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 (
    21535, 21536, 21537, 21538, 21539, 21540, 
    21541, 21542, 21543, 21544, 21545, 
    21546, 21547, 21548, 21549, 21550
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00037

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 (21535,21536,21537,21538,21539,21540,21541,21542,21543,21544,21545,21546,21547,21548,21549,21550) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
21535 Core Assembly-Evaporator<p>Part number 97927T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21536 Duct assembly - rear air conditioner NO.2<p>Part number 97935T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21537 Motor & Wheel Assembly<p>Part number 97945T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21538 Duct Assembly - Rear Air Conditioner<p>Part number 97930T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21539 Auto Transaxle and Torque Converter Assembly<p>Part number 450004JCA0</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21540 Tube Assembly-ATF<p>Part number 254704J000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21541 Bracket assembly-transmission support<p>Part number 4521047500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21542 Seal Assembly - Oil<p>Part number 452654F001</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21543 Flange Assembly - Companion<p>Part number 458204F010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21544 Shaft Assembly-Output<p>Part number 458104J000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21545 nut<p>Part number 458244F000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21546 Shaft Assembly - Propeller<p>Part number 49100T1600</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21547 Knuckle-front axle, left<p>Part number 51710T1100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21548 Knuckle-front axle, right<p>Part number 51711T1100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21549 Hub Assembly - Front Wheel<p>Part number 51750T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21550 bearing sensor<p>Part number 58980T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>