errors while working on xml functions in postgresql

Hello,

            I
installed postgresql on my sytem,I need to parse an xml file.When I am
working on xpath() it is showing an error that function is notfound and
i tried almost all xml functions in postgresql but it is giving me the
same result.why so ? should I have to install any xml library inorder
to avoid this?Please tell me its very important for me.

   these were the errors which i got

SELECT xpath('/my:a/text()', '<my:a
xmlns:my="http://example.com">test</my:a>', ARRAY[ARRAY['my',
'http://example.com']]);

ERROR:  function xpath("unknown", "unknown", text[]) does not exist at character 8

HINT:  No function matches the given name and argument types. You may need to add explicit type casts.

STATEMENT:  SELECT xpath('/my:a/text()', '<my:a
xmlns:my="http://example.com">test</my:a>', ARRAY[ARRAY['my',
'http://example.com']]);

ERROR:  function xpath("unknown", "unknown", text[]) does not exist

LINE 1: SELECT xpath('/my:a/text()', '<my:a xmlns:my="http://example...

               ^

HINT:  No function matches the given name and argument types. You may need to add explicit type casts.

Thanks,

Avin.