CREATE TABLE xmltest (
    id int,
    data xml
);
INSERT INTO xmltest VALUES (1, '<value>one</value>');
INSERT INTO xmltest VALUES (2, '<value>two</value>');
SELECT * FROM xmltest;
 id |        data        
----+--------------------
  1 | <value>one</value>
  2 | <value>two</value>
(2 rows)

SELECT xmlcomment('test');
 xmlcomment  
-------------
 <!--test-->
(1 row)

SELECT xmlcomment('-test');
  xmlcomment  
--------------
 <!---test-->
(1 row)

SELECT xmlcomment('test-');
ERROR:  invalid XML comment
SELECT xmlcomment('--test');
ERROR:  invalid XML comment
SELECT xmlcomment('te st');
  xmlcomment  
--------------
 <!--te st-->
(1 row)

SELECT xmlagg(data) FROM xmltest WHERE id > 10;
 xmlagg 
--------
 
(1 row)

SET XML OPTION DOCUMENT;
SET XML OPTION CONTENT;
SELECT xml '<!-- in SQL:2006+ a doc is content too--> <?y z?> <!DOCTYPE a><a/>';
                                xml                                 
--------------------------------------------------------------------
 <!-- in SQL:2006+ a doc is content too--> <?y z?> <!DOCTYPE a><a/>
(1 row)

SELECT xml '<?xml version="1.0"?> <!-- hi--> <!DOCTYPE a><a/>';
             xml              
------------------------------
  <!-- hi--> <!DOCTYPE a><a/>
(1 row)

SELECT xml '<!DOCTYPE a><a/>';
       xml        
------------------
 <!DOCTYPE a><a/>
(1 row)

SELECT xpath('//loc:piece/@id', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number one</local:piece><local:piece id="2" /></local:data>', ARRAY[ARRAY['loc', 'http://127.0.0.1']]);
 xpath 
-------
 {1,2}
(1 row)

SELECT xpath('//loc:piece', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number one</local:piece><local:piece id="2" /></local:data>', ARRAY[ARRAY['loc', 'http://127.0.0.1']]);
                                                                     xpath                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------
 {"<local:piece xmlns:local=\"http://127.0.0.1\" id=\"1\">number one</local:piece>","<local:piece xmlns:local=\"http://127.0.0.1\" id=\"2\"/>"}
(1 row)

SELECT xpath('//loc:piece', '<local:data xmlns:local="http://127.0.0.1" xmlns="http://127.0.0.2"><local:piece id="1"><internal>number one</internal><internal2/></local:piece><local:piece id="2" /></local:data>', ARRAY[ARRAY['loc', 'http://127.0.0.1']]);
                                                                                                   xpath                                                                                                    
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 {"<local:piece xmlns:local=\"http://127.0.0.1\" xmlns=\"http://127.0.0.2\" id=\"1\"><internal>number one</internal><internal2/></local:piece>","<local:piece xmlns:local=\"http://127.0.0.1\" id=\"2\"/>"}
(1 row)