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 (
    245, 239, 238, 74, 73, 71, 75, 72, 70, 76, 
    67, 86, 106, 105, 101, 69, 125, 120, 117, 
    113, 68
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00315

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": 21,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (245,239,238,74,73,71,75,72,70,76,67,86,106,105,101,69,125,120,117,113,68) and cscart_product_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}