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 (
    99396, 99397, 99398, 100358, 101872, 
    101873, 101874, 101875, 101876, 101877, 
    101878, 101879, 101880, 101881, 101882, 
    101883, 101884, 101885, 101886, 101887, 
    101888, 101889, 101890, 101891, 101892, 
    101893, 101894, 101895, 101896, 101897, 
    101898, 101899, 101900, 101901, 101902, 
    101903, 101904, 101905, 101906, 101907, 
    101908, 101909, 101910, 101911, 101912, 
    101913, 101914, 101915, 101916, 101917, 
    101918, 101919, 101920, 101921, 101922, 
    101923, 101924, 101925, 101926, 101927, 
    101932, 101933, 101934, 101935, 101936, 
    101937, 101938, 101939, 101940, 101941, 
    101942, 101943, 101944, 101945, 101946, 
    101947, 101948, 101949, 101950, 101951
  ) 
  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": "92.26"
    },
    "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": 80,
            "rows_produced_per_join": 80,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (99396,99397,99398,100358,101872,101873,101874,101875,101876,101877,101878,101879,101880,101881,101882,101883,101884,101885,101886,101887,101888,101889,101890,101891,101892,101893,101894,101895,101896,101897,101898,101899,101900,101901,101902,101903,101904,101905,101906,101907,101908,101909,101910,101911,101912,101913,101914,101915,101916,101917,101918,101919,101920,101921,101922,101923,101924,101925,101926,101927,101932,101933,101934,101935,101936,101937,101938,101939,101940,101941,101942,101943,101944,101945,101946,101947,101948,101949,101950,101951)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "48.01",
              "eval_cost": "8.00",
              "prefix_cost": "56.01",
              "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": 80,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "20.00",
              "eval_cost": "8.00",
              "prefix_cost": "84.01",
              "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": 80,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "8.00",
              "prefix_cost": "92.26",
              "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)"
          }
        }
      ]
    }
  }
}