Postgresql upsert with many constraints

1.sql

create table test (id int primary key, a int, b int, unique (a, b));
\d+ test

begin;
insert into test values (1, 1, 1);
do $$
declare
  _cn text;
begin
  insert into test values (1, 2, 2);
exception when others then
  get stacked diagnostics _cn = constraint_name;
  case
    when _cn = 'test_pkey' then
      update test set a = 2, b = 2 where id = 1;
    when _cn = 'test_a_b_key' then
      update test set id = 3 where a = 2 and b = 2;
  end case;
end;
$$ language plpgsql;

select * from test;

do $$
declare
  _cn text;
begin
  insert into test values (3, 2, 2);
exception when others then
  get stacked diagnostics _cn = constraint_name;
  case
    when _cn = 'test_pkey' then
      update test set a = 2, b = 2 where id = 1;
    when _cn = 'test_a_b_key' then
      update test set id = 3 where a = 2 and b = 2;
  end case;
  end;
$$ language plpgsql;

select * from test;

commit;