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 (
    395983, 395984, 395985, 395986, 395987, 
    395988, 395989, 395990, 395991, 395992, 
    395993, 395994, 395995, 396006, 396007, 
    396008, 396009, 396010, 396011, 396012, 
    396058, 396059, 396060, 396061, 396062, 
    396063, 396064, 396078, 396079, 396080, 
    396081, 396082, 396083, 396084, 396085, 
    414329, 414330, 414331, 414332, 414333, 
    414334, 414393, 414394, 414395, 414396, 
    414397, 414398, 414399, 414408, 414409, 
    414410, 414411, 414412, 414413, 414436, 
    414437, 414438, 414439, 414440, 414441, 
    431514, 431515, 431516, 431517, 431518, 
    431519, 431520, 431521, 431522, 431523, 
    431524, 431525, 431526, 431527, 431528, 
    431529, 431530, 431531, 431532, 431533, 
    431534, 431535, 431536, 431537
  ) 
GROUP BY 
  p.product_id

Query time 0.00100

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 (395983,395984,395985,395986,395987,395988,395989,395990,395991,395992,395993,395994,395995,396006,396007,396008,396009,396010,396011,396012,396058,396059,396060,396061,396062,396063,396064,396078,396079,396080,396081,396082,396083,396084,396085,414329,414330,414331,414332,414333,414334,414393,414394,414395,414396,414397,414398,414399,414408,414409,414410,414411,414412,414413,414436,414437,414438,414439,414440,414441,431514,431515,431516,431517,431518,431519,431520,431521,431522,431523,431524,431525,431526,431527,431528,431529,431530,431531,431532,431533,431534,431535,431536,431537)) 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"
            ]
          }
        }
      ]
    }
  }
}