toll_archive=# insert into foo values (1);
INSERT 0 1
Time: 0.985 ms
toll_archive=# insert into foo values (2);
INSERT 0 1
Time: 0.233 ms
toll_archive=# insert into foo values (3);
INSERT 0 1
Time: 0.230 ms
toll_archive=# insert into foo values (1);
ERROR:  duplicate key violates unique constraint "foo_pkey"
toll_archive=# insert into foo values (4);
ERROR:  current transaction is aborted, commands ignored until end of transaction block