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 (
    26977, 6554, 15218, 41399, 25730, 47599, 
    2015, 2255, 6098, 48826, 40526, 6555, 
    25180, 286, 4071, 26854, 405, 5226, 
    26604, 294, 33718, 2576, 26608, 26864, 
    2414, 21188, 2797, 2492, 26846, 2302, 
    6523, 38560
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00439

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": "11",
          "used_key_parts": ["product_id", "lang_code"],
          "rows": 32,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (26977,6554,15218,41399,25730,47599,2015,2255,6098,48826,40526,6555,25180,286,4071,26854,405,5226,26604,294,33718,2576,26608,26864,2414,21188,2797,2492,26846,2302,6523,38560) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}