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 (
    21005, 21006, 21007, 21008, 21009, 21010, 
    21011, 21012, 21013, 21014, 21015, 
    21016, 21017, 21018, 21019, 21020
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00051

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 (21005,21006,21007,21008,21009,21010,21011,21012,21013,21014,21015,21016,21017,21018,21019,21020) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21005 Case-inlet duct, lower part<p>Part number 97131T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21006 Case - heater & evaporator, lower part<p>Part number 97136T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21007 Door Assembly-Temperator (A)<p>Part number 97145T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21008 Door Assembly-Temperator (B)<p>Part number 97145T1500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21009 Door assembly - ventilator<p>Part number 971A0T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21010 Door Assembly-Defroster<p>Part number 97147T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21011 FILTER-AIR<p>Part number 97133AR000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21012 Clip-Case to Case<p>Part number 971682D000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21013 Transistor-Field Effect<p>Part number 97235D2000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21014 Insulation-Evaporate, lower part<p>Part number 97258T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21015 Actuator Motor-Mode Door<p>Part number 97226B1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21016 Door Assembly - Console Mode<p>Part number 971A1T1040</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21017 Door Assembly - Temperature<p>Part number 971A1T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21018 Door Assembly - Temperature<p>Part number 971A1T1020</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21019 Door Assembly-Defroster<p>Part number 97147T6050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21020 Door assembly - ventilator<p>Part number 971A0T6050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>