with hold cursor, cause function execute twice and wrong result

postgres=# select version();
                                                          version

------------------------------------------------------------------------------------------------------------------------
---
 PostgreSQL 9.0alpha5 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC) 4.1.2 20080704 (Asianux 3.0 4.1.2-44),
64-b
it

create table test_execute(id int,name varchar(40));
insert into test_execute values(1,'jack');
create or replace function p_test_execute() returns void
as
$$
begin
        raise notice 'hello world';
        update test_execute set id=id*2;
end;
$$ LANGUAGE plpgsql;

begin;
declare JDBC_CURS_1 cursor with hold for select p_test_execute() from test_execute;
fetch 50 from JDBC_CURS_1;
NOTICE:  hello world
end;
NOTICE:  hello world
COMMIT
select * from test_execute;
 id | name
----+------
  4 | jack

I expect id = 2, but it is **4** instead,

The reason is that the function p_test_execute is executed twice, when *fetch*, it is first executed, and when
transactioncommit, because the cursor is a *holdable* cursor, it is executed again. 

I read the code, for holdable cursor, when commit, following call will execute:
 CommitHoldablePortals-->PersistHoldablePortal-->ExecutorRewind.

Is *ExecutorRewind* necessary, is it the root of this bug?
Does *ExecutorRewind* cause plan re-execute?