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 (
    263981, 264764, 264765, 264766, 264767, 
    264102, 264969, 264970, 264971, 264972, 
    264973, 264974, 264975, 264976, 264977, 
    366690, 366691, 366692, 366693, 366760, 
    366761, 366762, 366771, 366772, 366773, 
    366774, 366775, 366776, 366815, 366816, 
    366817, 366818, 366819, 366820, 366821, 
    366827, 366828, 366829, 366830, 366831, 
    366832, 366833, 366834, 366835, 366836, 
    366837, 366838, 371861, 377772, 377773, 
    377774, 377775, 371868, 377776, 371873, 
    377777, 377778, 377779, 371878, 377780, 
    377781, 377782, 377783, 371884, 377784, 
    377785, 377786, 371890, 377787, 377788, 
    371895, 377789, 371898, 377790, 377791, 
    371902, 377792, 377793
  ) 
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 (263981,264764,264765,264766,264767,264102,264969,264970,264971,264972,264973,264974,264975,264976,264977,366690,366691,366692,366693,366760,366761,366762,366771,366772,366773,366774,366775,366776,366815,366816,366817,366818,366819,366820,366821,366827,366828,366829,366830,366831,366832,366833,366834,366835,366836,366837,366838,371861,377772,377773,377774,377775,371868,377776,371873,377777,377778,377779,371878,377780,377781,377782,377783,371884,377784,377785,377786,371890,377787,377788,371895,377789,371898,377790,377791,371902,377792,377793)) 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"
            ]
          }
        }
      ]
    }
  }
}