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 (
    99296, 99297, 99298, 99299, 99300, 99301, 
    99302, 99303, 99304, 99305, 99306, 
    99307, 99308, 99309, 99310, 99311, 
    99312, 100400, 100401, 100402, 100403, 
    100404, 100405, 100406, 100407, 100408, 
    100409, 100410, 100411, 100412, 100417, 
    100418, 100419, 100420, 100421, 100422, 
    100423, 100424, 100425, 100426, 100427, 
    101055, 101056, 101057, 101058, 101059, 
    101060, 101061, 101062, 101063, 101064, 
    101065, 101066, 101067, 101068, 101069, 
    101070, 101071, 101072, 101073, 101074, 
    101075, 101076, 101077, 101078, 101079, 
    101080, 101081, 101082, 101083, 101084, 
    101085, 101086, 101087, 101088, 101089, 
    101090, 101091, 101092
  ) 
  AND cscart_discussion.object_type = "P" 
GROUP BY 
  cscart_discussion.object_id

Query time 0.00126

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "91.11"
    },
    "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": 79,
            "rows_produced_per_join": 79,
            "filtered": "100.00",
            "index_condition": "((`nuie_scalesta_net`.`cscart_discussion`.`object_id` in (99296,99297,99298,99299,99300,99301,99302,99303,99304,99305,99306,99307,99308,99309,99310,99311,99312,100400,100401,100402,100403,100404,100405,100406,100407,100408,100409,100410,100411,100412,100417,100418,100419,100420,100421,100422,100423,100424,100425,100426,100427,101055,101056,101057,101058,101059,101060,101061,101062,101063,101064,101065,101066,101067,101068,101069,101070,101071,101072,101073,101074,101075,101076,101077,101078,101079,101080,101081,101082,101083,101084,101085,101086,101087,101088,101089,101090,101091,101092)) and (`nuie_scalesta_net`.`cscart_discussion`.`object_type` = 'P'))",
            "cost_info": {
              "read_cost": "47.41",
              "eval_cost": "7.90",
              "prefix_cost": "55.31",
              "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": 79,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "19.75",
              "eval_cost": "7.90",
              "prefix_cost": "82.96",
              "data_read_per_join": "34K"
            },
            "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": 79,
            "filtered": "100.00",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "7.90",
              "prefix_cost": "91.11",
              "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)"
          }
        }
      ]
    }
  }
}