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 (
    347, 346, 345, 344, 342, 341, 340, 339, 
    338, 335, 334, 333, 332, 331, 330, 321, 
    320, 319, 318, 317, 316, 315, 314, 313
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00040

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": 24,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (347,346,345,344,342,341,340,339,338,335,334,333,332,331,330,321,320,319,318,317,316,315,314,313) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}