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 (
    2282, 2285, 2904, 2900, 2909, 2887, 2890, 
    2888, 2886, 8380, 7741, 10237, 10140, 
    10197, 10020, 10299, 8379, 10167, 10344, 
    10032, 10177, 10178, 10345, 8439, 10249, 
    10222, 10035, 10147, 10172, 10048, 
    10193, 10276, 10039, 10095, 10228, 
    10192, 10058, 8345, 10239, 10148, 10156, 
    10023, 10103, 10026, 10074, 10088, 
    10174, 10233, 10226, 10219, 10244, 
    10105, 10157, 10160, 10059, 9972, 10274, 
    10294, 10022, 10104, 10247, 10261, 
    9952, 10159, 10161, 10225, 10254, 10255, 
    10273, 9937, 9968, 10071, 10272, 9969, 
    10047, 10050, 10056, 10093, 10232, 
    10238
  ) 
GROUP BY 
  p.product_id

Query time 0.00136

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.31"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 13,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.70",
              "eval_cost": "0.13",
              "prefix_cost": "1.83",
              "data_read_per_join": "582"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.33",
              "eval_cost": "0.01",
              "prefix_cost": "2.28",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_id` in (2282,2285,2904,2900,2909,2887,2890,2888,2886,8380,7741,10237,10140,10197,10020,10299,8379,10167,10344,10032,10177,10178,10345,8439,10249,10222,10035,10147,10172,10048,10193,10276,10039,10095,10228,10192,10058,8345,10239,10148,10156,10023,10103,10026,10074,10088,10174,10233,10226,10219,10244,10105,10157,10160,10059,9972,10274,10294,10022,10104,10247,10261,9952,10159,10161,10225,10254,10255,10273,9937,9968,10071,10272,9969,10047,10050,10056,10093,10232,10238)) and (`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "softwarepirmam_hewadelivard_cscart_4.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.02",
              "eval_cost": "0.01",
              "prefix_cost": "2.31",
              "data_read_per_join": "294"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}