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 (
    366688, 366689, 366690, 366691, 366692, 
    366693, 366724, 366725, 366729, 366730, 
    366760, 366761, 366762, 366765, 366766, 
    366767, 366768, 366771, 366772, 366773, 
    366774, 366775, 366776, 366791, 366792, 
    366793, 366794, 366796, 366797, 366798, 
    366809, 366810, 366811, 366812, 366813, 
    366814, 366815, 366816, 366817, 366818, 
    366819, 366820, 366821, 366827, 366828, 
    366829, 366830, 366831, 366832, 366833, 
    366834, 366835, 366836, 366837, 366838, 
    366841, 366842, 366843, 366844, 366851, 
    366852, 366853, 366854, 366857, 366858, 
    366859, 366860, 366861, 366862, 366875, 
    366876, 366877, 366878, 366879, 366880, 
    366881, 366882, 366883, 366884, 366885, 
    366886, 366888, 366889, 366890, 366891
  ) 
GROUP BY 
  p.product_id

Query time 0.00095

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 (366688,366689,366690,366691,366692,366693,366724,366725,366729,366730,366760,366761,366762,366765,366766,366767,366768,366771,366772,366773,366774,366775,366776,366791,366792,366793,366794,366796,366797,366798,366809,366810,366811,366812,366813,366814,366815,366816,366817,366818,366819,366820,366821,366827,366828,366829,366830,366831,366832,366833,366834,366835,366836,366837,366838,366841,366842,366843,366844,366851,366852,366853,366854,366857,366858,366859,366860,366861,366862,366875,366876,366877,366878,366879,366880,366881,366882,366883,366884,366885,366886,366888,366889,366890,366891)) 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"
            ]
          }
        }
      ]
    }
  }
}