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 (
    390977, 390984, 390981, 391005, 391003, 
    390987, 391006, 391004, 391007, 391028, 
    460952, 391015
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00029

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 (390977,390984,390981,391005,391003,390987,391006,391004,391007,391028,460952,391015) and cscart_product_descriptions.lang_code = 'ja'"
        }
      }
    ]
  }
}

Result

product_id short_description full_description unit_name
390977
390981
390984
390987
391003
391004
391005
391006
391007
391015
391028 Elevate your lifestyle with the Mi Smart Watch. Monitor your health, track fitness goals, and stay connected with smart notifications, all from your wrist.
460952 Elevate your lifestyle with the Mi Smart Watch. Monitor your health, track fitness goals, and stay connected with smart notifications, all from your wrist.