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 (
    81624, 
    81625, 
    81627, 
    81634, 
    81636, 
    81756, 
    81762, 
    81764, 
    81765, 
    81766, 
    81768, 
    81773, 
    81776, 
    81806, 
    81812, 
    81814, 
    81816, 
    81817, 
    81826, 
    81860, 
    81862, 
    81863, 
    82052, 
    82061, 
    82069, 
    82071, 
    82072, 
    82073, 
    82074, 
    82095, 
    82096, 
    82114, 
    82115, 
    82116, 
    81611, 
    149176, 
    309571, 
    359740, 
    359742, 
    366754, 
    366755, 
    366841, 
    366842, 
    366843, 
    366844, 
    368409, 
    372040, 
    371843, 
    377765, 
    377766, 
    377767, 
    371905, 
    377794, 
    377795, 
    371912, 
    377798, 
    377799, 
    377800, 
    377801, 
    371917, 
    377802, 
    377803, 
    377735, 
    377804, 
    377805, 
    377806, 
    377807, 
    377755, 
    377808, 
    377809, 
    377810, 
    371945, 
    379703, 
    371984, 
    379707, 
    379708, 
    379709, 
    372021, 
    379711, 
    379712, 
    379713, 
    379714, 
    381634, 
    387082, 
    392485, 
    392486, 
    392487, 
    396013, 
    396014, 
    396015, 
    396016, 
    396017, 
    396065, 
    396066, 
    396067, 
    414342, 
    414343, 
    414344, 
    414345, 
    414346, 
    414347, 
    414348, 
    414349, 
    414350, 
    414351, 
    414352, 
    414353, 
    414354, 
    414355, 
    414356, 
    414357, 
    414358, 
    414359, 
    414502, 
    414504, 
    414525, 
    414527, 
    414528, 
    414529, 
    414530, 
    414531, 
    414534, 
    414535, 
    414536, 
    414537, 
    414538, 
    414541, 
    414542, 
    414543, 
    414544, 
    414545, 
    431667, 
    431668, 
    431669, 
    431670, 
    431671
  ) 
GROUP BY 
  p.product_id

Query time 0.00128

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 (81624,81625,81627,81634,81636,81756,81762,81764,81765,81766,81768,81773,81776,81806,81812,81814,81816,81817,81826,81860,81862,81863,82052,82061,82069,82071,82072,82073,82074,82095,82096,82114,82115,82116,81611,149176,309571,359740,359742,366754,366755,366841,366842,366843,366844,368409,372040,371843,377765,377766,377767,371905,377794,377795,371912,377798,377799,377800,377801,371917,377802,377803,377735,377804,377805,377806,377807,377755,377808,377809,377810,371945,379703,371984,379707,379708,379709,372021,379711,379712,379713,379714,381634,387082,392485,392486,392487,396013,396014,396015,396016,396017,396065,396066,396067,414342,414343,414344,414345,414346,414347,414348,414349,414350,414351,414352,414353,414354,414355,414356,414357,414358,414359,414502,414504,414525,414527,414528,414529,414530,414531,414534,414535,414536,414537,414538,414541,414542,414543,414544,414545,431667,431668,431669,431670,431671)) 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"
            ]
          }
        }
      ]
    }
  }
}