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 (
    81655, 81657, 81702, 81704, 81737, 81739, 
    81826, 81860, 81862, 81863, 82052, 
    82061, 82069, 82071, 82072, 82073, 
    82074, 82101, 82103, 82038, 148882, 
    148883, 148884, 148885, 148886, 148887, 
    148888, 148889, 148890, 148891, 148892, 
    148893, 148894, 148895, 148896, 148897, 
    148898, 148899, 148900, 148901, 148902, 
    148903, 148904, 148905, 148906, 148907, 
    148908, 148909, 148910, 148911, 148912, 
    148913, 148914, 148915, 148916, 148917, 
    148918, 148919, 148920, 148921, 148922, 
    148923, 148924, 148925, 148926, 148927, 
    148928, 148929, 148930, 148931, 81948, 
    149025, 149026, 149027, 149028, 149029, 
    149030, 149031, 149032, 149033, 149034, 
    149035, 149036, 149037, 149038, 149039, 
    149040, 149041, 82118, 149043, 149044, 
    149045, 149046, 149047, 149048, 149049, 
    82009, 149101, 149102, 149103, 149104, 
    149105, 149106, 149107, 149108, 149109, 
    149110, 149111, 149112, 149113, 149114, 
    81647, 149123, 149124, 149125, 149126, 
    149127, 149128, 149129, 149130
  ) 
GROUP BY 
  p.product_id

Query time 0.00100

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 (81655,81657,81702,81704,81737,81739,81826,81860,81862,81863,82052,82061,82069,82071,82072,82073,82074,82101,82103,82038,148882,148883,148884,148885,148886,148887,148888,148889,148890,148891,148892,148893,148894,148895,148896,148897,148898,148899,148900,148901,148902,148903,148904,148905,148906,148907,148908,148909,148910,148911,148912,148913,148914,148915,148916,148917,148918,148919,148920,148921,148922,148923,148924,148925,148926,148927,148928,148929,148930,148931,81948,149025,149026,149027,149028,149029,149030,149031,149032,149033,149034,149035,149036,149037,149038,149039,149040,149041,82118,149043,149044,149045,149046,149047,149048,149049,82009,149101,149102,149103,149104,149105,149106,149107,149108,149109,149110,149111,149112,149113,149114,81647,149123,149124,149125,149126,149127,149128,149129,149130)) 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"
            ]
          }
        }
      ]
    }
  }
}