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 (
    5215, 5303, 5360, 5311, 5231, 5292, 5219, 
    5341, 5217, 5230, 5301, 5222, 5318, 
    5216, 5224, 5302, 5342, 5317, 5324, 
    5298, 5305, 5227, 5293, 5322, 5343, 
    5310, 5320, 5229, 5299, 5319, 5294, 
    5306, 5325, 5220, 5321, 5221, 5362, 
    5226, 5300, 5225, 5223, 5345, 5351, 
    5295, 5323, 5228, 5347, 5218, 5314, 
    5304, 5357, 5296, 5355, 5232, 5361, 
    5327, 5340, 5354, 5315, 5353, 5308, 
    5312, 5326, 5316, 5344, 5356, 5359, 
    5339, 5350, 5313, 5349, 5352, 5358, 
    5307, 5348, 5337, 5338, 5297
  ) 
GROUP BY 
  p.product_id

Query time 0.00068

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 (5215,5303,5360,5311,5231,5292,5219,5341,5217,5230,5301,5222,5318,5216,5224,5302,5342,5317,5324,5298,5305,5227,5293,5322,5343,5310,5320,5229,5299,5319,5294,5306,5325,5220,5321,5221,5362,5226,5300,5225,5223,5345,5351,5295,5323,5228,5347,5218,5314,5304,5357,5296,5355,5232,5361,5327,5340,5354,5315,5353,5308,5312,5326,5316,5344,5356,5359,5339,5350,5313,5349,5352,5358,5307,5348,5337,5338,5297)) 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"
            ]
          }
        }
      ]
    }
  }
}