SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  cscart_discussion 
  INNER JOIN cscart_products as p ON (
    cscart_discussion.object_id = p.product_id
  ) 
  INNER JOIN cscart_discussion_posts as dp ON (
    cscart_discussion.thread_id = dp.thread_id 
    AND cscart_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    431225, 431483, 431058, 431207, 431430, 
    431475, 431496, 431086, 431479, 430986, 
    431036, 431282, 431351, 431391, 431450, 
    431223, 431059, 431227, 431437, 430716, 
    431294, 431492, 430996, 431278, 431296, 
    431396, 431435, 431460, 435511, 431413, 
    430777, 431367, 431221, 431261, 431323, 
    431331, 431485, 431276, 431258, 431307, 
    431312, 431348, 431471, 431382, 431448, 
    431095, 431477, 431499, 381714, 431214, 
    431310, 431375, 431449, 431266, 431299, 
    430728, 431491, 431347, 431366, 431436, 
    431441, 431075, 431418, 431489, 430709, 
    431228, 431265, 431321, 431400, 431363, 
    431486, 431189, 430670, 431206, 431370, 
    431374, 431388, 387053, 430774, 430779, 
    431209, 431232, 431256, 431257, 431379, 
    387048, 430710, 431179, 431404, 431445, 
    430668, 431293, 431277, 431286, 431462, 
    431317, 431473, 431476, 431320, 431390
  ) 
GROUP BY 
  p.product_id

Query time 0.00105

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (431225,431483,431058,431207,431430,431475,431496,431086,431479,430986,431036,431282,431351,431391,431450,431223,431059,431227,431437,430716,431294,431492,430996,431278,431296,431396,431435,431460,435511,431413,430777,431367,431221,431261,431323,431331,431485,431276,431258,431307,431312,431348,431471,431382,431448,431095,431477,431499,381714,431214,431310,431375,431449,431266,431299,430728,431491,431347,431366,431436,431441,431075,431418,431489,430709,431228,431265,431321,431400,431363,431486,431189,430670,431206,431370,431374,431388,387053,430774,430779,431209,431232,431256,431257,431379,387048,430710,431179,431404,431445,430668,431293,431277,431286,431462,431317,431473,431476,431320,431390)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}