select (fn()).* executes function multiple times

PostgreSQL 10.5, 11.1:

create table fn_ret_test_storage (f1 int default 1, f2 int default 2, f3 int default 3);

create or replace function fn_ret_test(out f1 int, out f2 int, out f3 int)

returns record

language plpgsql

as

$$

begin

insert into fn_ret_test_storage default values

returning * into f1, f2, f3;

end

$$;

do

$$

declare

_count int;

begin

select count(*) into _count from fn_ret_test_storage;

perform (fn_ret_test()).*;

select count(*) - _count into _count from fn_ret_test_storage;

raise notice '----------------- % rows inserted -----------------', _count;

end

$$;