How index are running and how to optimise ?

Hi,

I have may be a stupid question, but I'm a little surprised with some explains
I have, using date fields ...

I would like to understand exactly when index are used ...
I'm using PostgresQL 7.4.1

I have a table with 351 000 records.
I have about 300 to 600 new records by day
I have an index like this :
ix_contracts_start_stop_date btree (start_date, stop_date)

I want to simply do something like this :

select o.id_contract
   from contracts o
 where o.start_date <= '2001-10-31'
    and (o.stop_date > '2001-11-06' or stop_date is null);

OK I get an explain like this :
                                                  QUERY PLAN
--------------------------------------------------------------------------------------------------------------
 Seq Scan on contracts o  (cost=0.00..12021.80 rows=160823 width=4)
   Filter: ((start_date <= '2001-10-31'::date) AND ((stop_date >
'2001-11-06'::date) OR (stop_date IS NULL)))

I understand that the OR could make the no use of the stop_date index ..., but
why I'm not using the index for the start_date part ?

Index are used only if I use an egality like this :

select o.id_contract
   from contracts o
 where o.start_date = '2001-10-31'
    and o.stop_date = '2001-11-06';

                                          QUERY PLAN
------------------------------------------------------------------------------------------------
 Index Scan using ix_contracts_start_stop_date on contracts o
(cost=0.00..6.00 rows=1 width=4)
   Index Cond: ((start_date = '2001-10-31'::date) AND (stop_date =
'2001-11-06'::date))

Could you please explain me why index are not used with <, > and how I can
optimise my request ... I have no idea but I'm using this request to do
insert in another table and this segmentation take 13 hours for making the
insert ! :o((

Thanks for help,
--
Hervé Piedvache

Elma Ingénierie Informatique
6 rue du Faubourg Saint-Honoré
F-75008 - Paris - France
Pho. 33-144949901
Fax. 33-144949902