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 (
    21391, 21392, 21393, 21394, 21395, 21396, 
    21397, 21398, 21399, 21400, 21401, 
    21402, 21403, 21404, 21405, 21406
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00048

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 (21391,21392,21393,21394,21395,21396,21397,21398,21399,21400,21401,21402,21403,21404,21405,21406) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21391 Funnel assembly<p>Part number 98616T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21392 Motor & Pump Assembly - Windshield Washer<p>Part number 985102W500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21393 Reservoir & Pump Assembly-Washer<p>Part number 98610T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21394 Funnel-Windshield Washer<p>Part number 98617T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21395 hose-funnel<p>Part number 98619T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21396 Reservoir Assembly - Windshield Washer<p>Part number 98611T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21397 Grommet-Funnel<p>Part number 98635T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21398 cable-bend<p>Part number 911113B000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21399 Bolt & Rubber Assembly - Motor Mounting<p>Part number 98718T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21400 Motor & Linkage Assembly - Rear Wiper<p>Part number 98700T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21401 Rubber Pad - Rear Wiper<p>Part number 987173J000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21402 Arm assembly - rear wiper<p>Part number 98820T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21403 Cab - Rear Wiper Arm<p>Part number 98812T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21404 Blade assembly - wiper, rear<p>Part number 98850T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21405 Grommet & Hose Assembly-Rear Washer<p>Part number 98960T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21406 Hose Assembly - Rear Washer<p>Part number 98950T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>