Re: xpath processing brain dead - Mailing list pgsql-hackers

Hannu Krosing wrote:
> On Sun, 2009-03-01 at 10:13 -0500, Andrew Dunstan wrote:
>   
>> Hannu Krosing wrote:
>>     
>>>> Some of the functions, including some specified in the standard, produce 
>>>> fragments. That's why we have the 'IS DOCUMENT' test.
>>>>     
>>>>         
>>> But then you could use xmlfragments as the functions return type, no ?
>>>
>>> Does tha standard require that the same field type must store both
>>> documents and fragments ?
>>>
>>>   
>>>       
>> Yes, the standard very explicitly provides for one XML type which need 
>> not be an XML document. We have no choice about that.
>>     
>
> What is it then ? A sequence of XML elements ?
>   

In its typically oblique way, the 2003 draft says this:
   NOTE 1 — An XML root information item is similar to an XML document   information item, with the following
modifications:an   XML root information item does not have a [document element]   property, a [base URI] property, a
[characterencoding scheme] property,   or an [all declarations processed] property; and the [children]   property
permitsmore than one XML element information item.
 
   An SQL/XML information item is either an XML root information item   or one of the following (defined in   Subclause
3.1.3,“Definitions provided in Part 14”): an XML   attribute information item, an XML character   information item, an
XMLcomment information item, an XML document   type declaration information item,   an XML element information item, an
XMLnamespace information item,   an XML notation information item,   an XML processing instruction information item, an
XMLunexpanded   entity reference information item, or   an XML unparsed entity information item.
 
   An XML value is either the null value, or a collection of SQL/XML   information items, consisting of exactly   one
XMLroot information item, plus any other SQL/XML information   items that can be reached recursively   by traversing
theproperties of the SQL/XML information items.
 


> Which standard does postgreSQL XML type need to confirm to - general XML
> DB, Xpath or some other XML ?
>   

I think the XML type needs to conform to the SQL/XML spec. However, we 
are trying to apply XPath, which has a different data model, to that 
type - hence the impedance mismatch.

I think that the best we can do (for 8.4, having fixed 8.3 as best we 
can without adversely changing behaviour) is to throw the responsibility 
for ensuring that the XML passed to the function is an XML document back 
on the programmer. Anything else, especially any mangling of the XPath 
expression, presents a very real danger of breaking on correct input.

cheers

andrew