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 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    2403, 2413, 2434, 781, 1572, 1600, 1635, 
    1660, 1688, 1689, 1701, 1704
  ) 
  AND cscart_product_descriptions.lang_code = 'de'

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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"],
      "rows": 12,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (2403,2413,2434,781,1572,1600,1635,1660,1688,1689,1701,1704) and cscart_product_descriptions.lang_code = 'de'"
    }
  }
}