Index on immutable function call

Hi,

I'm trying to figure out how to use an index on an immutable function
call in order to speed up queries.

I came up with this small test:

----------------------------------------
--create database foo;

--drop table indexed_table;

create table indexed_table (
id serial primary key,
data1 integer,
data2 integer
);

create or replace function this_is_a_long_transformation(d integer)
returns integer as $$
declare
l integer;
begin
-- wait
l = 0;
while l < 100 loop
l = l + 1;
end loop;
-- return same value
return d;
end
$$
language plpgsql immutable;

-- insert data into table
insert into indexed_table
select
i,
cast((select random() * 1000 * i) as integer),
cast((select random() * 1000 * i) as integer)
from generate_series(1, 100000) as i;

-- create index
create index long_transformation_index on indexed_table
(this_is_a_long_transformation(data2));

--select * from indexed_table WHERE data1 > data2;
select * from indexed_table WHERE data1 >
this_is_a_long_transformation(data2);
----------------------------------------

My goal is to make query...

select * from indexed_table WHERE data1 >
this_is_a_long_transformation(data2);

... as fast as

select * from indexed_table WHERE data1 > data2;

... with the help of the index "long_transformation_index".

Unfortunately, Postgreql does not use the index at all.

What am I doing wrong? I use the default query tuning options of
Postgresql 8.3.7.

Best regards,

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