> Sure -- use CREATE UNIQUE INDEX.
>
> alvherre=# create function singleton(int) returns int immutable language sql
as $$ select 1 $$;
> CREATE FUNCTION
> alvherre=# create table singleton (a int);
> CREATE TABLE
> alvherre=# create unique index only_one on singleton (singleton(a));
> CREATE INDEX
> alvherre=# insert into singleton values (3);
> INSERT 0 1
> alvherre=# insert into singleton values (6);
> ERROR: llave duplicada viola restricción de unicidad «only_one»
And it's possible to make this check deferrable? Uniqueness will be validated
on commit or SET ALL CONSTRAINTS IMMEDIATE command?
>
>
> The reason it's not supported in table_constraint, IIUC, is that the
> standard doesn't allow it, and that syntax is standards-mandated.
> CREATE INDEX, however, is not.
