 100744, 100745, 
    100746, 100747, 100748, 100749, 100750, 
    100751, 100752, 100753, 100754, 100755, 
    100756, 100757, 100758, 100759, 100760, 
    100761, 100762, 100763, 100764, 100765, 
    100766, 100767, 100768, 100769, 100770, 
    100771, 100772, 100773, 100774, 100775, 
    100776, 100777, 100778, 100779, 100780, 
    100781, 100782, 100783, 100784, 100785, 
    100786, 100787, 100788, 100841, 100842, 
    100843, 100844, 100845, 100846, 100847, 
    100848, 100856
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00062

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "94.56"
    },
    "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": 82,
            "rows_produced_per_join": 82,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (100716,100717,100718,100719,100720,100721,100722,100723,100724,100725,100726,100727,100728,100729,100730,100731,100732,100733,100734,100735,100736,100737,100738,100739,100740,100741,100742,100743,100744,100745,100746,100747,100748,100749,100750,100751,100752,100753,100754,100755,100756,100757,100758,100759,100760,100761,100762,100763,100764,100765,100766,100767,100768,100769,100770,100771,100772,100773,100774,100775,100776,100777,100778,100779,100780,100781,100782,100783,100784,100785,100786,100787,100788,100841,100842,100843,100844,100845,100846,100847,100848,100856)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "49.21",
              "eval_cost": "8.20",
              "prefix_cost": "57.41",
              "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": 82,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "20.50",
              "eval_cost": "8.20",
              "prefix_cost": "86.11",
              "data_read_per_join": "35K"
            },
            "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": 82,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "8.20",
              "prefix_cost": "94.56",
              "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)"
          }
        }
      ]
    }
  }
}