I cannot explain why this works

deferred_constraints.sql

create table users (
  name text not null primary key
);

create table groups (
  name text not null primary key,
  owner text not null references users
);

create table groups_users (
  id serial primary key,
  user_name text not null references users,
  group_name text not null references groups,
  unique (user_name, group_name)
);

alter table groups
  add foreign key (name, owner) references groups_users (group_name, user_name);

insert into users (name) values ('my name');

-- this fails
insert into groups (name, owner) values ('my group', 'my name');
insert into groups_users (group_name, user_name) values ('my group', 'my name');

-- this fails
begin;
insert into groups (name, owner) values ('my group', 'my name');
insert into groups_users (group_name, user_name) values ('my group', 'my name');
commit;

-- this works
with new_group as (
  insert into groups (name, owner)
  values ('my group', 'my name')
  returning name, owner
)
insert into groups_users (group_name, user_name) 
select new_group.name, new_group.owner
from new_group;

--cleanup
drop table groups_users cascade;
drop table groups cascade;
drop table users cascade