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 (
    303, 304, 305, 310, 311, 312, 313, 314, 
    315, 316, 317, 318, 319, 320, 321, 291, 
    292, 307, 296, 297
  ) 
  AND cscart_product_descriptions.lang_code = 'vn'

Query time 0.00036

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": 20,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (303,304,305,310,311,312,313,314,315,316,317,318,319,320,321,291,292,307,296,297) and cscart_product_descriptions.lang_code = 'vn'"
        }
      }
    ]
  }
}