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, 344, 343, 342, 341, 340, 339, 338, 
    335, 334, 333, 332, 331, 330, 328, 321, 
    320, 319, 318, 317, 316, 315, 314, 313, 
    312, 311, 310, 309, 308, 307, 306, 305, 
    304, 303, 302, 301, 300, 299, 298, 297, 
    296, 295, 294, 293, 292, 291
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00088

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": 46,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (347,344,343,342,341,340,339,338,335,334,333,332,331,330,328,321,320,319,318,317,316,315,314,313,312,311,310,309,308,307,306,305,304,303,302,301,300,299,298,297,296,295,294,293,292,291) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}