xsl:merge

The purpose of the instruction is to allow merging of two or more pre-sorted input files, optionally using streamed processing of any or all of the inputs.

Available in XSLT 3.0. From Saxon 9.8, available in all editions. Implemented in Saxon-PE and Saxon-EE since Saxon 9.4.

• Category: instruction
• Content: ( xsl:merge-source+, xsl:merge-action, xsl:fallback* )
• Permitted parent elements: any XSLT element whose content model is sequence-constructor; any literal result element
• Element has no attributes

Notes on the Saxon implementation

The xsl:merge instruction is new in XSLT 3.0, and was first implemented in Saxon-EE 9.4. There have been a number of changes to the specification in successive working drafts; since Saxon 9.8, the implementation conforms with the final XSLT 3.0 Recommendation.

Saxon 9.5 implemented the sort-before-merge attribute, which allows the input to be sorted before merging.

Saxon 9.6 introduced support for streamed merging. There was one departure from the specification: the nodes selected for merging using the xsl:merge-source/@select are copies of the nodes in the source document (in the sense of the copy-of() function), rather than snapshots (as defined by the snapshot() function). This means that ancestors of the selected nodes, and attributes of ancestors, are not available.

Streamed processing requires Saxon-EE, but the instruction is implemented without streaming in Saxon-PE and Saxon-HE.

Details

Each kind of input source is described in an xsl:merge-source child element of the xsl:merge instruction; if there are multiple instances of that kind of input source, they are selected in the for-each-item or for-each-source attribute of the xsl:merge-source element, while the select attribute selects the actual nodes forming the input sequence. The processing to be carried out on each group of input items sharing a value for the merge key is defined in an xsl:merge-action element.

Examples

Example 1

Merges a homogenous collection of log files, each already sorted by timestamp:

<xsl:merge>
    <xsl:merge-source for-each-item="collection('log-collection')" select="events/event"/>
        <xsl:merge-key select="@timestamp" order="ascending"/>
    </xsl:merge-source>
    <xsl:merge-action>
        <xsl:sequence select="current-merge-group()"/>
    </xsl:merge-action>
</xsl:merge>

Example 2

Merges two log files with different internal structure:

<xsl:merge>
    <xsl:merge-source select="doc('log1.xml')/transactions/transaction"/>
        <xsl:merge-key select="xs:dateTime(@date, @time)" order="ascending"/>
    </xsl:merge-source>
    <xsl:merge-source select="doc('log2.xml')/eventdata/transfer"/>
        <xsl:merge-key select="@timestamp" order="ascending"/>
    </xsl:merge-source>
    <xsl:merge-action>
        <xsl:apply-templates select="current-merge-group()"/>
    </xsl:merge-action>
</xsl:merge>

Links

• XSLT 3.0 Specification
• Saxon

See also

• xsl:merge-action
• xsl:merge-source

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