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 (
    47599, 21464, 2015, 1836, 2255, 3451, 
    40526, 6555, 25180, 286, 4071, 26604, 
    3004, 3765, 2509, 294, 33718, 2576, 
    26608, 5577, 2414, 2797, 2492, 2302, 
    50233, 47603, 48844, 38560, 6607, 1732, 
    36834, 2448
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00408

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 (47599,21464,2015,1836,2255,3451,40526,6555,25180,286,4071,26604,3004,3765,2509,294,33718,2576,26608,5577,2414,2797,2492,2302,50233,47603,48844,38560,6607,1732,36834,2448) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}