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 (
    1, 228, 8, 9, 10, 8483, 71, 4, 2359, 1532, 
    8499, 8502, 8501, 2459, 1528, 2432, 
    2455, 1517, 2438, 8498, 7624, 8484, 
    8497, 2812, 7608, 8485, 1538, 8488, 
    8486, 8492, 8487, 7614, 8503, 8505, 
    7613, 8496, 8489, 7248, 7615, 2465, 
    7290, 8490, 7283, 8491, 8493, 7625, 
    7285, 7294, 7261, 7611, 7612, 2441, 
    7616, 7293, 7279, 7617, 7620, 7618, 
    7619, 7281, 7610, 7277, 7609, 7623, 
    7621, 7622, 2832, 8478, 8474, 8475, 
    6380, 10136, 10137, 8512, 3506, 8513, 
    8510, 4949, 10135, 5985
  ) 
GROUP BY 
  p.product_id

Query time 0.00134

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 (1,228,8,9,10,8483,71,4,2359,1532,8499,8502,8501,2459,1528,2432,2455,1517,2438,8498,7624,8484,8497,2812,7608,8485,1538,8488,8486,8492,8487,7614,8503,8505,7613,8496,8489,7248,7615,2465,7290,8490,7283,8491,8493,7625,7285,7294,7261,7611,7612,2441,7616,7293,7279,7617,7620,7618,7619,7281,7610,7277,7609,7623,7621,7622,2832,8478,8474,8475,6380,10136,10137,8512,3506,8513,8510,4949,10135,5985)) 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"
            ]
          }
        }
      ]
    }
  }
}