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 (
    3354, 3308, 3397, 3841, 3212, 3928, 3660, 
    3828, 3518, 3918, 3400, 3232, 3740, 
    3743, 3741, 3440, 3441, 10078, 3768, 
    3747, 3406, 3827, 3738, 3403, 3443, 
    3737, 3655, 3658, 3769, 3745, 3767, 
    3950, 3659, 3766, 3739, 3444, 3785, 
    3442, 3787, 3770, 3447, 3793, 3450, 
    3772, 3795, 3788, 3439, 3446, 3794, 
    3948, 3449, 3871, 3786, 3867, 3951, 
    3949, 3881, 11629, 8597, 8280, 8443, 
    8596, 11632, 8405, 8584, 8588, 8437, 
    8515, 8542, 10446, 10492, 11623, 11624, 
    11625, 11626, 11627, 11628, 11631
  ) 
GROUP BY 
  p.product_id

Query time 0.00091

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 (3354,3308,3397,3841,3212,3928,3660,3828,3518,3918,3400,3232,3740,3743,3741,3440,3441,10078,3768,3747,3406,3827,3738,3403,3443,3737,3655,3658,3769,3745,3767,3950,3659,3766,3739,3444,3785,3442,3787,3770,3447,3793,3450,3772,3795,3788,3439,3446,3794,3948,3449,3871,3786,3867,3951,3949,3881,11629,8597,8280,8443,8596,11632,8405,8584,8588,8437,8515,8542,10446,10492,11623,11624,11625,11626,11627,11628,11631)) 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"
            ]
          }
        }
      ]
    }
  }
}