问如何使用Postgres排除约束来防止插入两个主行？

EN

 CREATE TABLE emails (
     id integer NOT NULL,
     email character varying(255),
     "primary" boolean DEFAULT false,
     user_id integer,
 );

 ALTER TABLE emails ADD CONSTRAINT one_primary_email_per_user EXCLUDE USING gist (user_id WITH =, "primary" WITH &);

 ERROR:  data type boolean has no default operator class for access method "gist"
 HINT:  You must specify an operator class for the index or define a default operator class for the data type.

 ALTER TABLE emails ADD CONSTRAINT one_primary_email_per_user EXCLUDE  (user_id WITH =, (case "primary" when 't' then '1'::bit else '0'::bit end) WITH &);

 ERROR:  operator &(bit,bit) is not a member of operator family "bit_ops"
 DETAIL:  The exclusion operator must be related to the index operator class for the constraint.