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 (
    415288, 269868, 269870, 265423, 265427, 
    265433, 265422, 265411, 270140, 269842, 
    269869, 269838, 265431, 269871, 269853, 
    269845, 269872, 269860, 269867, 265430, 
    265412, 269843, 269835, 265403, 265414, 
    270143, 269840, 265419, 265413, 265424, 
    269848, 392318, 269851, 270139, 269863, 
    269837, 269865, 270155, 270141, 392269, 
    270142, 270144, 265428, 265408, 392330, 
    265401, 265420, 392264, 265465, 392324, 
    392270, 392272, 392314, 392325, 392273, 
    392329, 392317, 392326, 392332, 392333, 
    392335, 392313, 392319, 392320, 392328, 
    392262, 392323, 392316, 392271, 392331, 
    392266, 392268, 392274, 392275, 392327, 
    392321, 392334, 392315, 392322, 392312
  ) 
GROUP BY 
  p.product_id

Query time 0.00144

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 (415288,269868,269870,265423,265427,265433,265422,265411,270140,269842,269869,269838,265431,269871,269853,269845,269872,269860,269867,265430,265412,269843,269835,265403,265414,270143,269840,265419,265413,265424,269848,392318,269851,270139,269863,269837,269865,270155,270141,392269,270142,270144,265428,265408,392330,265401,265420,392264,265465,392324,392270,392272,392314,392325,392273,392329,392317,392326,392332,392333,392335,392313,392319,392320,392328,392262,392323,392316,392271,392331,392266,392268,392274,392275,392327,392321,392334,392315,392322,392312)) 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"
            ]
          }
        }
      ]
    }
  }
}