index scan and functions

Hi,
In a table, I've some geoip informations with indexes to two colums

\d geoip
Table « public.geoip »
Colonne | Type | Modificateurs
----------+--------------+---------------
begin_ip | bigint |
end_ip | bigint |
country | character(2) |
Index :
"geoip_begin_idx" btree (begin_ip)
"geoip_end_idx" btree (end_ip)

when I try to select stuffs form this table, request is fast:

syj=> explain select * from geoip where begin_ip <= 2130706433 and end_ip >= 2130706433;
QUERY PLAN
-----------------------------------------------------------------------------------
Index Scan using geoip_end_idx on geoip (cost=0.00..1448.46 rows=26967 width=19)
Index Cond: (end_ip >= 2130706433)
Filter: (begin_ip <= 2130706433)
(3 lignes)

But when using a custom function to compute my where parameter, request is slow:

syj=> explain select * from geoip where begin_ip <= inet_to_bigint('127.0.0.1') and end_ip >= inet_to_bigint('127.0.0.1');
QUERY PLAN
---------------------------------------------------------------------------------------------------------------
Seq Scan on geoip (cost=0.00..67654.95 rows=14418 width=19)
Filter: ((begin_ip <= inet_to_bigint('127.0.0.1'::inet)) AND (end_ip >= inet_to_bigint('127.0.0.1'::inet)))
(2 lignes)

inet_to_bigint is a function that transform an inet address its integer representation.

Is there a way, either to put function return value in a variable, or to tell
postgres to still use a sequential scan ?

thanks