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 (
    100984, 100985, 100986, 100987, 100988, 
    100989, 100990, 100991, 100992, 100993, 
    100994, 100995, 100996, 100997, 100998, 
    100999, 101000, 101001, 101002, 101003, 
    101004, 101005, 101006, 101007, 101008, 
    101009, 101010, 101037, 101093, 101094, 
    101095, 101096, 101097, 101098, 101099, 
    101100, 101101, 101102, 101103, 101104, 
    101536, 101537, 101538, 101539, 101540, 
    101541, 101542, 101543, 101544, 101545, 
    101546, 101628, 101629, 101630, 101631, 
    101632, 101633, 101634, 101678, 101679, 
    101680, 101681, 101740, 101741, 101742, 
    101743, 101854, 101855, 101856, 101857, 
    101858, 101859, 101860, 101861, 101862, 
    101863, 101864, 101865, 101866, 101867, 
    101868, 101869, 101870, 101871, 101928, 
    101929, 101930, 101931, 101956, 101957, 
    101958, 101959, 101960, 101961, 101962
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00135

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "109.51"
    },
    "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": 95,
            "rows_produced_per_join": 95,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (100984,100985,100986,100987,100988,100989,100990,100991,100992,100993,100994,100995,100996,100997,100998,100999,101000,101001,101002,101003,101004,101005,101006,101007,101008,101009,101010,101037,101093,101094,101095,101096,101097,101098,101099,101100,101101,101102,101103,101104,101536,101537,101538,101539,101540,101541,101542,101543,101544,101545,101546,101628,101629,101630,101631,101632,101633,101634,101678,101679,101680,101681,101740,101741,101742,101743,101854,101855,101856,101857,101858,101859,101860,101861,101862,101863,101864,101865,101866,101867,101868,101869,101870,101871,101928,101929,101930,101931,101956,101957,101958,101959,101960,101961,101962)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "57.01",
              "eval_cost": "9.50",
              "prefix_cost": "66.51",
              "data_read_per_join": "2K"
            },
            "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": 95,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "23.75",
              "eval_cost": "9.50",
              "prefix_cost": "99.76",
              "data_read_per_join": "41K"
            },
            "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": 95,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "9.50",
              "prefix_cost": "109.51",
              "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)"
          }
        }
      ]
    }
  }
}