RE: [xsl] xslt performance issue position() function used in predicate very slow

["Michael Kay" <mike@xxxxxxxxxxxx>]

Firstly, the fact that ./v[$p] doesn't work, but ./v[$p + 0] does work,
means that the value of $p is not a number. Perhaps it is a string, or
perhaps it is a result tree fragment. Either way, you would probably get an
immediate performance improvement by changing it to be a number, to avoid
the conversion costs each time the value is used.

Incidentally, the expression "./v" is exactly the same as "v". If your
processor is particularly stupid, the more complex expression might take
longer to evaluate.

It's of course entirely dependent on your XSLT processor whether the
expression v[$p] takes constant time, or time proportional to $p. It looks
as if in your case, it's taking time proportional to $p. That would be the
case if you were using Saxon. In Saxon you could get round this by using a
variable:

<xsl:variable name="vseq" select="v"/>
<xsl:for-each....
  <xsl:value-of  select="$vseq[$p]"

But that wouldn't necessarily help on a different processor. Note also that
this only works if $p is known at compile time to be a number - so you need
to fix that problem first.

Michael Kay
http://www.saxonica.com/
 

> -----Original Message-----
> From: krzysztof@xxxxxxxxxxxx [mailto:krzysztof@xxxxxxxxxxxx] 
> Sent: 25 August 2006 12:20
> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> Subject: [xsl] xslt performance issue position() function 
> used in predicate very slow
> 
> Hi,
> 
> I'm processing xml files that have 150kB size they are 
> composed from xml blocks that contain 350 elements with int 
> values here is example
> 
> &lt; obj name="obj1">
>  &lt;v>1&lt;/v>
>  &lt;v>2&lt;/v>
>  &lt;v>3&lt;/v>
>  ....
>  &lt;v>350&lt;/v>
> &lt;/obj>
> 
> obj blocks are  repeated many times
> in general my xslt selects some  set of &lt;v> rows and saves 
> in csv format e.g. select  1,2,3,4,15,28,71,17,19 and save to 
> csv in my xslt I'm using below expression to select v rows
> 
> &lt;xsl:value-of select="./v[position()=$p]"/>
> 
> ( in for-each loop I assign value to $p variable)
> 
> on P4 2.8 Ghz 512MB ram it takes 20 seconds! to parse 150KB file !!!
> if I comment out  this line it takes 1 second so definitely 
> it's issue with
> position()
> and selecting this v values.
> 
> BTW abbreviated version doesn't wok  i.e. select="./v[$p]" 
> but select="./v[$p+0]" works is this some bug ???
> 
> Thanks for any tips how to improve performance 
> 
> Chris