How to add a value to the existing element value and return it as a new value - xquery

Question

This is the xml file.
<?xml version="1.0" encoding="UTF-8"?>
<root>
<AtcoCode> System-Start-Date= 2018-05-16T12:35:48.6929328-04:00, " ", System-End-Date = 9999-12-31, " ", 150042010003</AtcoCode>
<NaptanCode>esxatgjd</NaptanCode>
<PlateCode>
</PlateCode>
<CleardownCode>
</CleardownCode>
<CommonName>Upper Park</CommonName>
<CommonNameLang>
</CommonNameLang>
<ShortCommonName>
</ShortCommonName>
<ShortCommonNameLang>
</ShortCommonNameLang>
<Landmark>Upper Park</Landmark>
<LandmarkLang>
</LandmarkLang>
<Street>High Road</Street>
<StreetLang>
</StreetLang>
<Crossing>
</Crossing>
<CrossingLang>
</CrossingLang>
<Indicator>adj</Indicator>
<IndicatorLang>
</IndicatorLang>
<Bearing>NE</Bearing>
<NptgLocalityCode>E0046286</NptgLocalityCode>
<LocalityName>Loughton</LocalityName>
<ParentLocalityName>
</ParentLocalityName>
<GrandParentLocalityName>
</GrandParentLocalityName>
<Town>Loughton</Town>
<TownLang>
</TownLang>
<Suburb>
</Suburb>
<SuburbLang>
</SuburbLang>
<LocalityCentre>1</LocalityCentre>
<GridType>U</GridType>
<Easting>541906</Easting>
<Northing>195737</Northing>
<Co-ordinates>51.64255,0.04944</Co-ordinates>
<StopType>BCT</StopType>
<BusStopType>MKD</BusStopType>
<TimingStatus>OTH</TimingStatus>
<DefaultWaitTime>
</DefaultWaitTime>
<Notes>
</Notes>
<NotesLang>
</NotesLang>
<AdministrativeAreaCode>080</AdministrativeAreaCode>
<CreationDateTime>2006-11-06T00:00:00</CreationDateTime>
<ModificationDateTime>2010-01-16T07:58:02</ModificationDateTime>
<RevisionNumber>5</RevisionNumber>
<Modification>rev</Modification>
<Status>act</Status>
</root>
How to achieve this?
Question: Create the path range index for the status element and fetch all the documents that has status del
after fetching all the documents, you need to create the new element called currentreservationnumber under RevisionNumber element.
The value of the currentrevisionnumber will be +1 to the RevisionNumber.

Answer 1

I think the warning about sequential numbers is related to system-wide unique numbers/ids (like Oracle sequence), so not a worry in this case?
If you only ever have one RevisionNumber, and you can find it without a path index, you can maybe get by with element-value query on the RevisionNumber since it's already indexed.
Given that you get the document somehow, it could be as simple as:
let $doc := fn:doc ('/foo.xml')
let $rev-node := $doc/root/RevisionNumber
return xdmp:node-insert-after ($rev-node, <currentreservationnumber>{$rev-node + 1}</currentreservationnumber>)
though remember to consider locking if you are doing a big query/update. And you might need to switch to node-replace if there is already a currentreservationnumber.