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 (
    21423, 21424, 21425, 21426, 21427, 21428, 
    21429, 21430, 21431, 21432, 21433, 
    21434, 21435, 21436, 21437, 21438
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

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 (21423,21424,21425,21426,21427,21428,21429,21430,21431,21432,21433,21434,21435,21436,21437,21438) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21423 Arm-Flower Door<p>Part number 97185T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21424 Lever-Temporature Door<p>Part number 97158T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21425 Lever-Temporature Door<p>Part number 97158T6050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21426 Actuator Assembly - Suction<p>Part number 97124M5000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21427 Seal-intake<p>Part number 97127T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21428 Case - heater & evaporator, left<p>Part number 97134T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21429 Case - heater & evaporator, right<p>Part number 97135T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21430 Case - heater & evaporator, lower part<p>Part number 97137T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21431 Case-intake, upper<p>Part number 97121T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21432 Case-intake, lower part<p>Part number 97131T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21433 Door Assembly-Temperator (A)<p>Part number 97145T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21434 Door Assembly-Temperator (B)<p>Part number 97145T6500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21435 Door Assembly - Temperature<p>Part number 97146T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21436 Door Assembly-Foot<p>Part number 97148T6550</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21437 Cover-Heater Core<p>Part number 97141T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21438 Insulation-Evaporate, lower part<p>Part number 97258T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>