Re: xpath improvement suggestion - Mailing list pgsql-hackers

Sorry for the previous NUUUB post, didn't now the mailing list doesn't
support html ;(
Robert Haas wrote:
> On Tue, Jan 5, 2010 at 6:09 PM, Arie Bikker <arie@abikker.nl> wrote:
>
>> Hi all,
>>
>> Well I had  to burn some midnight oil trying to figure out why a construct
>> like
>> SELECT xpath('name()','<a/>');
>> doesn't give the expected result. Kept getting an empty array:
>>  xpath
>> -------------
>> {}
>> instead of the expected "{a}"
>> BugID 4294 and the TODO item "better handling of XPath data types" pointed
>> in the right direction.
>> whithin src/backend/utils/adt/xml.c in the function xpath the result of the
>> call to xmlXPathCompiledEval is not handled optimally. In fact, the result
>> is assumed to be a nodeset without consulting the ->type member of the
>> result. I've made some minor changes to xml.c to handle some non-nodeset
>> results of xmlXPathCompiledEval.
>> Essentially, the revised code makes an array of all the nodes in the
>> xpathobj result in case this is a nodeset, or an array with a single element
>> in case the reult is a number/string/boolean. The problem cases mentioned in
>> http://archives.postgresql.org/pgsql-hackers/2008-06/msg00616.php now work
>> as expected.
>> Revision of the code involves:
>> - A switch statement to handle the result type of xmlXPathCompiledEval.
>> - an additional function xmlpathobjtoxmltype.
>>
>> diff of the revisioned code with respect to original is in attached file.
>>
>> kind regards, Arie Bikker
>>
>
> Hi,
>
> Could you please resend this as a context diff and add it to our patch
> management application?
>
> http://wiki.postgresql.org/wiki/Submitting_a_Patch
> https://commitfest.postgresql.org/action/commitfest_view/open
>
> Thanks!
>
> ...Robert
>
Hope this is the right attachement type (I'm new at this)
BTW. here a some nice examples:

- Get the number of attributes of the first childnode:

select ( xpath('count(@*)',(xpath('*[1]','<a b="c"><d e="f"
g="j"/></a>'))[1]))[1];

- an alternative for xpath_exist('/a/d')
select (xpath('boolean(/a/d)','<a b="c"><d e="f" g="j"/></a>'))[1];

- fixes bug 4206

select xpath('//text()',xmlparse(document '<?xml
version="1.0"?><elem1><elem2>one</elem2><elem2>two</elem2><elem2>three</elem2><elem3att="2"/></elem1>'));

- fixes bug 4294

select xpath('name(/my:a/*[last()])', '<a
xmlns="http://myns.com/ns"><b>text1</b><c>text2</c></a>',
ARRAY[ARRAY['my','http://myns.com/ns']]);

kind regards, Arie Bikker
*** src/backend/utils/adt/xml.c.old    Fri Sep  4 12:49:43 2009
--- src/backend/utils/adt/xml.c    Wed Jan  6 21:32:22 2010
***************
*** 111,116 ****
--- 111,117 ----
  static xmlDocPtr xml_parse(text *data, XmlOptionType xmloption_arg,
            bool preserve_whitespace, int encoding);
  static text *xml_xmlnodetoxmltype(xmlNodePtr cur);
+ static text *xml_xmlpathobjtoxmltype(xmlXPathObjectPtr cur);
  #endif   /* USE_LIBXML */

  static StringInfo query_to_xml_internal(const char *query, char *tablename,
***************
*** 3265,3270 ****
--- 3266,3312 ----

      return result;
  }
+
+ /*
+  * Convert XML pathobject to text for non-nodeset objects
+  */
+ static text *
+ xml_xmlpathobjtoxmltype(xmlXPathObjectPtr cur)
+ {
+     xmltype    *result;
+
+     if (cur->type == XPATH_BOOLEAN)
+     {
+         PG_TRY();
+         {
+             result = cstring_to_text((char *)(xmlXPathCastToBoolean(cur)?"t":"f"));
+         }
+         PG_CATCH();
+         {
+             PG_RE_THROW();
+         }
+         PG_END_TRY();
+     }
+     else
+     {
+         xmlChar    *str;
+
+         str = xmlXPathCastToString(cur);
+         PG_TRY();
+         {
+             result = (xmltype *) cstring_to_text((char *) str);
+         }
+         PG_CATCH();
+         {
+             xmlFree(str);
+             PG_RE_THROW();
+         }
+         PG_END_TRY();
+         xmlFree(str);
+     }
+
+     return result;
+ }
  #endif


***************
*** 3418,3442 ****
              xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
                          "could not create XPath object");

!         /* return empty array in cases when nothing is found */
!         if (xpathobj->nodesetval == NULL)
!             res_nitems = 0;
!         else
!             res_nitems = xpathobj->nodesetval->nodeNr;
!
!         if (res_nitems)
!         {
!             for (i = 0; i < xpathobj->nodesetval->nodeNr; i++)
              {
!                 Datum        elem;
!                 bool        elemisnull = false;
!
!                 elem = PointerGetDatum(xml_xmlnodetoxmltype(xpathobj->nodesetval->nodeTab[i]));
!                 astate = accumArrayResult(astate, elem,
!                                           elemisnull, XMLOID,
!                                           CurrentMemoryContext);
              }
          }
      }
      PG_CATCH();
      {
--- 3460,3504 ----
              xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
                          "could not create XPath object");

!         switch (xpathobj->type) {
!         case XPATH_NODESET: {
!             /* return empty array in cases when nothing is found */
!             if (xpathobj->nodesetval == NULL)
!                 res_nitems = 0;
!             else
!                 res_nitems = xpathobj->nodesetval->nodeNr;
!
!             if (res_nitems)
              {
!                 for (i = 0; i < xpathobj->nodesetval->nodeNr; i++)
!                 {
!                     Datum        elem;
!                     bool        elemisnull = false;
!
!                     elem = PointerGetDatum(xml_xmlnodetoxmltype(xpathobj->nodesetval->nodeTab[i]));
!                     astate = accumArrayResult(astate, elem,
!                                               elemisnull, XMLOID,
!                                               CurrentMemoryContext);
!                 }
              }
+             break;
          }
+         case XPATH_BOOLEAN:
+         case XPATH_NUMBER:
+         case XPATH_STRING: {
+             Datum        elem;
+             bool        elemisnull = false;
+
+             elem = PointerGetDatum(xml_xmlpathobjtoxmltype(xpathobj));
+             astate = accumArrayResult(astate, elem,
+                               elemisnull, XMLOID,
+                               CurrentMemoryContext);
+             break;
+         }
+         default: {
+             ereport(WARNING, (errmsg("Unknown PathObjectType (%d)", xpathobj->type)));
+         }
+         }
      }
      PG_CATCH();
      {
***************
*** 3460,3466 ****
      xmlFreeDoc(doc);
      xmlFreeParserCtxt(ctxt);

!     if (res_nitems == 0)
          PG_RETURN_ARRAYTYPE_P(construct_empty_array(XMLOID));
      else
          PG_RETURN_ARRAYTYPE_P(makeArrayResult(astate, CurrentMemoryContext));
--- 3522,3528 ----
      xmlFreeDoc(doc);
      xmlFreeParserCtxt(ctxt);

!     if (astate == NULL)
          PG_RETURN_ARRAYTYPE_P(construct_empty_array(XMLOID));
      else
          PG_RETURN_ARRAYTYPE_P(makeArrayResult(astate, CurrentMemoryContext));