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 (
    430746, 431068, 430813, 430925, 431085, 
    431103, 431163, 431164, 430817, 430654, 
    430902, 431313, 431053, 431148, 430751, 
    431044, 431120, 431456, 430660, 430825, 
    430678, 430680, 430959, 431013, 431127, 
    430691, 430756, 430820, 431109, 431161, 
    430737, 431035, 431041, 431194, 430875, 
    431049, 430819, 431083, 430808, 430989, 
    431080, 430635, 430650, 430629, 430860, 
    431126, 430657, 430807, 430921, 430922, 
    430843, 430812, 430868, 431073, 431144, 
    430789, 430795, 430758, 431082, 430818, 
    430901, 430928, 430950, 431156, 430658, 
    430673, 431110, 430684, 431066, 431114, 
    431118, 430689, 430822, 431064, 430769, 
    430892, 431043, 431169, 430628, 430664, 
    430842, 430834, 430859, 430906, 431459, 
    430768, 430890, 430988, 431183, 431250, 
    431008, 430703, 430838, 430962, 430909, 
    431063, 431159, 430695, 430904, 430923
  ) 
GROUP BY 
  p.product_id

Query time 0.00094

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 (430746,431068,430813,430925,431085,431103,431163,431164,430817,430654,430902,431313,431053,431148,430751,431044,431120,431456,430660,430825,430678,430680,430959,431013,431127,430691,430756,430820,431109,431161,430737,431035,431041,431194,430875,431049,430819,431083,430808,430989,431080,430635,430650,430629,430860,431126,430657,430807,430921,430922,430843,430812,430868,431073,431144,430789,430795,430758,431082,430818,430901,430928,430950,431156,430658,430673,431110,430684,431066,431114,431118,430689,430822,431064,430769,430892,431043,431169,430628,430664,430842,430834,430859,430906,431459,430768,430890,430988,431183,431250,431008,430703,430838,430962,430909,431063,431159,430695,430904,430923)) 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"
            ]
          }
        }
      ]
    }
  }
}