incrementing on errors

Question

How do I suppress the 'id' in this table from incrementing when an error occurs?

db=> CREATE TABLE test (id serial primary key, info text, UNIQUE(info));
NOTICE:  CREATE TABLE will create implicit sequence "test_id_seq" for serial column "test.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "test_pkey" for table "test"
NOTICE:  CREATE TABLE / UNIQUE will create implicit index "test_info_key" for table "test"
CREATE TABLE

db=> INSERT INTO test (info) VALUES ('hello') ;
INSERT 0 1

db=> INSERT INTO test (info) VALUES ('hello') ;
ERROR:  duplicate key violates unique constraint "test_info_key"

db=> INSERT INTO test (info) VALUES ('hello') ;
ERROR:  duplicate key violates unique constraint "test_info_key"

db=> INSERT INTO test (info) VALUES ('goodbye') ;
INSERT 0 1

db=> SELECT * from test; SELECT last_value from test_id_seq;

 id |  info   
----+---------
  1 | hello
  4 | goodbye
(2 rows)

 last_value 
------------
          4
(1 row)

Answer 1

You cannot suppress this - and there is nothing wrong with having gaps in your ID values.

The primary key is a totally meaningless value that is only used to uniquely identify one row in a table.

You cannot rely on the ID to never have any gaps - just think what happens if you delete a row.

Simply ignore it - nothing is wrong

Edit
Just wanted to mention that this behaviour is also clearly stated in the manual:

To avoid blocking concurrent transactions that obtain numbers from the same sequence, a nextval operation is never rolled back

http://www.postgresql.org/docs/current/static/functions-sequence.html
(Scroll to the bottom)