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 (
    414351, 377755, 152080, 149667, 396065, 
    359747, 431667, 82115, 81776, 149585, 
    359740, 149670, 82074, 415550, 387078, 
    152069, 185415, 149665, 81611, 371912, 
    381626, 414342, 149481, 309571, 81862, 
    149617, 359779, 396013, 265423, 81627, 
    372021, 414534, 387080, 393018, 377735, 
    387083, 430340, 359751, 391691, 366903, 
    82114, 82069, 265427, 152068, 81773, 
    414541, 81817, 149605, 81636, 81812, 
    265433, 391704, 427476, 152085, 368409, 
    427477, 387082, 414525, 414322, 366900, 
    152067, 371917, 366968, 391071, 372041, 
    81624, 371984, 371905, 81806, 82099, 
    392486, 82052, 415554, 81756, 81816, 
    82096, 372040, 393020, 366754, 152071, 
    371843, 392485, 393026, 391278, 381634, 
    392484, 366841, 81814, 265422, 265411, 
    393022, 414502, 149664, 392444, 359756, 
    371799, 390761, 371945, 270140, 392487
  ) 
GROUP BY 
  p.product_id

Query time 0.00070

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 (414351,377755,152080,149667,396065,359747,431667,82115,81776,149585,359740,149670,82074,415550,387078,152069,185415,149665,81611,371912,381626,414342,149481,309571,81862,149617,359779,396013,265423,81627,372021,414534,387080,393018,377735,387083,430340,359751,391691,366903,82114,82069,265427,152068,81773,414541,81817,149605,81636,81812,265433,391704,427476,152085,368409,427477,387082,414525,414322,366900,152067,371917,366968,391071,372041,81624,371984,371905,81806,82099,392486,82052,415554,81756,81816,82096,372040,393020,366754,152071,371843,392485,393026,391278,381634,392484,366841,81814,265422,265411,393022,414502,149664,392444,359756,371799,390761,371945,270140,392487)) 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"
            ]
          }
        }
      ]
    }
  }
}