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 (
    20685, 20686, 20687, 20688, 20689, 20690, 
    20691, 20692, 20693, 20694, 20695, 
    20696, 20697, 20698, 20699, 20700
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00035

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 (20685,20686,20687,20688,20689,20690,20691,20692,20693,20694,20695,20696,20697,20698,20699,20700) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
20685 Keyboard Assembly - AVN<p>Part number 96540T1010VCS</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20686 Keyboard Assembly - AVN<p>Part number 96540T1210VCS</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20687 Air Ventilator Assy-Console<p>Part number 97040T1100OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20688 Control assembly - rear heater<p>Part number 97950T1000UV2</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20689 Socket Assembly-Accessories<p>Part number 95120T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20690 bolt washer assembly<p>Part number 84434T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20691 Bar Assembly - Cowl Cross<p>Part number 84410T1100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20692 glove box assembly<p>Part number 84510T1000OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20693 Crash pad assembly-main<p>Part number 84701T1300OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20694 Panel assembly - center fascia, upper part<p>Part number 84835T1000OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20695 Grille Assembly - Center Speaker<p>Part number 84715T1000OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20696 Panel assembly - lower crash pad, left<p>Part number 84750T1000OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20697 Panel Assembly-Lower Crash Pad<p>Part number 84730T1001OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20698 Garnish-crash pad, left<p>Part number 84790T1200RWD</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20699 Garnish-crash pad, right<p>Part number 84795T1200RWD</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20700 Cover Assy - Head Up Display<p>Part number 84775T1000NBD</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>