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 (
    21165, 21166, 21167, 21168, 21169, 21170, 
    21171, 21172, 21173, 21174, 21175, 
    21176, 21177, 21178, 21179, 21180
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00099

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 (21165,21166,21167,21168,21169,21170,21171,21172,21173,21174,21175,21176,21177,21178,21179,21180) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}

Result

product_id short_description full_description unit_name
21165 Housing Assembly - Outside Rear Venue Mirror, Left<p>Part number 87613T6010EB</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21166 Mirror assembly - outside rear view, left<p>Part number 87610T6040UYH</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21167 Mirror assembly - outside rear view, right<p>Part number 87620T6040UYH</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21168 Housing assembly - outside mirror, right<p>Part number 87623T6010EB</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21169 Lamp assembly-puddle, left<p>Part number 87619T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21170 Base pad assembly, left<p>Part number 87635T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21171 Housing cover - outer side rear view mirror, left<p>Part number 87615T6000EB</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21172 Base pad assembly, right<p>Part number 87645T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21173 G/Holder Assy-O/S Rear View, RH<p>Part number 87621T6010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21174 G/Holder Assy-O/S Rear View, LH<p>Part number 87611T6010</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21175 Lamp assembly-puddle, right<p>Part number 87629T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21176 Housing cover - outer side rear view mirror, right<p>Part number 87625T6000EB</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21177 Base lower cover-outside mirror, left<p>Part number 87634T6000EB</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21178 Base lower cover - outside mirror, right<p>Part number 87644T6000EB</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21179 Camera assembly - side view, left<p>Part number 99220T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>
21180 Camera assembly - side view, right<p>Part number 99230T6000</p><p>If you are not sure about compatibility, please contact us in advance to confirm the part number.</p>