postgres=# create sequence my_seq start - 20000  minvalue - 20000 ;
CREATE SEQUENCE
postgres=# select nextval('my_seq');
 nextval
---------
  - 20000 
( 1  запись)

postgres=# select nextval('my_seq');
 nextval
---------
  - 19999 
( 1  запись)
....
postgres=# insert into t1 values ( nextval('my_seq'), '1');
INSERT  0   1 
postgres=# insert into t1 values ( nextval('my_seq'), '2');
INSERT  0   1 
...
postgres=# alter sequence my_seq restart with  0 ;
ALTER SEQUENCE
postgres=# insert into t1 values ( nextval('my_seq'), '7');
INSERT  0   1 
postgres=# insert into t1 values ( nextval('my_seq'), '8');
INSERT  0   1 
...
postgres=# select * from t1 order by id;
   id   | val
--------+--------
 - 19970  |  1 
 - 19969  |  2 
 - 19968  |  3 
 - 19967  |  4 
 - 19966  |  5 
 - 19965  |  6 
       0  |  7 
       1  |  8 
       2  |  9 
       3  |  10 
( 10  rows)

postgres=# select * from t1 order by id desc;
   id   | val
--------+--------
       3  |  10 
       2  |  9 
       1  |  8 
       0  |  7 
 - 19965  |  6 
 - 19966  |  5 
 - 19967  |  4 
 - 19968  |  3 
 - 19969  |  2 
 - 19970  |  1 
( 10  rows)