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 (
    398, 500, 146, 147, 148, 759, 149, 150, 
    151, 152, 153, 3, 507, 509, 481, 510, 
    511, 449, 482, 513, 484, 492, 493, 494, 
    495, 515, 516, 517, 518, 519
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.05295482,
    "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": 30,
          "cost": 0.05295482,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (398,500,146,147,148,759,149,150,151,152,153,3,507,509,481,510,511,449,482,513,484,492,493,494,495,515,516,517,518,519) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}