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 (
    20621, 20622, 20623, 20624, 20625, 20626, 
    20627, 20628, 20629, 20630, 20631, 
    20632, 20633, 20634, 20635, 20636
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (20621,20622,20623,20624,20625,20626,20627,20628,20629,20630,20631,20632,20633,20634,20635,20636) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
20621 Hook-rear door curtain<p>Part number 83917T10004X</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20622 Label-Fuel Economy<p>Part number 324303N027</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20623 Latch assembly - rear door, left<p>Part number 81410T1410</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20624 Latch sub assembly - rear door, left<p>Part number 81401T1300</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20625 Latch assembly - rear door, right<p>Part number 81420T1410</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20626 Latch sub assembly - rear door, right<p>Part number 81402T1300</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20627 Glass assembly - rear door, left<p>Part number 83410T1800</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20628 Grip Assembly - Rear Door Glass<p>Part number 834123M000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20629 Glass assembly-rear, right<p>Part number 83420T1010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20630 Panel assembly - rear door module, left<p>Part number 83471T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20631 Panel assembly - rear door module, right<p>Part number 83481T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20632 Retainer - rear door latch, left<p>Part number 83472T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20633 Adapter - rear door latch, left<p>Part number 83486T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20634 Retainer - rear door latch, right<p>Part number 83482T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20635 Adapter - rear door latch, right<p>Part number 83496T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20636 Run-rear door window glass, right<p>Part number 83540T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>