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, 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, 149042, 
    149050, 149051, 149052, 149053, 149054, 
    149055, 149056, 149057, 149058, 149059, 
    149060, 149061, 149062, 149063, 149064, 
    149065, 149066, 149067, 149068, 149069, 
    149070, 149071, 149072, 149073, 149074, 
    149075, 149076, 149077, 149078, 149079, 
    149080, 149081, 149082, 149083, 149084, 
    149085, 149086, 149087, 149088, 149089, 
    149090, 149091, 149092, 149093, 149094, 
    149095, 149096, 149097, 149098, 149099, 
    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 0.00102

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 (81655,81657,81702,81704,81737,81739,81826,81860,81862,81863,82052,82061,82069,82071,82072,82073,82074,82101,82103,81948,149025,149026,149027,149028,149029,149030,149031,149032,149033,149034,149035,149036,149037,149038,149039,149040,149041,82118,149043,149044,149045,149046,149047,149048,149049,149042,149050,149051,149052,149053,149054,149055,149056,149057,149058,149059,149060,149061,149062,149063,149064,149065,149066,149067,149068,149069,149070,149071,149072,149073,149074,149075,149076,149077,149078,149079,149080,149081,149082,149083,149084,149085,149086,149087,149088,149089,149090,149091,149092,149093,149094,149095,149096,149097,149098,149099,82009,149101,149102,149103,149104,149105,149106,149107,149108,149109,149110,149111,149112,149113,149114,81647,149123,149124,149125,149126,149127,149128,149129,149130)) 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"
            ]
          }
        }
      ]
    }
  }
}