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 (
    357038, 358186, 350654, 350889, 351111, 
    359307, 355184, 357857, 350727, 354634, 
    357680, 350718, 355245, 358130, 358146, 
    359166, 351020, 355292, 357278, 359195, 
    359388, 350728, 350801, 357311, 357839, 
    359272, 359313, 351692, 354594, 357977, 
    357996, 358128, 359146, 357042, 358216, 
    350872, 351492, 352774, 351516, 355179, 
    355205, 352287, 357172, 351056, 358166, 
    350828, 350900, 350942, 351017, 357299, 
    357850, 359169, 353933, 357041, 357069, 
    350768, 350823, 350887, 350923, 351129, 
    356169, 357053, 357750, 358206, 350635, 
    351514, 352292, 351499, 354909, 355214, 
    355331, 357099, 351171, 351655, 357257, 
    351219, 356141, 357847, 351041, 351045, 
    356934, 357933, 359097, 351006, 351669, 
    351702, 354910, 355213, 350670, 350809, 
    353938, 357717, 351712, 350755, 351167, 
    351482, 354938, 355301, 357755, 357764
  ) 
GROUP BY 
  p.product_id

Query time 0.00087

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 (357038,358186,350654,350889,351111,359307,355184,357857,350727,354634,357680,350718,355245,358130,358146,359166,351020,355292,357278,359195,359388,350728,350801,357311,357839,359272,359313,351692,354594,357977,357996,358128,359146,357042,358216,350872,351492,352774,351516,355179,355205,352287,357172,351056,358166,350828,350900,350942,351017,357299,357850,359169,353933,357041,357069,350768,350823,350887,350923,351129,356169,357053,357750,358206,350635,351514,352292,351499,354909,355214,355331,357099,351171,351655,357257,351219,356141,357847,351041,351045,356934,357933,359097,351006,351669,351702,354910,355213,350670,350809,353938,357717,351712,350755,351167,351482,354938,355301,357755,357764)) 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"
            ]
          }
        }
      ]
    }
  }
}