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 (
    396044, 396045, 396046, 396047, 396048, 
    396049, 396050, 396078, 396079, 396080, 
    396081, 396082, 396083, 396084, 396085, 
    414329, 414330, 414331, 414332, 414333, 
    414334, 414335, 414336, 414337, 414338, 
    414339, 414340, 414341, 414393, 414394, 
    414395, 414396, 414397, 414398, 414399, 
    414429, 414430, 414431, 414432, 414433, 
    414434, 414435, 414514, 414516, 414517, 
    414518, 414519, 414520, 414521, 431514, 
    431515, 431516, 431517, 431518, 431519, 
    431520, 431521, 431522, 431523, 431524, 
    431525, 431526, 431527, 431528, 431529, 
    431530, 431531, 431532, 431533, 431534, 
    431535, 431536, 431537, 431552, 431553, 
    431554, 431555, 431556, 431557, 431558, 
    431559, 431560, 431687, 431688, 431689, 
    431690, 431691, 431692, 431693, 431694, 
    431695, 431696, 431697
  ) 
GROUP BY 
  p.product_id

Query time 0.00098

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`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": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (396044,396045,396046,396047,396048,396049,396050,396078,396079,396080,396081,396082,396083,396084,396085,414329,414330,414331,414332,414333,414334,414335,414336,414337,414338,414339,414340,414341,414393,414394,414395,414396,414397,414398,414399,414429,414430,414431,414432,414433,414434,414435,414514,414516,414517,414518,414519,414520,414521,431514,431515,431516,431517,431518,431519,431520,431521,431522,431523,431524,431525,431526,431527,431528,431529,431530,431531,431532,431533,431534,431535,431536,431537,431552,431553,431554,431555,431556,431557,431558,431559,431560,431687,431688,431689,431690,431691,431692,431693,431694,431695,431696,431697)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}