Actualice toda la tabla con el número de secuencia en Oracle

merge into foo
using
(
   select rowid as rid,
          row_number() over (order by date_value) as seqno
   from foo
) t on (foo.rowid = t.rid)
when matched then update
   set srno = t.seqno;

Demostración de SQLFiddle:http://sqlfiddle.com/#!4/d8cc5/2