Sandbox

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 (200) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "const",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result

product_id	short_description	full_description
200		<p> 印象的でポップな旋律によって、各国のアマチュア合唱団の間で絶大な人気を誇るジョン・ラターのクリスマスアルバム、</p> <p> 収録内容</p> <p> 1. Ding Dong! Merrily on High </p> <p> 2. The Colours of Christmas </p> <p> 3. In Dulci Jubilo </p> <p> 4. What is this lovely fragrance? </p> <p> 5. Away in a Manger </p> <p> 6. Hodie Christus natus est </p> <p> 7. Riu riu chiu </p> <p> 8. I wonder as I wander </p> <p> 9. Star Carol </p> <p> 10. Once in Royal David's City </p> <p> 11. Silent Night </p> <p> 12. In the bleak midwinter </p> <p> 13. Gabriel's Message </p> <p> 14. Fanfare - O come, all ye faithful </p> <p> 15. O Holy Night </p> <p> 16. A New Year Carol </p> <p> 17. Joy to the World </p> <p> 18. Sleigh Ride </p> <p> 19. Deck the Hall </p> <p> 20. The Christmas Song </p> <p> 21. The Twelve Days Of Christmas </p> <p> 22. Have yourself a merry little Christmas </p> <p> 23. Fanfare - Hark! the herald angels sing</p>