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 (
    353384, 357241, 350560, 354163, 355107, 
    350285, 353055, 353876, 354140, 355911, 
    359349, 350064, 353374, 353147, 354708, 
    354776, 354951, 355543, 356170, 357888, 
    350458, 350994, 353210, 350263, 358184, 
    349660, 355639, 357128, 349702, 350699, 
    351138, 351520, 353258, 358744, 353100, 
    353555, 354131, 354326, 356774, 350290, 
    350647, 357439, 353287, 351089, 352301, 
    352662, 354333, 354366, 355101, 356088, 
    356092, 356812, 358698, 349902, 351739, 
    357227, 350836, 352279, 359316, 351055, 
    352182, 352700, 353658, 357400, 349916, 
    349917, 350501, 351130, 351367, 353862, 
    353896, 354390, 350428, 353403, 351308, 
    351990, 353866, 356551, 353088, 353184, 
    355269, 355812, 357924, 358553, 358879, 
    352124, 353004, 355518, 356096, 350618, 
    350676, 351366, 353693, 357701, 358570, 
    353353, 354026, 350223, 350480, 358046
  ) 
GROUP BY 
  p.product_id

Query time 0.00181

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 (353384,357241,350560,354163,355107,350285,353055,353876,354140,355911,359349,350064,353374,353147,354708,354776,354951,355543,356170,357888,350458,350994,353210,350263,358184,349660,355639,357128,349702,350699,351138,351520,353258,358744,353100,353555,354131,354326,356774,350290,350647,357439,353287,351089,352301,352662,354333,354366,355101,356088,356092,356812,358698,349902,351739,357227,350836,352279,359316,351055,352182,352700,353658,357400,349916,349917,350501,351130,351367,353862,353896,354390,350428,353403,351308,351990,353866,356551,353088,353184,355269,355812,357924,358553,358879,352124,353004,355518,356096,350618,350676,351366,353693,357701,358570,353353,354026,350223,350480,358046)) 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"
            ]
          }
        }
      ]
    }
  }
}