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 (
    400943, 439400, 443680, 427742, 455495, 
    457174, 416538, 434799, 443084, 417214, 
    449549, 421275
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00117

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0216566,
    "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": 12,
          "cost": 0.0216566,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (400943,439400,443680,427742,455495,457174,416538,434799,443084,417214,449549,421275) and cscart_product_descriptions.lang_code = 'ja'"
        }
      }
    ]
  }
}