SELECT xpath('//st:SecondName/text()', '<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<Root xmlns="urn:student">Студент группы <Groups>98 ВМК 1</Groups> <Inicial><SerName>Полищук</SerName> <FirstName>Юрий</FirstName> <SecondName>Владимирович</SecondName></Inicial>. Возраст <Age>20</Age> лет.</Root>', ARRAY[ARRAY['st', 'urn:student']]);

CREATE TABLE test2
( id integer NOT NULL,
  "xml" text,
  CONSTRAINT pk2 PRIMARY KEY (id));

SELECT xpath('//st:SecondName/text()', xml, ARRAY[ARRAY['st', 'urn:student']])
FROM test2
WHERE id= 2 ;