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 (
    20813, 20814, 20815, 20816, 20817, 20818, 
    20819, 20820, 20821, 20822, 20823, 
    20824, 20825, 20826, 20827, 20828
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00032

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 (20813,20814,20815,20816,20817,20818,20819,20820,20821,20822,20823,20824,20825,20826,20827,20828) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
20813 Sealing pad-back panel molding NO.1<p>Part number 87321T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20814 Sealing pad-back panel molding NO.2<p>Part number 87322T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20815 Sealing pad-back panel molding NO.3<p>Part number 87323T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20816 Sealing pad-back panel molding NO.4<p>Part number 87324T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20817 Pad-Back Panel Molding<p>Part number 87378T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20818 Pad-Back Panel Molding<p>Part number 87379T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20819 Molding Subassembly - Back Panel<p>Part number 87380T1000MDY</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20820 Bracket-Tailgate Garnish<p>Part number 87395T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20821 Unit Assembly - Rear View Camera<p>Part number 99240T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20822 Unit Assembly - Rear View Camera<p>Part number 99240T1010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20823 Lamp assembly-puddle, left<p>Part number 87619T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20824 Base pad assembly, left<p>Part number 87635T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20825 Base pad assembly, right<p>Part number 87645T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20826 G/Holder Assy-O/S Rear View, RH<p>Part number 87621JI000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20827 G/Holder Assy-O/S Rear View, LH<p>Part number 87611JI000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20828 Lamp assembly-puddle, right<p>Part number 87629T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>