Postgresql 9.2 xml namespace prefix error

Question

On PostgreSQL 9.1 i can execute this two queries with out any errors, with correct result:

SELECT xpath('/a', '<a s:dd="11"><c>test</c></a>')
SELECT xpath('/a', '<a s:dd="11"><c>test</c></a>',ARRAY[ARRAY['s', 'http://example.com']])

On PostgreSQL 9.2 same queries throws error:

ERROR:  could not parse XML document
DETAIL:  line 1: Namespace prefix s for dd on a is not defined

Only this query works fine:

SELECT xpath('/a', '<a xmlns:s="ddd" s:dd="11"><c>test</c></a>')

How can i parse XML file, whth out modifying XML code?

Problems starts when i want to make xpath query for xml element, recived from previous query.

For example xml document:

 <some xmlns:my="somens">
      <a>
          <b my:param="11" />
      </a>
 </some>

And i want to do something like this:

 elem = xpath('/a',doc);
 elem2= xpath('//b',elem[0]);

And second row throws error, becouse of unknown prefix my. Any ideas?

did you try my suggestion?

Trygve Laugstøl
– Trygve Laugstøl

2013-01-05 10:39:40 +00:00
Commented Jan 5, 2013 at 10:39 — Trygve Laugstøl
– Trygve Laugstøl, Commented Jan 5, 2013 at 10:39

Trygve Laugstøl · Accepted Answer · 2012-12-31 10:11:33Z

1

I fail to see the problem you're having other than PostgreSQL breaking features. In the first document

This is the expected behavior from a strict XML parser. The fact that PostgreSQL changed behavior is bad, but something we just have to deal with I guess.

This

SELECT xpath('/a', '<a s:dd="11"><c>test</c></a>')

fails because the s namespace is not declared in the XML document. This works:

# SELECT xpath('/a', '<a xmlns:s="http://example.com" s:dd="11"><c>..</c></a>');
                      xpath                       
--------------------------------------------------
 {"<a xmlns:s=\"http://example.com\" s:dd=\"11\">+
   <c>..</c>                                   +
 </a>"}
(1 row)

What you're doing here:

SELECT xpath('/a', '<a xmlns:s="http://example.com" s:dd="11"><c>..</c></a>',
    ARRAY[ARRAY['s', 'http://example.com']]);

is to bind the s namespace to http://example.com enabling you to run xpath expressions in that namespace. Observe:

This is the original query but where the a tag is in the http://example.com namespace. Your query (/a) doesn't match any document as you're querying for a elements in the default namespace:

# SELECT xpath('/a', '<s:a xmlns:s="http://example.com" s:dd="11"><c>test</c></s:a>');
 xpath 
-------
 {}
(1 row)

This however, selects the root element:

# SELECT xpath('/x:a', '<s:a xmlns:s="http://example.com" s:dd="11"><c>..</c></s:a>',
    ARRAY[ARRAY['x', 'http://example.com']]);
                       xpath                        
----------------------------------------------------
 {"<s:a xmlns:s=\"http://example.com\" s:dd=\"11\">+
   <c>test</c>                                     +
 </s:a>"}
(1 row)

Notice how s and x are bound to the same namespace http://example.com. Using different namespace bindings is confusing, but I just wanted to show you how it works.

answered Dec 31, 2012 at 10:11

Trygve Laugstøl

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2 Comments

Yavanosta Over a year ago

Sory, it's holidays in russia, until 9th january. I'll try, when i'll return to office, and write a response here.

Trygve Laugstøl Over a year ago

Ah, no problem. Just wondering if was correct/helpful as I'm new to postgresql and xml myself.

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