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 (
    339484, 339755, 430807, 431068, 339697, 
    431359, 430875, 431041, 430825, 430868, 
    431085, 431156, 339681, 339828, 430925, 
    431053, 431164, 381705, 430819, 430820, 
    430901, 431013, 431049, 339596, 430654, 
    430658, 431144, 431148, 430691, 430989, 
    430746, 430808, 430812, 430834, 430629, 
    430680, 431136, 339774, 339765, 430843, 
    339728, 339836, 339843, 431110, 339725, 
    339734, 339816, 431080, 339655, 339839, 
    339599, 431118, 431161, 339466, 430673, 
    430890, 339845, 430789, 430859, 430906, 
    430922, 339582, 339723, 431082, 431126, 
    430657, 430678, 339702, 430737, 430822, 
    430860, 431111, 339674, 430689, 431044, 
    339754, 381704, 430795, 430832, 430892, 
    430914, 430921, 430635, 339802, 339821, 
    430663, 430838, 430854, 431174, 339486, 
    430650, 430959, 431456, 430815, 339727, 
    381697, 431169, 430672, 430695, 430950
  ) 
GROUP BY 
  p.product_id

Query time 0.00132

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 (339484,339755,430807,431068,339697,431359,430875,431041,430825,430868,431085,431156,339681,339828,430925,431053,431164,381705,430819,430820,430901,431013,431049,339596,430654,430658,431144,431148,430691,430989,430746,430808,430812,430834,430629,430680,431136,339774,339765,430843,339728,339836,339843,431110,339725,339734,339816,431080,339655,339839,339599,431118,431161,339466,430673,430890,339845,430789,430859,430906,430922,339582,339723,431082,431126,430657,430678,339702,430737,430822,430860,431111,339674,430689,431044,339754,381704,430795,430832,430892,430914,430921,430635,339802,339821,430663,430838,430854,431174,339486,430650,430959,431456,430815,339727,381697,431169,430672,430695,430950)) 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"
            ]
          }
        }
      ]
    }
  }
}