Sorry, I guess I wasn't clear. I have an existing table in my DB, and it doesn't have a column with serial values (actually it did originally, but due to later deletions of about 2/3 of the rows the column now has "holes"). I realize I could add a new serial column, but prefer not to change table + the new column would also become nonconsecutive after further deletions. The nice thing about Oracle's "rownum" is that it' a pseudo-column", not a real one, and AFAIK is always valid.
change the default of that column to use sequence. For instance, lookup CREATE SEQUENCE in manual, and ALTER TABLE .. SET DEFAULT ..
for example of how it looks, just create table foo(a serial), and check its definition with \d+ foo