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 (
    6945, 6947, 6948, 6959, 6971, 6975, 7019, 
    7060, 7062, 7064, 7065, 7094, 7107, 
    7109, 7122, 7136, 7140, 7158, 7161, 
    7204, 7206, 7209, 7297, 6966, 6846, 
    6873, 6883, 6896, 6907, 6908, 6909, 
    6914, 6927, 6950, 6955, 6956, 6963, 
    7003, 7061, 7104, 7164, 7165, 7179, 
    7181, 7273, 7275, 7305, 7310, 7311, 
    6897, 7106, 6967, 6898, 6901, 6915, 
    6932, 6964, 6969, 7090, 7108, 7201, 
    7203, 7207, 7301, 7312, 6890, 6894, 
    6910, 6919, 6974, 6998, 7114, 7298, 
    7327, 7143, 7309, 7314, 7063
  ) 
GROUP BY 
  p.product_id

Query time 0.00058

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 (6945,6947,6948,6959,6971,6975,7019,7060,7062,7064,7065,7094,7107,7109,7122,7136,7140,7158,7161,7204,7206,7209,7297,6966,6846,6873,6883,6896,6907,6908,6909,6914,6927,6950,6955,6956,6963,7003,7061,7104,7164,7165,7179,7181,7273,7275,7305,7310,7311,6897,7106,6967,6898,6901,6915,6932,6964,6969,7090,7108,7201,7203,7207,7301,7312,6890,6894,6910,6919,6974,6998,7114,7298,7327,7143,7309,7314,7063)) 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"
            ]
          }
        }
      ]
    }
  }
}