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 (
    21261, 21262, 21263, 21264, 21265, 21266, 
    21267, 21268, 21269, 21270, 21271, 
    21272, 21273, 21274, 21275, 21276
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00035

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 (21261,21262,21263,21264,21265,21266,21267,21268,21269,21270,21271,21272,21273,21274,21275,21276) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21261 Protector-wiring<p>Part number 91961T6110</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21262 Protector-wiring<p>Part number 91961T6120</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21263 Protector-wiring<p>Part number 91961T6130</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21264 Protector-wiring<p>Part number 91961T6140</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21265 Protector-wiring<p>Part number 91961T6150</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21266 grommet<p>Part number 919803F070</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21267 grommet<p>Part number 91981T6010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21268 cap<p>Part number 91981T6020</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21269 grommet<p>Part number 919812B010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21270 Wiring assembly - rear door left<p>Part number 91620T6051</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21271 Wiring assembly - rear door right<p>Part number 91630T6051</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21272 Grommet to door wiring<p>Part number 91981T6030</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21273 Wiring assembly - front door (driver's seat)<p>Part number 91600T6380</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21274 Wiring assembly - front door (passenger seat)<p>Part number 91610T6380</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21275 BOLT<p>Part number 1127308207K</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21276 Bracket-Connect Junction Box Side<p>Part number 919G5T6310</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>