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 (
    309585, 
    309586, 
    359805, 
    435519, 
    359744, 
    359746, 
    359752, 
    359753, 
    359772, 
    395983, 
    395984, 
    395985, 
    395986, 
    395987, 
    395988, 
    395989, 
    395990, 
    395991, 
    395992, 
    395993, 
    395994, 
    395995, 
    396002, 
    396003, 
    396004, 
    396005, 
    396006, 
    396007, 
    396008, 
    396009, 
    396010, 
    396011, 
    396012, 
    396018, 
    396019, 
    396020, 
    396021, 
    396022, 
    396023, 
    396024, 
    396025, 
    396027, 
    396028, 
    396029, 
    396030, 
    396031, 
    396032, 
    396033, 
    396034, 
    396035, 
    414408, 
    414409, 
    414410, 
    414411, 
    414412, 
    414413, 
    414436, 
    414437, 
    414438, 
    414439, 
    414440, 
    414441, 
    431542, 
    431543, 
    431544, 
    431545, 
    431546, 
    431547, 
    431548, 
    431549, 
    431550, 
    431551, 
    431677, 
    431678, 
    431679, 
    431680, 
    431681, 
    431682, 
    431683, 
    431684, 
    431685, 
    431686, 
    431561, 
    431562, 
    431563, 
    431564, 
    431565, 
    431566, 
    431567, 
    431568, 
    431569, 
    431587, 
    431588, 
    431589, 
    431590, 
    431591, 
    431592, 
    431593, 
    431594, 
    431595, 
    431596, 
    431597, 
    431598, 
    431599, 
    431600, 
    431601, 
    431602, 
    431603, 
    431604, 
    431605, 
    431606, 
    431607, 
    431608, 
    431609, 
    431610, 
    431611, 
    431612, 
    431613, 
    431614, 
    431659, 
    431660, 
    431661, 
    431662, 
    431663, 
    431664, 
    431665, 
    431666, 
    431734, 
    431735, 
    431736, 
    431737, 
    431738, 
    431739, 
    431740, 
    431741, 
    431742, 
    431767, 
    431768, 
    431769, 
    431770, 
    431771, 
    431772, 
    431773, 
    431774, 
    431775
  ) 
GROUP BY 
  p.product_id

Query time 0.00209

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 (309585,309586,359805,435519,359744,359746,359752,359753,359772,395983,395984,395985,395986,395987,395988,395989,395990,395991,395992,395993,395994,395995,396002,396003,396004,396005,396006,396007,396008,396009,396010,396011,396012,396018,396019,396020,396021,396022,396023,396024,396025,396027,396028,396029,396030,396031,396032,396033,396034,396035,414408,414409,414410,414411,414412,414413,414436,414437,414438,414439,414440,414441,431542,431543,431544,431545,431546,431547,431548,431549,431550,431551,431677,431678,431679,431680,431681,431682,431683,431684,431685,431686,431561,431562,431563,431564,431565,431566,431567,431568,431569,431587,431588,431589,431590,431591,431592,431593,431594,431595,431596,431597,431598,431599,431600,431601,431602,431603,431604,431605,431606,431607,431608,431609,431610,431611,431612,431613,431614,431659,431660,431661,431662,431663,431664,431665,431666,431734,431735,431736,431737,431738,431739,431740,431741,431742,431767,431768,431769,431770,431771,431772,431773,431774,431775)) 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"
            ]
          }
        }
      ]
    }
  }
}