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 (
    49398, 49663, 49400, 49399, 49540, 49503, 
    49501, 49502, 49550, 49383, 49541, 
    49543, 49542, 49504, 49547, 49800, 
    49664, 49551, 49666, 49661, 49549, 
    49665, 49604, 49662, 49548, 49785, 
    50235, 50218, 50234, 49788
  ) 
  AND cscart_product_descriptions.lang_code = 'vi'

Query time 0.00232

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": 30,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (49398,49663,49400,49399,49540,49503,49501,49502,49550,49383,49541,49543,49542,49504,49547,49800,49664,49551,49666,49661,49549,49665,49604,49662,49548,49785,50235,50218,50234,49788) and cscart_product_descriptions.lang_code = 'vi'"
        }
      }
    ]
  }
}