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 (
    394196, 431190, 430574, 431092, 431106, 
    431147, 430849, 431162, 431128, 431099, 
    431132, 431202, 430637, 430639, 431096, 
    431168, 431141, 431153, 431137, 431155, 
    431033, 431130, 430911, 430640, 431134, 
    431125, 431154, 431090, 431146, 431065, 
    431067, 431187, 431048, 431094, 430697, 
    430767, 431050, 431104, 430745, 431124, 
    431117, 431191, 431157, 431160, 430681, 
    431116, 430743, 430850, 430908, 431084, 
    431107, 430998, 431026, 430655, 430744, 
    431100, 431115, 430915, 430916, 430636, 
    430924, 431195, 430747, 430803, 431176, 
    431203, 430748, 431204, 431133, 430648, 
    431196, 430823, 431158, 431199, 431122, 
    430903, 430848, 431139, 430831, 431150, 
    430665, 430696, 431145, 431102, 430771, 
    430907, 430649, 430646, 430647, 430912, 
    430935, 430638, 430766, 431097, 431151, 
    430821, 430824, 431359, 430857, 431201
  ) 
GROUP BY 
  p.product_id

Query time 0.00079

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 (394196,431190,430574,431092,431106,431147,430849,431162,431128,431099,431132,431202,430637,430639,431096,431168,431141,431153,431137,431155,431033,431130,430911,430640,431134,431125,431154,431090,431146,431065,431067,431187,431048,431094,430697,430767,431050,431104,430745,431124,431117,431191,431157,431160,430681,431116,430743,430850,430908,431084,431107,430998,431026,430655,430744,431100,431115,430915,430916,430636,430924,431195,430747,430803,431176,431203,430748,431204,431133,430648,431196,430823,431158,431199,431122,430903,430848,431139,430831,431150,430665,430696,431145,431102,430771,430907,430649,430646,430647,430912,430935,430638,430766,431097,431151,430821,430824,431359,430857,431201)) 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"
            ]
          }
        }
      ]
    }
  }
}