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, 
    396044, 
    396045, 
    396046, 
    396047, 
    396048, 
    396049, 
    396050, 
    396058, 
    396059, 
    396060, 
    396061, 
    396062, 
    396063, 
    396064, 
    396078, 
    396079, 
    396080, 
    396081, 
    396082, 
    396083, 
    396084, 
    396085, 
    414329, 
    414330, 
    414331, 
    414332, 
    414333, 
    414334, 
    414376, 
    414377, 
    414378, 
    414379, 
    414380, 
    414381, 
    414382, 
    414383, 
    414384, 
    414385, 
    414386, 
    414387, 
    414388, 
    414393, 
    414394, 
    414395, 
    414396, 
    414397, 
    414398, 
    414399, 
    414408, 
    414409, 
    414410, 
    414411, 
    414412, 
    414413, 
    414422, 
    414423, 
    414424, 
    414425, 
    414426, 
    414427, 
    414428, 
    414436, 
    414437, 
    414438, 
    414439, 
    414440, 
    414441, 
    414514, 
    414516, 
    414517, 
    414518, 
    414519, 
    414520, 
    414521, 
    431514, 
    431515, 
    431516, 
    431517, 
    431518, 
    431519, 
    431520, 
    431521, 
    431522, 
    431523, 
    431524, 
    431525, 
    431526, 
    431527, 
    431528, 
    431529, 
    431530, 
    431531, 
    431532, 
    431533, 
    431534, 
    431535, 
    431536, 
    431537, 
    431552, 
    431553, 
    431554, 
    431555, 
    431556, 
    431557, 
    431558, 
    431559, 
    431560, 
    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, 
    431659, 
    431660, 
    431661, 
    431662, 
    431663, 
    431664, 
    431665, 
    431666, 
    431687, 
    431688, 
    431689, 
    431690, 
    431691, 
    431692, 
    431693, 
    431694, 
    431695, 
    431696, 
    431697, 
    431783, 
    431784, 
    431785, 
    431786, 
    431787, 
    431788, 
    431789, 
    431790, 
    431791, 
    431802, 
    431803, 
    431804, 
    431805, 
    431806, 
    431807, 
    431808, 
    431809, 
    431810
  ) 
GROUP BY 
  p.product_id

Query time 0.00169

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 (395983,395984,395985,395986,395987,395988,395989,395990,395991,395992,395993,395994,395995,396006,396007,396008,396009,396010,396011,396012,396044,396045,396046,396047,396048,396049,396050,396058,396059,396060,396061,396062,396063,396064,396078,396079,396080,396081,396082,396083,396084,396085,414329,414330,414331,414332,414333,414334,414376,414377,414378,414379,414380,414381,414382,414383,414384,414385,414386,414387,414388,414393,414394,414395,414396,414397,414398,414399,414408,414409,414410,414411,414412,414413,414422,414423,414424,414425,414426,414427,414428,414436,414437,414438,414439,414440,414441,414514,414516,414517,414518,414519,414520,414521,431514,431515,431516,431517,431518,431519,431520,431521,431522,431523,431524,431525,431526,431527,431528,431529,431530,431531,431532,431533,431534,431535,431536,431537,431552,431553,431554,431555,431556,431557,431558,431559,431560,431561,431562,431563,431564,431565,431566,431567,431568,431569,431587,431588,431589,431590,431591,431592,431593,431594,431595,431596,431597,431598,431599,431600,431601,431602,431603,431604,431659,431660,431661,431662,431663,431664,431665,431666,431687,431688,431689,431690,431691,431692,431693,431694,431695,431696,431697,431783,431784,431785,431786,431787,431788,431789,431790,431791,431802,431803,431804,431805,431806,431807,431808,431809,431810)) 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"
            ]
          }
        }
      ]
    }
  }
}