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 (
    21631, 21632, 21633, 21634, 21635, 21636, 
    21637, 21638, 21639, 21640, 21641, 
    21642, 21643, 21644, 21645, 21646
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00041

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 (21631,21632,21633,21634,21635,21636,21637,21638,21639,21640,21641,21642,21643,21644,21645,21646) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}

Result

product_id short_description full_description unit_name
21631 Holder - Fuel Tube<p>Part number 31359T1060</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21632 Holder - Fuel Tube<p>Part number 31360T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21633 Auto Transaxle and Torque Converter Assembly<p>Part number 4500047EG0</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21634 seal<p>Part number 2547947101</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21635 Tube Assembly-ATF<p>Part number 2547047101</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21636 Case-transmission<p>Part number 4524147351</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21637 gear-rear line<p>Part number 4577047000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21638 gear-rear line<p>Part number 4577147600</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21639 Carrier assembly - rear planet<p>Part number 4578047600</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21640 Gear Assembly - Rear Anulus<p>Part number 4579647530</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21641 Diff Assembly-Front<p>Part number 4740047600</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21642 Shaft Assembly - Propeller<p>Part number 49100T1500</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21643 Shaft Assembly - Drive Right<p>Part number 49501T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21644 Bearing bracket & shaft assembly<p>Part number 49560T1400</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21645 Shaft assembly-drive, left<p>Part number 49500T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21646 Shaft assembly - drive rear, left<p>Part number 49600T1200</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>