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 (
    265000, 40404, 262379, 265262, 392209, 
    339537, 114747, 271509, 339493, 265255, 
    339524, 339540, 339536, 339491, 265241, 
    114761, 265230, 339517, 339522, 339526, 
    339520, 285971, 81644, 265236, 81606, 
    81700, 271508, 265251, 271499, 224457, 
    271501, 339490, 392218, 265238, 265240, 
    392216, 265253, 392217, 81601, 366896, 
    339741, 381654, 265243, 265264, 81687, 
    339498, 271507, 339449, 265233, 32880, 
    339528, 224454, 381657, 81703, 391799, 
    265234, 265257, 381658, 339497, 339689, 
    224455, 265259, 285967, 81679, 339448, 
    265249, 265252, 339450, 265260, 359820, 
    339435, 392151, 265239, 285969, 381656, 
    339622, 339442, 381655, 81666, 339616, 
    339443, 152086, 339436, 170242, 339574, 
    259708, 339523, 339507, 205875, 339437, 
    339504, 339451, 339512, 265035, 81683, 
    139611, 339740, 339609, 339500, 339532
  ) 
GROUP BY 
  p.product_id

Query time 0.00112

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 (265000,40404,262379,265262,392209,339537,114747,271509,339493,265255,339524,339540,339536,339491,265241,114761,265230,339517,339522,339526,339520,285971,81644,265236,81606,81700,271508,265251,271499,224457,271501,339490,392218,265238,265240,392216,265253,392217,81601,366896,339741,381654,265243,265264,81687,339498,271507,339449,265233,32880,339528,224454,381657,81703,391799,265234,265257,381658,339497,339689,224455,265259,285967,81679,339448,265249,265252,339450,265260,359820,339435,392151,265239,285969,381656,339622,339442,381655,81666,339616,339443,152086,339436,170242,339574,259708,339523,339507,205875,339437,339504,339451,339512,265035,81683,139611,339740,339609,339500,339532)) 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"
            ]
          }
        }
      ]
    }
  }
}