SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '' 
    OR cscart_product_descriptions.short_description IS NULL, 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    403905, 410883, 408899, 408775, 440269, 
    435840, 444141, 410125, 453374, 429849, 
    427100, 430100, 427619, 444547, 432847, 
    403496, 431636, 455406, 435103, 415659, 
    443503, 401160, 406539, 408349
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00065

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.04252208,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["product_id", "lang_code"],
          "loops": 1,
          "rows": 24,
          "cost": 0.04252208,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (403905,410883,408899,408775,440269,435840,444141,410125,453374,429849,427100,430100,427619,444547,432847,403496,431636,455406,435103,415659,443503,401160,406539,408349) and cscart_product_descriptions.lang_code = 'ja'"
        }
      }
    ]
  }
}