Re: [PATCH] Add XMLEXISTS function from the SQL/XML standard

Quoting Peter Eisentraut <peter_e@gmx.net>:

> It would be nice to make XMLEXISTS work as in the standard, seeing how
> many others are providing the same interface.
>
...
>
> I think providing XPath is enough, at least for now.
>

Hi Peter,

 From piecing together sections 8.4 (<XML exists predicate>) and 6.18  
(<XML query>), I believe the full call signature would be (apologies  
if my bracketing isn't right):

XMLEXISTS
(
  xquery_expression
  [
   PASSING {BY REF|BY VALUE}
     { (xml_expression [BY REF|BY VALUE])
       | (xml_expression AS identifier)
     }
     [, ...]
  ]
)

Seeing as we're only intereseted in XPath for the moment and most of  
the syntax above is only really relevant to XQuery, I would suggest  
the following as accepatable until full XQuery support comes:

XMLEXISTS
(
  xpath_expression
   [
    PASSING BY REF xml_expression [BY REF]
   ]
)

There is one drawback in this approach, namely xml namespace support.  
I think borrowing the solution used in the xpath() (i.e. passing a  
nsarray) is out of the question as it won't be future proofed against  
full XQuery support. This is because the declaration of namespaces in  
XQuery is actually part of the query itself. I also think it  
inappropriate to attempt to parse the xpath_expression looking for  
XQuery style namespace declartions. So I think we ignore declared  
namespace support for the moment and just get the syntax correct ready  
for XQuery support in the hopefully near future.

Do you agree with what I have proposed?

Regards,
--
Mike Fowler