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 (
    20044, 20036, 20088, 20082, 20041, 20035, 
    20067, 20040, 20043, 20037, 20042, 
    20038, 20079
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00068

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": 13,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (20044,20036,20088,20082,20041,20035,20067,20040,20043,20037,20042,20038,20079) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
20035
20036
20037
20038
20040
20041
20042
20043
20044
20067
20079
20082
20088