Streaming: should using a step with /copy-of() make a difference to a map operator with !copy-of()?

Saxon Java EE 10.5 tells me that

    <xsl:template match="/">
        <xsl:sequence select="count(//team/aussies/cricketer/copy-of()[role = 'Batsman'])"/>
    </xsl:template>

is streamable while

    <xsl:template match="/">
        <xsl:sequence select="count(//team/aussies/cricketer!copy-of()[role = 'Batsman'])"/>
    </xsl:template>

is not:

Error on line 12 of count-example2.xsl:
  XTSE3430  Template rule is not streamable
  * Expression
  descendant::cricketer[bytecode(parent::aussies[bytecode(parent::team)])]!xsl:copy-of[bytecode(role = "Batsman")] is not streamable because the select expression is crawling and the body is consuming
  * The result of the template rule can contain streamed nodes
Template rule is not streamable
  * Expression descendant::cricketer[bytecode(parent::aussies[bytecode(parent::team)])]!xsl:copy-of[bytecode(role = "Batsman")] is not streamable because the select expression is crawling and the body is consuming
  * The result of the template rule can contain streamed nodes

With streaming I am always happy if I find a way or path that is streamable but in this case I don't quite understand why doing !copy-of() makes a difference to /copy-of().

Replies (1)

RE: Streaming: should using a step with /copy-of() make a difference to a map operator with !copy-of()? - Added by Michael Kay over 2 years ago

It might be because !copy-of(.) has to deliver the copies in document order of cricketers, which requires evaluating //team/aussies/cricketer in document order (which is the famous crawling/consuming //section/head problem); whereas /copy-of() effectively allows the copies to be delivered in any order you like, because the last "/" sorts into document order, and the document order of separate trees is implementation-dependent.

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