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 (
    431132, 339796, 391798, 339457, 265042, 
    339477, 339541, 431162, 339700, 339811, 
    431202, 339489, 392172, 431099, 431128, 
    339458, 339623, 339469, 431155, 430639, 
    339453, 339797, 339798, 339606, 392167, 
    431141, 339699, 339643, 431130, 431168, 
    339482, 339685, 431153, 431154, 339730, 
    431096, 431137, 339731, 392175, 339463, 
    381703, 339709, 430637, 431125, 431124, 
    339478, 381725, 339460, 430911, 339468, 
    431094, 431146, 430640, 431134, 339467, 
    430745, 431048, 431187, 339753, 431033, 
    339810, 339808, 339696, 430767, 339805, 
    431067, 339757, 339787, 339701, 339806, 
    430697, 431191, 339794, 431090, 431117, 
    339464, 431065, 431104, 431107, 431157, 
    431116, 381701, 392166, 381687, 431204, 
    339452, 431196, 431084, 339752, 339750, 
    339695, 339814, 430655, 430744, 430850, 
    431203, 431160, 431176, 431100, 339707
  ) 
GROUP BY 
  p.product_id

Query time 0.00099

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 (431132,339796,391798,339457,265042,339477,339541,431162,339700,339811,431202,339489,392172,431099,431128,339458,339623,339469,431155,430639,339453,339797,339798,339606,392167,431141,339699,339643,431130,431168,339482,339685,431153,431154,339730,431096,431137,339731,392175,339463,381703,339709,430637,431125,431124,339478,381725,339460,430911,339468,431094,431146,430640,431134,339467,430745,431048,431187,339753,431033,339810,339808,339696,430767,339805,431067,339757,339787,339701,339806,430697,431191,339794,431090,431117,339464,431065,431104,431107,431157,431116,381701,392166,381687,431204,339452,431196,431084,339752,339750,339695,339814,430655,430744,430850,431203,431160,431176,431100,339707)) 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"
            ]
          }
        }
      ]
    }
  }
}