gistfile1.txt

omarqureshi=# create unique index foos_some_field_idx on foos(some_field);
CREATE INDEX
omarqureshi=# insert into foos(some_field) values (null);
INSERT 0 1
omarqureshi=# insert into foos(some_field) values (null);
INSERT 0 1
omarqureshi=# insert into foos(some_field) values ('a');
INSERT 0 1
omarqureshi=# insert into foos(some_field) values ('a');
ERROR:  duplicate key value violates unique constraint "foos_some_field_idx"
DETAIL:  Key (some_field)=(a) already exists.