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 (
    189, 192, 168, 188, 166, 203, 196, 3, 165, 
    167, 221, 184, 193, 224, 160, 820, 169, 
    200
  ) 
  AND cscart_product_descriptions.lang_code = 'de'

Query time 0.00032

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.03208934,
    "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": 18,
          "cost": 0.03208934,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (189,192,168,188,166,203,196,3,165,167,221,184,193,224,160,820,169,200) and cscart_product_descriptions.lang_code = 'de'"
        }
      }
    ]
  }
}