drop table if exists tab;
create unlogged table tab (a int4, b int4, f3 int, f4 int, f5 text, f6 float8)
partition by range(a);
do $$
begin
for i in 0..9 loop
  execute 'create unlogged table tab'||i||' partition of tab for values from
 ('||i*1000000||') to ('||(i+1)*1000000||');';
end loop;
end$$;

\timing on
insert into tab select g, g, g, g, g::text, g from generate_series(1, 10000000-1) g;
vacuum tab;
explain verbose update tab set b = b, f6 = f6;
update tab set b = b, f6 = f6;