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 (
    3516, 33744, 1980, 31634, 3518, 40442, 
    31631, 31639, 15462, 2289, 3570, 3569, 
    31636, 1979, 1088, 398, 1981, 31637, 
    5278, 1277, 31635, 1580, 3876, 120, 
    40443, 37850, 3577, 41590, 40149, 2290, 
    34674, 34668
  ) 
  AND cscart_product_descriptions.lang_code = 'vi'

Query time 0.00425

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 (3516,33744,1980,31634,3518,40442,31631,31639,15462,2289,3570,3569,31636,1979,1088,398,1981,31637,5278,1277,31635,1580,3876,120,40443,37850,3577,41590,40149,2290,34674,34668) and cscart_product_descriptions.lang_code = 'vi'"
        }
      }
    ]
  }
}