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 (197) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00030

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
197 <p> 収録内容:</p> <p> - Tenderly</p> <p> - I Want To Hold Your Hand</p> <p> - My Cherie Amour</p> <p> - Naima</p> <p> - Tequila</p> <p> - Don&#39;t Know Why</p> <p> - Lady In My Life</p> <p> - My One And Only Love</p> <p> - Paper Moon</p> <p> - Danny Boy</p> <p> - Since I Fell For You</p> <p> - Fingerlero</p>