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 (
    424699, 381632, 430332, 81747, 392446, 
    371941, 269842, 431672, 414375, 392445, 
    269869, 391070, 371909, 371988, 432451, 
    393012, 185391, 366701, 149587, 269838, 
    81768, 396068, 392483, 82116, 269871, 
    265431, 82095, 371947, 81672, 415548, 
    81764, 371929, 435209, 393013, 185376, 
    81765, 435210, 269853, 427473, 432224, 
    269845, 432223, 429770, 366897, 269872, 
    269934, 185375, 81762, 269946, 269860, 
    371933, 392222, 185380, 435270, 185377, 
    81766, 185395, 269972, 366785, 414480, 
    434596, 371944, 430344, 269928, 269932, 
    371949, 269930, 414400, 185385, 372042, 
    269960, 269867, 185416, 269961, 269945, 
    185379, 185392, 430333, 185393, 430573, 
    371935, 430346, 185386, 270059, 430347, 
    435211, 391702, 265430, 265412, 269950, 
    185422, 185378, 269937, 185402, 269935, 
    371940, 185388, 185383, 269933, 269843
  ) 
GROUP BY 
  p.product_id

Query time 0.00176

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 (424699,381632,430332,81747,392446,371941,269842,431672,414375,392445,269869,391070,371909,371988,432451,393012,185391,366701,149587,269838,81768,396068,392483,82116,269871,265431,82095,371947,81672,415548,81764,371929,435209,393013,185376,81765,435210,269853,427473,432224,269845,432223,429770,366897,269872,269934,185375,81762,269946,269860,371933,392222,185380,435270,185377,81766,185395,269972,366785,414480,434596,371944,430344,269928,269932,371949,269930,414400,185385,372042,269960,269867,185416,269961,269945,185379,185392,430333,185393,430573,371935,430346,185386,270059,430347,435211,391702,265430,265412,269950,185422,185378,269937,185402,269935,371940,185388,185383,269933,269843)) 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"
            ]
          }
        }
      ]
    }
  }
}