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 (15381) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00018

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
15381 <p>Coasters made from sustainably harvested palm fronds, naturally dyed and handcrafted in various traditional Botswana designs. Perfect for an elegant coaster or decor in the home. </p> <p>Pattern description: Arguably the most popular pattern, Forehead of the Zebra is a literal interpretation of a zebra's face. It specifically highlights the star-shaped marking on their foreheads. <br /><br /><br /><br />*All products are made by hand with love and may vary slightly in color and size<br /><br />Handmade in Botswana. Sold in sets of 2.<br /><br />Dimensions: <br /><br />D: 9 cm / 3.5 inches (NXX)</p>