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 (
    366926, 366931, 366929, 366928, 366927, 
    366909, 366917, 366924, 366910, 366923, 
    366918, 366919, 366911, 366916, 366925, 
    366930, 366920, 366908, 366922, 366912, 
    366949, 366948, 366947, 415241, 366950, 
    366921, 286678, 357379, 357932, 353932, 
    350961, 435488, 351123, 352254, 350949, 
    351969, 357984, 351136, 355255, 352236, 
    350951, 351987, 350960, 351168, 350630, 
    350982, 357357, 387062, 355293, 350617, 
    354066, 350597, 352264, 357154, 351074, 
    351124, 357903, 357961, 357316, 355232, 
    358167, 354932, 357413, 351707, 350616, 
    387063, 351134, 357165, 350599, 355294, 
    350838, 357995, 357351, 355312, 357724, 
    350954, 357686, 351207, 357272, 357364, 
    357851, 350956, 355197, 350917, 350955, 
    357285, 350947, 356939, 350622, 357308, 
    351671, 357332, 351651, 351023, 357363, 
    351982, 357976, 357276, 357910, 355368
  ) 
GROUP BY 
  p.product_id

Query time 0.00113

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 (366926,366931,366929,366928,366927,366909,366917,366924,366910,366923,366918,366919,366911,366916,366925,366930,366920,366908,366922,366912,366949,366948,366947,415241,366950,366921,286678,357379,357932,353932,350961,435488,351123,352254,350949,351969,357984,351136,355255,352236,350951,351987,350960,351168,350630,350982,357357,387062,355293,350617,354066,350597,352264,357154,351074,351124,357903,357961,357316,355232,358167,354932,357413,351707,350616,387063,351134,357165,350599,355294,350838,357995,357351,355312,357724,350954,357686,351207,357272,357364,357851,350956,355197,350917,350955,357285,350947,356939,350622,357308,351671,357332,351651,351023,357363,351982,357976,357276,357910,355368)) 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"
            ]
          }
        }
      ]
    }
  }
}