[xml] Simple Element+Attribute Lookup with XPath - performance ?

[Martin Trappel <0xCDCDCDCD gmx at>]

-------- Original Message --------
Subject: Simple Element+Attribute Lookup with XPath - performance ?
Date: Tue, 15 Jul 2008 16:16:18 +0200
From: 0xCDCDCDCD gmx at
To: xml gnome org

Hi.

Given a document tree I have to find all (direct) children of a certain
element given an attribute value.
<ROOT>
  <FOO Name="a">
    <BAR Id="2"/>
    <BAR Id="1"/>
    <BAR Id="4"/>
    ...
    <BAR Id="2345"/>
  </FOO>
  <FOO Name="b">
    ...
  </FOO>
  ...
</ROOT>

I have an xmlNodePtr pointing to a certain FOO element and now would
like to find a certain BAR element of this FOO (lets say the one with id=4).

Possibility a)
Use the xpath search expression 'BAR[ id="4"]' and use the returned nodeset.
###
 // take_start_time();
 xmlXPathContext* xpc = xmlXPathNewContext(doc);
 xpc->node = my_foo_node;
 m_xmlXPathObject* xpo =
  xmlXPathEvalExpression("BAR[ id=\"4\"]", xpc);
 assert( xpo->nodesetval->nodeNr == 1 );
 // take_end_time();
###

b) Use xpath search expression 'BAR' and for-loop over the result set,
reading the attribute with xmlGetProp and collecting the ones where the
id matches.
###
 // take_start_time();
 xmlXPathContext* xpc = xmlXPathNewContext(doc);
 xpc->node = my_foo_node;
 m_xmlXPathObject* xpo =
   xmlXPathEvalExpression("BAR", xpc);
 xmlNodePtr found_node = NULL;
 for(int i=0, e=xpo->nodesetval->nodeNr; i<e; ++i) {
   xmlChar* p_val = xmlGetProp(xpo->nodesetval->nodeTab[i], "id");
   if(p_val && 0==strcmp("4", p_val)) {
     found_node = xpo->nodesetval->nodeTab[i];
     // Note: Do _not_ break; for loop when found
   }
   if(p_val) xmlFree(p_val);
 }
 assert( found_node );
 // take_end_time();
###

The second version is about 2-3 times faster than the first version with
the attribute lookup in the XPath expression. (it's factor 5-10 in the
debug version, but I guess that is highly compiler dependant)
Is there an easy explanation for this?
Is my XPath expression somehow to general?
Does XPath have to do significantly more when a predicate is present?

( I'm running this with Visual Studio 2005 / VC8 )

thanks, br,
Martin