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 (
    396008, 395983, 309587, 395989, 414436, 
    414408, 414393, 431514, 396058, 396078, 
    396044, 414329, 414422, 431590, 431659, 
    359772, 309590, 359805, 309585, 359762, 
    396002, 414335, 431542, 414429, 414468, 
    359808, 431767, 414459, 309561, 414360, 
    414401, 431605, 309550, 359744, 414415, 
    396052, 431734, 396027, 359752, 392229, 
    392230, 392210, 392231, 392232, 396018, 
    359758, 392255, 392254, 392252, 392253, 
    431506, 396074, 309576, 431615, 389637, 
    366904, 414316, 414488, 395996, 414323, 
    414443, 359478, 396037, 415577, 431579, 
    414507, 391690, 414326, 309567, 359747, 
    396065, 415550, 387078, 431667, 414351, 
    381626, 309571, 359779, 414342, 393018, 
    387080, 396013, 387083, 414534, 430340, 
    359751, 391691, 414541, 391704, 427476, 
    427477, 387082, 414322, 414525, 415554, 
    393020, 372040, 393026, 381634, 393022
  ) 
GROUP BY 
  p.product_id

Query time 0.00093

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 (396008,395983,309587,395989,414436,414408,414393,431514,396058,396078,396044,414329,414422,431590,431659,359772,309590,359805,309585,359762,396002,414335,431542,414429,414468,359808,431767,414459,309561,414360,414401,431605,309550,359744,414415,396052,431734,396027,359752,392229,392230,392210,392231,392232,396018,359758,392255,392254,392252,392253,431506,396074,309576,431615,389637,366904,414316,414488,395996,414323,414443,359478,396037,415577,431579,414507,391690,414326,309567,359747,396065,415550,387078,431667,414351,381626,309571,359779,414342,393018,387080,396013,387083,414534,430340,359751,391691,414541,391704,427476,427477,387082,414322,414525,415554,393020,372040,393026,381634,393022)) 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"
            ]
          }
        }
      ]
    }
  }
}