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 (
    2227, 2226, 2522, 2655, 2719, 2728, 3112, 
    3113, 3153, 3289, 3377, 3378, 3382, 
    3383, 3385, 3386, 3664, 3665, 3832, 
    3831, 4110, 4111, 3811, 5948, 6128, 
    6127, 6126, 6129, 6130, 6131, 6132, 
    6133, 6134, 6135, 6041, 6042, 6043, 
    6044, 6045, 6046, 6047, 6054, 6055, 
    6056, 6057, 6058, 6059, 5956, 6087, 
    6088, 6089, 6090, 6091, 6092, 6093, 
    5683, 6152, 6153, 6154, 6155, 6196, 
    6195, 6194, 6197, 6198, 6199, 6200, 
    6201, 6202
  ) 
GROUP BY 
  p.product_id

Query time 0.00081

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 (2227,2226,2522,2655,2719,2728,3112,3113,3153,3289,3377,3378,3382,3383,3385,3386,3664,3665,3832,3831,4110,4111,3811,5948,6128,6127,6126,6129,6130,6131,6132,6133,6134,6135,6041,6042,6043,6044,6045,6046,6047,6054,6055,6056,6057,6058,6059,5956,6087,6088,6089,6090,6091,6092,6093,5683,6152,6153,6154,6155,6196,6195,6194,6197,6198,6199,6200,6201,6202)) 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"
            ]
          }
        }
      ]
    }
  }
}