select using an index

Hi

Recently i created an index in a table using a function (not a column) as
following

create index H2IAb on precalc (idr(p1,p4,p6,p7,p9,'H-2*IAb'));,

where idr is a function that returns a real number,

as a result i got the following table

mhc2db=> \d precalc;
Table "public.precalc"
Column | Type | Modifiers
-------------+-------------------+------------------------------------------------------

id | integer | not null default
nextval('precalc_id_seq'::regclass)
p1 | character(1) |
p4 | character(1) |
p6 | character(1) |
p7 | character(1) |
p9 | character(1) |
Indexes:
"h2iab" btree (idr(p1, p4, p6, p7, p9, 'H-2*IAb'::character varying)),

now, i would like to perform a query using this index, something like

Select * from precalc where h2iab>2

but obviously h2iab is not a column...

some of you knows what i should do?

thanks

--
Pau Marc Muñoz Torres
Laboratori de Biologia Computacional
Institut de Biotecnologia i Biomedicina Vicent
Villar
Universitat Autonoma de Barcelona
E-08193 Bellaterra (Barcelona)

telèfon: 93 5812807
Email : paumarc.munoz@bioinf.uab.cat

--
Pau Marc Muñoz Torres

Laboratori de Biologia Computacional
Institut de Biotecnologia i Biomedicina Vicent
Villar
Universitat Autonoma de Barcelona
E-08193 Bellaterra (Barcelona)

telèfon: 93 5812807
Email : paumarc.munoz@bioinf.uab.cat