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An XPath query is executed slowly when referencing the parent node using .. /../<node name>

I want to get all read nodes and their values ​​from each session in my XML document, and also get the session identifier from the parent node. I am running this on an instance of SQL Server 2008.

My xml setup:

<Sessions>
<SessionID>99</SessionID>
<Readings>
  <Reading>
    <Point>
      <Lat>-40.411558</Lat>
      <Lng>175.63504</Lng>
    </Point>
    <Heading>54</Heading>
    <Speed>0.1</Speed>
    <Height>0</Height>
    <FlowRate>0.69</FlowRate>
    <AppRate>74</AppRate>
  </Reading>
  ... more readings
</Readings>
<Sessions>

My initial attempt (which works) was shown below, however it seems very slow when I run it. However, when I take out a line that adds SessionID ie./../SessionID, it works much faster. It takes a request for 12 seconds to two seconds.

        SELECT
            [SessionID] = c.value('(../../SessionID)[1]', 'int'),
            [Heading] = c.value('(Heading)[1]', 'float'),
            [Speed] = c.value('(Speed)[1]', 'float'),
            [Height] = c.value('(Height)[1]', 'float'),
            [FlowRate] = c.value('(FlowRate)[1]', 'float'),
            [AppRate] = c.value('(AppRate)[1]', 'float'),
            [Latitude] = c.value('(Point/Lat)[1]', 'float'),
            [Longtitude] = c.value('(Point/Lng)[1]', 'float')
        FROM
            @XMLData.nodes('/Sessions/Readings/Reading') XMLData(c)

In any case, I can do this without having to reference the SessionID using the notation .. /../. I need a SessionID, as this query is used to insert records into a temporary table:

insert into #MyTempTable
SELECT
            [SessionID] = c.value('(../../SessionID)[1]', 'int'),
            [Heading] = c.value('(Heading)[1]', 'float'),
            [Speed] = c.value('(Speed)[1]', 'float'),
            [Height] = c.value('(Height)[1]', 'float'),
            [FlowRate] = c.value('(FlowRate)[1]', 'float'),
            [AppRate] = c.value('(AppRate)[1]', 'float'),
            [Latitude] = c.value('(Point/Lat)[1]', 'float'),
            [Longtitude] = c.value('(Point/Lng)[1]', 'float')
        FROM
            @XMLData.nodes('/Sessions/Readings/Reading') XMLData(c)
+3
1

../../ CROSS APPLY .

, INSERT , SELECT INTO :

SELECT
    s.value('(SessionID)[1]', 'int') SessionID,             
    r.value('(Heading)[1]', 'float') Heading,
    r.value('(Speed)[1]', 'float') Speed,
    r.value('(Height)[1]', 'float') Height,
    r.value('(FlowRate)[1]', 'float') FlowRate,
    r.value('(AppRate)[1]', 'float') AppRate,
    r.value('(Point/Lat)[1]', 'float') Latitude,
    r.value('(Point/Lng)[1]', 'float') Longtitude
INTO #Reading
FROM
    @XMLData.nodes('/Sessions') as S(s)
    CROSS APPLY 
    s.nodes('./Readings/Reading') XMLData(r)
+4

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