SQL/XML functions slow down queries?

Does someone could explain why the SQL/XML functions slow down queryies so dramatically?

This piece of code takes a few seconds to complete:

declare
  type recType is record (
    col1 varchar2(80),
    col2 date
  ) ;
  type arrType is table of recType ;
  arr arrType ;
begin
  select col1,col2 bulk collect into arr from bigtable order by col1,col2;   dbms_output.put_line('count=>'||arr.count) ; end ;
/

While this seem to take forever:

declare
  type xmlArrayType is table of xmlType ;   xmlArr xmlArrayType ;
begin
  select xmlElement("ROW",xmlForest(col1,col2)) bulk collect into xmlArr from bigtable order by col1,col2;
  dbms_output.put_line('count=>'||xmlArr.count) ; end ;
/

I'm using Oracle 10.2.0.3 on Linux. Bigtable has 2072904 rows and the optimizer uses the first two columns of the primary key to sort on col1 and col2 on both cases.

Thank you. Kind regards, Yossarian. Received on Wed Sep 03 2008 - 09:29:49 CDT