Subject: Re: grouping (was: if or template?) From: "Steve Muench" <smuench@xxxxxxxxxxxxx> Date: Tue, 9 May 2000 01:10:53 -0700 |
| ><xsl:key name="tid" use="tracker-id" select="."/> | > | ><xsl:for-each | >select="//tracker-id[generate-id(.)=generate-id(key('tid',.)[1])]"> | > | >I hope Steve will forgive me for announcing this discovery | >before he does, | >I'm quite excited by it because it gives much better performance. | | All it does to me is make me scratch my head! | Steve/Mike, would you give us the idiots view on this please, | whats happening? *Why* does it provide the unique tracker-id please? When you're doing grouping, you basically want to select exactly one of each unique thing. //tracker-id would select all tracker-id elements in the document. Declaring a 'tid' key like: <xsl:key name="tid" use="tracker-id" select="."/> The key('tid','tidvalue') function looks up all nodes having tracker-id = 'tidvalue'. In order to support this lookup, the processor will be keeping a list in memory like this: "tid" Key lookup Table ====================== tracker-id Ref to tracker-id elements value having that value ----------- -------------------------- abc123 node(109),node(344),node(496) def456 node(15) hij332 node(89),node(101) Where the notation node(nnn) means "the node whose node-id is nnn" as defined by generated-id(). To be concrete, the processor is likely keeping some kind of Hashtable with the tracker-id *value* as the hash key, and a node-list as the hash value. //tracker-id[generate-id(.)=generate-id(key('tid',.)[1])] selects all tracker-id elements in the document having a node-id equal to the node-id of the first node in the "key lookup table's" list of nodes having the current tracker-id. Said more simply, it selects the first tracker-id element for each unique tracker-id value. Or even more simply, it selects a list of distinct tracker-id values. Here's an example. Take the "Task.xml" File below... <Tasks> <Task><Desc>Task1</Desc><Owner>Steve</Owner></Task> <Task><Desc>Task2</Desc><Owner>Mike</Owner></Task> <Task><Desc>Task3</Desc><Owner>Dave</Owner></Task> <Task><Desc>Task4</Desc><Owner>Steve</Owner></Task> <Task><Desc>Task5</Desc><Owner>Mike</Owner></Task> <Task><Desc>Task9</Desc><Owner>Mike</Owner></Task> </Tasks> The stylesheet: <?xml version="1.0"?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output indent="yes"/> <xsl:key name="xxx" match="/Tasks/Task/Owner" use="."/> <xsl:template match="/"> <Tasks> <xsl:for-each select="/Tasks/Task/Owner[generate-id(.)=generate-id(key('xxx',.))]"> <xsl:sort select="."/> <Owner name="{.}"> <xsl:for-each select="key('xxx',.)/.."> <xsl:copy-of select="."/> </xsl:for-each> </Owner> </xsl:for-each> </Tasks> </xsl:template> </xsl:stylesheet> Produces a sorted, grouped list of tasks by owner and is much faster than the equivalent "scan-my-preceding" approach... <?xml version = '1.0' encoding = 'UTF-8'?> <Tasks> <Owner name="Dave"> <Task> <Desc>Task3</Desc> <Owner>Dave</Owner> </Task> </Owner> <Owner name="Mike"> <Task> <Desc>Task2</Desc> <Owner>Mike</Owner> </Task> <Task> <Desc>Task5</Desc> <Owner>Mike</Owner> </Task> <Task> <Desc>Task9</Desc> <Owner>Mike</Owner> </Task> </Owner> <Owner name="Steve"> <Task> <Desc>Task1</Desc> <Owner>Steve</Owner> </Task> <Task> <Desc>Task4</Desc> <Owner>Steve</Owner> </Task> </Owner> </Tasks> For testing, here is a slower.xsl stylesheet that does the same job without using the key() technique: <?xml version="1.0"?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <!-- Slower.xsl --> <xsl:output indent="yes"/> <xsl:template match="/"> <Tasks> <xsl:for-each select="/Tasks/Task[not(preceding-sibling::Task/Owner=./Owner)]/Owner"> <xsl:sort select="."/> <xsl:variable name="owner" select="."/> <Owner for="{.}"> <xsl:for-each select="/Tasks/Task[Owner = $owner ]"> <xsl:copy-of select="."/> </xsl:for-each> </Owner> </xsl:for-each> </Tasks> </xsl:template> </xsl:stylesheet> as you scale up the size of the Task.xml input file, the performance difference can be dramatic. Try copy/pasting the elements in the Task.xml above to creates a couple thousand <Task> elements to give it a spin... ______________________________________________________________ Steve Muench, Lead XML Evangelist & Consulting Product Manager Business Components for Java & XSQL Servlet Development Teams Oracle Rep to the W3C XSL Working Group XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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