Numbering rows by date

["Andrus" <kobruleht2@xxxxxx>]

I have table

create Document ( docdate date, docorder integer )

I need update docorder column with numbers 1,2 in docdate date order
Something like

i = 1;
UPDATE Document SET docorder = i++
  ORDER BY docdate;


How to do this is PostgreSQL 8.2 ?

Andrus.