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.