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,

-----------------------------------------------------------------------
Philippe Lang                   Web    : www.attiksystem.ch
Attik System                    Email  : philippe.lang@attiksystem.ch
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