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 (
    5474, 5452, 5453, 5446, 5435, 5454, 5449, 
    5444, 5429, 5430, 5448, 5436, 5522, 
    5451, 5478, 5442, 5437, 5441, 5434, 
    6219, 6670, 5455, 5428, 5440, 6664, 
    5438, 5443, 5476, 6533, 6576, 5447, 
    6795, 5439, 6218, 6777, 6244, 5475, 
    6691, 5433, 6797, 5432, 6681, 6790, 
    6246, 6682, 5431, 6211, 6536, 6808, 
    6238, 6544, 6547, 6532, 6572, 6543, 
    6534, 6545, 6684, 6792, 6666, 10012, 
    6571, 6551, 6683, 6573, 6714, 6680, 
    6789, 6796, 6694, 6778, 6793, 10420, 
    6575, 6718, 6816, 6548, 6809, 6715, 
    6717
  ) 
GROUP BY 
  p.product_id

Query time 0.00115

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 (5474,5452,5453,5446,5435,5454,5449,5444,5429,5430,5448,5436,5522,5451,5478,5442,5437,5441,5434,6219,6670,5455,5428,5440,6664,5438,5443,5476,6533,6576,5447,6795,5439,6218,6777,6244,5475,6691,5433,6797,5432,6681,6790,6246,6682,5431,6211,6536,6808,6238,6544,6547,6532,6572,6543,6534,6545,6684,6792,6666,10012,6571,6551,6683,6573,6714,6680,6789,6796,6694,6778,6793,10420,6575,6718,6816,6548,6809,6715,6717)) 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"
            ]
          }
        }
      ]
    }
  }
}