Background information

Oracle supports the DBDBMS_XMLGEN package and its functions for manipulation of xml-type data.
SQL> SELECT dbms_xmlgen.newcontext('select * from dbmgr.xmldemo') FROM dual;
DBMS_XMLGEN.NEWCONTEXT('SELECT*FROMDBMGR.XMLDEMO')
--------------------------------------------------
                                                 1
SQL> select dbms_xmlgen.getxml(1) from dual;
DBMS_XMLGEN.GETXML(1)
--------------------------------------------------------------------------------
<? xml version="1.0"? >
<ROWSET>
 <ROW>
  <A>10</A>
  <B>first line</B>
 </ROW>
 <

Solutions

Although PolarDB-O does not support the DBMS_XMLGEN package and its functions, PolarDB provides XML-related functions, such as xmlagg, xmlroot, xmlforest, xmlelement, xmlconcat, and xmlcomment. You can use these functions to implement similar functions.
 van=> SELECT xmlforest(a AS A, b AS B) from xmldemo;
         xmlforest          
---------------------------- 
<a>10</a><b>first line</b> 
<a>20</a><b>line 2</b>
(2 rows)

For more information, see https://www.postgresql.org/docs/11/functions-xml.html.