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 (
    430818, 339755, 430807, 339483, 339484, 
    431068, 431359, 339697, 430825, 431053, 
    431041, 431085, 339828, 430868, 430925, 
    431013, 431148, 431156, 431164, 430654, 
    430820, 431049, 381705, 430658, 430819, 
    431144, 339596, 430691, 430901, 430989, 
    430629, 430746, 430808, 339774, 430680, 
    430812, 430834, 430843, 431136, 339765, 
    431110, 339728, 339836, 339843, 431161, 
    339725, 339734, 339816, 431080, 339655, 
    339845, 430673, 430906, 431118, 339839, 
    339599, 430890, 339466, 430789, 431082, 
    339802, 430657, 430678, 430737, 430859, 
    430922, 431111, 339702, 339582, 339723, 
    430822, 431126, 430860, 430921, 339674, 
    431044, 430689, 430795, 430832, 430892, 
    339754, 381704, 430635, 430838, 430854, 
    430914, 339821, 430663, 431174, 431456, 
    430650, 339486, 430815, 430959, 430664, 
    431169, 339727, 381697, 430950, 430672
  ) 
GROUP BY 
  p.product_id

Query time 0.00115

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 (430818,339755,430807,339483,339484,431068,431359,339697,430825,431053,431041,431085,339828,430868,430925,431013,431148,431156,431164,430654,430820,431049,381705,430658,430819,431144,339596,430691,430901,430989,430629,430746,430808,339774,430680,430812,430834,430843,431136,339765,431110,339728,339836,339843,431161,339725,339734,339816,431080,339655,339845,430673,430906,431118,339839,339599,430890,339466,430789,431082,339802,430657,430678,430737,430859,430922,431111,339702,339582,339723,430822,431126,430860,430921,339674,431044,430689,430795,430832,430892,339754,381704,430635,430838,430854,430914,339821,430663,431174,431456,430650,339486,430815,430959,430664,431169,339727,381697,430950,430672)) 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"
            ]
          }
        }
      ]
    }
  }
}