Postgres conditional unique index

gistfile1.sql

create table unique_test (
  number integer not null, 
  active boolean not null
)

create unique index unique_test_index on unique_test(number, boolean) where active;

insert into unique_test values (1, true);
insert into unique_test values (1, true);
-- ERROR:  duplicate key value violates unique constraint "unique_test_index"

insert into unique_test values (1, false);
insert into unique_test values (1, false);
insert into unique_test values (1, false);

select * from unique_test ;

-- number | active 
----------+--------
--      1 | t
--      1 | f
--      1 | f
--      1 | f
--(4 rows)