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 (
    101115, 101121, 101122, 101123, 101124, 
    101130, 101131, 101132, 101133, 101514, 
    101515, 101516, 101517, 101523, 101524, 
    101525, 101526, 101532, 101533, 101534, 
    101535, 101552, 101553, 101554, 101555, 
    101561, 101562, 101563, 101564, 101570, 
    101571, 101572, 101573, 101814, 101815, 
    101816, 101817, 101818, 101819, 101820, 
    101821, 101822, 101823, 101824, 101825, 
    101826, 101827, 101828, 101829, 101830, 
    101831, 101832, 101833, 101834, 101835, 
    101836, 101837, 101838, 101839, 101840, 
    101841, 101842, 101843, 101844, 101845, 
    101846, 101847, 101848, 101849, 101850, 
    101851, 101852, 101853
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "84.21"
    },
    "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": 73,
            "rows_produced_per_join": 73,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (101115,101121,101122,101123,101124,101130,101131,101132,101133,101514,101515,101516,101517,101523,101524,101525,101526,101532,101533,101534,101535,101552,101553,101554,101555,101561,101562,101563,101564,101570,101571,101572,101573,101814,101815,101816,101817,101818,101819,101820,101821,101822,101823,101824,101825,101826,101827,101828,101829,101830,101831,101832,101833,101834,101835,101836,101837,101838,101839,101840,101841,101842,101843,101844,101845,101846,101847,101848,101849,101850,101851,101852,101853)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "43.81",
              "eval_cost": "7.30",
              "prefix_cost": "51.11",
              "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": 73,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "18.25",
              "eval_cost": "7.30",
              "prefix_cost": "76.66",
              "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": 73,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "7.30",
              "prefix_cost": "84.21",
              "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)"
          }
        }
      ]
    }
  }
}