log temp files are created twice in PL/pgSQL function

Hi,

Why does log temp files are created twice when query is executed in PL/pgSQL function?
Would you please explain it to me?

As below test result. Log temp files are created twice when SELECT statement is put
into a PL/pgSQL function. It led a little of performance degradation.
Is there any way to define PL/pgSQL function to avoid this issue?
# I am using PostgreSQL 9.3.9

my test results
-----
[postgres(at)test]$ psql -c "select test_tempfiles();" > /dev/null
LOG: temporary file: path "base/pgsql_tmp/pgsql_tmp2223.0", size 3244032
CONTEXT: PL/pgSQL function test_cursor() line 3 at RETURN QUERY
LOG: temporary file: path "base/pgsql_tmp/pgsql_tmp23223.1", size 2828062
LOG: duration: 421.426 ms statement: select test_tempfiles();

[postgres(at)test]$ psql -c "select name from testtbl order by id" > /dev/null
LOG: temporary file: path "base/pgsql_tmp/pgsql_tmp25586.0", size 2850816
LOG: duration: 389.054 ms statement: select random from testtbl order by random
-----

test_tempfiles() function is defined as below
-----
CREATE OR REPLACE FUNCTION public.test_tempfiles()
RETURNS TABLE(name text)
LANGUAGE plpgsql
AS
$function$
begin
return query execute "select name from testtbl order by id ";
end;
$function$
-----

Thanks and best regrards,
bocap