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 (
    21117, 21118, 21119, 21120, 21121, 21122, 
    21123, 21124, 21125, 21126, 21127, 
    21128, 21129, 21130, 21131, 21132
  ) 
  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 (21117,21118,21119,21120,21121,21122,21123,21124,21125,21126,21127,21128,21129,21130,21131,21132) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21117 Glass-Tail Gate<p>Part number 87111T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21118 Terminal-Rear Window Glass Defogger<p>Part number 87116G8000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21119 Nut-Washer Assembly<p>Part number 1338206007K</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21120 SPOILER ASSY-RR<p>Part number 87210T6000UYH</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21121 Pad 1 - Rear Spoiler<p>Part number 87213T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21122 Pad 2 - Rear Spoiler<p>Part number 87214T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21123 Pad 3 - Rear Spoiler<p>Part number 87215T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21124 Pad 4 - Rear Spoiler<p>Part number 87216T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21125 Pad 5 - Rear Spoiler<p>Part number 87217T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21126 Bolt-Insert<p>Part number 87255D9000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21127 Pad-rear spoiler NO.07<p>Part number 87218T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21128 Pad 6 - Rear Spoiler<p>Part number 87218Q5000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21129 Pad 7 - Rear Spoiler<p>Part number 87219J7000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21130 Bolt-Insert<p>Part number 87223T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21131 Bolt-Rear Spoiler<p>Part number 87299J7300</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21132 Pad-rear spoiler<p>Part number 87256G3000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>