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 (
    20637, 20638, 20639, 20640, 20641, 20642, 
    20643, 20644, 20645, 20646, 20647, 
    20648, 20649, 20650, 20651, 20652
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

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 (20637,20638,20639,20640,20641,20642,20643,20644,20645,20646,20647,20648,20649,20650,20651,20652) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
20637 Run-rear door window glass, left<p>Part number 83530T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20638 Channel assembly - rear door rear, left<p>Part number 83550T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20639 Channel assembly - rear door rear, right<p>Part number 83560T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20640 Molding assembly - rear door frame, left<p>Part number 83850T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20641 Molding assembly - rear door frame, right<p>Part number 83860T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20642 Bolt-Washer Assembly<p>Part number 1128110206B</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20643 Insulator-rear floor<p>Part number 84115T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20644 Cover-side upper, left<p>Part number 841A5T1001</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20645 Cover-side upper, right<p>Part number 841B5T1001</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20646 Strut bar - front, left<p>Part number 841A2T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20647 Support assembly - upper partition, left<p>Part number 841A7T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20648 Support assembly - upper partition, right<p>Part number 841A8T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20649 Under cover assy-floor front, left<p>Part number 84213T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20650 Under cover assy-floor rear, left<p>Part number 84237T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20651 Pad assembly-isolation dash panel<p>Part number 84120T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20652 Undercover Assy-Floor Front, Right<p>Part number 84223T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>