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 (
    20989, 20990, 20991, 20992, 20993, 20994, 
    20995, 20996, 20997, 20998, 20999, 
    21000, 21001, 21002, 21003, 21004
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00090

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 (20989,20990,20991,20992,20993,20994,20995,20996,20997,20998,20999,21000,21001,21002,21003,21004) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
20989 hose<p>Part number 98619T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20990 Cap-Windshield Washer Reservoir<p>Part number 98623G6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20991 Wiring Assembly - Air Conditioner<p>Part number 97258D2700</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20992 nut<p>Part number 972322C000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20993 Heater & Evaporator Assembly<p>Part number 97205T1011</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20994 Door assembly - ventilator left<p>Part number 97146T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20995 Door Assembly - Ventilator Light<p>Part number 97146T1550</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20996 Door Assembly - Floor Left<p>Part number 97148T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20997 Door Assembly - Floor Light<p>Part number 97148T1550</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20998 Door assembly-temperature, left<p>Part number 971B1T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
20999 Door assembly-temperature, right<p>Part number 971B1T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21000 Case - blower, lower part<p>Part number 97112T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21001 Case-Blower, Top<p>Part number 97114T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21002 Core & Seal Assemblies - Heaters<p>Part number 97138T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21003 O-ring<p>Part number 971414H900</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21004 Seal-Heater to Def Duct<p>Part number 971A6T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>