choosing index to use

Greetings,
 How can I control which indexes will or won't be used by query ?
 I never think, that I'll need something like this.

Short version:
 Simple SELECT query perfomance speeds up (10x-20x) after _removing_
one of indexes. Because (as EXPLAIN shows), after removing, query
switches to another index. How to make such without index removing ?
 PostgreSQL 8.0.2, OpenBSD 3.7, i386 
Thank you.
Ilya A. Kovalenko
Special EQ SW section
JSC Oganer-Service

Details:

CREATE TABLE traffic_stat
( time      timestamptz NOT NULL, client    inet NOT NULL, remote    inet NOT NULL, count_in  int8 NOT NULL, count_out
int8NOT NULL
 
) WITHOUT OIDS;

CREATE INDEX traffic_client_idx ON traffic_stat USING btree (client);
CREATE INDEX traffic_date_idx ON traffic_stat USING btree ("time");
CREATE INDEX traffic_remote_idx ON traffic_stat USING btree (remote);
CREATE INDEX traffic_multy_idx ON traffic_stat USING btree ("time", client, remote);
CREATE INDEX traffic_date_client_idx ON traffic_stat USING btree ("time", client);

SELECT count(*) FROM traffic_stat;

135511

Query is:
SELECT to_char(time, 'DD.MM.YYYY HH24:MI.SS'), remote, count_in, count_out FROM traffic_stat WHERE client =
'192.168.xxx.xxx'AND time > '2005-05-16' AND time < '2005-05-16'::date + '1 days'::interval ORDER BY time;
 

Case 1:
SELECT ...

Total query runtime: 2643 ms.
Data retrieval runtime: 20 ms.
39 rows retrieved.

EXPLAIN SELECT ...

Index Scan using traffic_date_idx on traffic_stat  (cost=0.00..3.08 rows=1 width=35) Index Cond: (("time" > '2005-05-16
00:00:00+08'::timestampwith time zone) AND ("time" < '2005-05-17 00:00:00'::timestamp without time zone)) Filter:
(client= '192.168.114.31'::inet)
 

Case 2:

DROP INDEX traffic_date_idx;

SELECT ...

Total query runtime: 290 ms.
Data retrieval runtime: 20 ms.
41 rows retrieved.

EXPLAIN SELECT ...

Index Scan using traffic_date_client_idx on traffic_stat  (cost=0.00..4.37 rows=1 width=35) Index Cond: (("time" >
'2005-05-1600:00:00+08'::timestamp with time zone) AND ("time" < '2005-05-17 00:00:00'::timestamp without time zone)
AND(client = '192.168.114.31'::inet))