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 (
    21615, 21616, 21617, 21618, 21619, 21620, 
    21621, 21622, 21623, 21624, 21625, 
    21626, 21627, 21628, 21629, 21630
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00033

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 (21615,21616,21617,21618,21619,21620,21621,21622,21623,21624,21625,21626,21627,21628,21629,21630) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21615 Cover-After-Sales Service<p>Part number 29113T1200</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21616 Lower cover, back<p>Part number 29140T1041</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21617 Tank Assembly - Fuel<p>Part number 31150T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21618 Sender Assembly-Fuel Pump<p>Part number 94460T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21619 Tube-Fuel Feed<p>Part number 31310T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21620 Tube Assembly - Fuel Vapor<p>Part number 31312T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21621 Protect-Fuel<p>Part number 31315T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21622 Support-fuel tube<p>Part number 31325T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21623 Support-Tube Clamp<p>Part number 31327T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21624 Tube-Fuel Vapor<p>Part number 31340T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21625 Tube Assembly - Fuel Vapor<p>Part number 31342T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21626 Clip<p>Part number 31351J5100</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21627 Holder - Fuel Tube<p>Part number 31355T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21628 Holder - Fuel Tube<p>Part number 31357T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21629 Holder - Fuel Tube<p>Part number 31358T1000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21630 Holder - Fuel Tube<p>Part number 31359T1050</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>