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 (
    13445, 13473, 13446, 13474, 13447, 13475, 
    13448, 13476, 13449, 13477, 13450, 
    13478, 13899, 13900, 13901, 13919, 
    13920, 13963, 13964, 13965, 13966, 
    13967, 13968, 14140, 14141, 18508, 
    18509, 18510, 18511, 18513, 18514, 
    18515, 18516, 18517, 18518, 18519
  ) 
GROUP BY 
  p.product_id

Query time 0.00034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.015057387,
    "filesort": {
      "sort_key": "p.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "dp",
              "access_type": "index",
              "possible_keys": ["thread_id", "thread_id_2"],
              "key": "thread_id_2",
              "key_length": "6",
              "used_key_parts": ["thread_id", "status"],
              "loops": 1,
              "rows": 2,
              "cost": 0.00693929,
              "filtered": 100,
              "attached_condition": "dp.`status` = 'A'",
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "cscart_discussion",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "object_id"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["thread_id"],
              "ref": ["u508912950_dev_2025.dp.thread_id"],
              "loops": 2,
              "rows": 1,
              "cost": 0.00342728,
              "filtered": 100,
              "attached_condition": "cscart_discussion.object_type = 'P'"
            }
          },
          {
            "table": {
              "table_name": "p",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["u508912950_dev_2025.cscart_discussion.object_id"],
              "loops": 2,
              "rows": 1,
              "cost": 0.00342728,
              "filtered": 100,
              "attached_condition": "p.product_id in (13445,13473,13446,13474,13447,13475,13448,13476,13449,13477,13450,13478,13899,13900,13901,13919,13920,13963,13964,13965,13966,13967,13968,14140,14141,18508,18509,18510,18511,18513,18514,18515,18516,18517,18518,18519)"
            }
          }
        ]
      }
    }
  }
}