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 (
    291, 292, 293, 294, 295, 296, 297, 298, 
    300, 302, 303, 304, 305, 306, 307, 308, 
    309, 310, 311, 312, 313, 314, 315, 316
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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"],
          "rows": 24,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (291,292,293,294,295,296,297,298,300,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}