Fwd: duplicate key value violates unique constraint

Hi all,

how to fix a problem, suppose there is a table with id and username

if I set the id to bigint so the limit is 9223372036854775807
if I insert for example 3 rows
id username
-- --------------
1 abc
2 def
3 ghi

if I delete all rows and insert one another it is like

id username
-- --------------
4 jkl

So it doesn't start again from non-available id 1, so what is needed to do
to make the new inserts go into non-available id numbers?

and if the id reaches the limit and maybe there is some ids that are not
used.