Re: [xslt] Performance issue when matching text()|*

[Kasimier Buchcik <kbuchcik 4commerce de>]

Daniel Veillard wrote:
> On Fri, Jan 28, 2005 at 08:12:00AM -0800, Jerome Pesenti wrote:
>
> > We encountered a strange performance problem related
> > to the cost of building large nodesets.  In
>
>
>   You are not building large node sets. You are building 1 huge text
> node a few bytes at a time. XPath model requires coalescing of adjacent
> text nodes and apparently the fix to make it linear doesn't seems to work
> in all cases see also http://bugzilla.gnome.org/show_bug.cgi?id=165107

Just out of curiosity; the spec says:

"text() selects all text node children of the context node"

"Function: number count(node-set)
The count function returns the number of nodes in the argument
node-set."

Why does this build 1 huge text node?

> > particular, it seems that the cost of building a
> > nodeset is not linear in the size.  To demonstrate
>
>
>   it's not a node set it's 
>
>   make text node, size 10, 
>   while ...
>       add a text node sibling size 10
>       coalesce both text nodes
>
> coalescing without trick means: realloc() existing string to 
>  the sum of both string size, copy the new string , free the new
>  text node.
> And that is of course quadratic if you don't prealocate memory and
> keep len(old_string) around.
> There is a fix in libxslt to avoid this. Unfortunately your XSLt code
> seems to break that optimization.
>
> Daniel

Kasimier