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 (
    21839, 21840, 21841, 21842, 21843, 21844, 
    21845, 21846, 21847, 21848, 21849, 
    21850, 21851, 21852, 21853, 21854
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00042

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 (21839,21840,21841,21842,21843,21844,21845,21846,21847,21848,21849,21850,21851,21852,21853,21854) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21839 Bracket assembly - Chald lower anchor outside, right side<p>Part number 89798T1100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21840 Seat Belt Assembly - Front Left<p>Part number 88810T1000NWA</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21841 Seat Belt Assembly - Front Right<p>Part number 88820T1000NWA</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21842 Heater-rear seat cushion, right<p>Part number 89252T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21843 Wiring harness-rear seat, left<p>Part number 89561T1040</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21844 Wiring harness-rear seat, right<p>Part number 89661T1040</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21845 Heater ECU - Rear seat, left<p>Part number 89B70T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21846 Heater ECU - Rear seat, right<p>Part number 89B71T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21847 Headrest Assembly-Rear Seat<p>Part number 89700T1300JK6</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21848 Guide assembly-rear seat headrest<p>Part number 89370J6000OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21849 Guide assembly-rear seat headrest<p>Part number 89372J6000OWN</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21850 Bezel assembly-rear seat armrest<p>Part number 89917T1010RWD</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21851 Charger Assembly-USB<p>Part number 96125T11004X</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21852 Charger Assembly-USB<p>Part number 96125T10504X</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21853 Switch assembly - rear remote control<p>Part number 96730T1010VCS</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21854 Seat belt assembly - rear, center<p>Part number 89850T1000NWA</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>