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 (
    238, 239, 245, 71, 70, 69, 68, 67, 86, 106, 
    105, 101, 113, 117, 125, 120, 74, 73, 
    75, 72, 76
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00047

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": 21,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (238,239,245,71,70,69,68,67,86,106,105,101,113,117,125,120,74,73,75,72,76) and cscart_product_descriptions.lang_code = 'ja'"
    }
  }
}