queryoptimizer force index use

Hello!

We have  postgres 6.5.3 . We will update to 7.1.3on march 2002, because
the database is in heavy use.
Every night we make a vacuum and delete and recreate the indexes.

We 've got that query, wich is very slow:

chili=> explain SELECT study.*,series.*,image.* FROM study, series,
image, relstudy_series000 r0, relseries_image000 r1 WHERE
(study.chiliOID='0xaec1c13f.8271.1000122247.1') AND
(study.chiliOID=r0.parentOID AND series.chiliOID=r0.childOID) and
(series.chiliOID=r1.parentOID and image.chiliOID = r1.childOID) AND
image.imageType ~* 'LOCALIZER';
NOTICE:  QUERY PLAN:

Hash Join  (cost=488316.53 rows=19345 width=528)
  ->  Nested Loop  (cost=20999.69 rows=265624 width=360)
        ->  Nested Loop  (cost=906.51 rows=524 width=336)
              ->  Nested Loop  (cost=35.26 rows=425 width=224)
                    ->  Index Scan using indstudy_oid on study
(cost=2.05 rows=2 width=200)
                    ->  Index Scan using indstudy_series_parent on
relstudy_series000 r0  (cost=16.61 rows=263398 width=24)
              ->  Index Scan using indseries_oid on series  (cost=2.05
rows=263399 width=112)
        ->  Index Scan using indseries_image_parent on
relseries_image000 r1  (cost=38.35 rows=4780608 width=24)
  ->  Hash  (cost=406837.75 rows=347439 width=168)
        ->  Seq Scan on image  (cost=406837.75 rows=347439 width=168)


----Why is here a hash Join??????? and a Seq scan on image, the index
indimage_oid is still existing.
Why is the index indimage_oid not used?
----But without the last constraint the Query is very faster!!!

chili=> explain SELECT study.*,series.*,image.* FROM
chili-> study,series,image,relstudy_series000 r0, relseries_image000 r1
WHERE
chili-> (study.chiliOID='0xaec1c13f.8271.1000122247.1') AND
chili-> (study.chiliOID=r0.parentOID AND series.chiliOID=r0.childOID)
and
chili-> (series.chiliOID=r1.parentOID and image.chiliOID = r1.childOID)
;
NOTICE:  QUERY PLAN:

Nested Loop  (cost=565528.88 rows=265624 width=528)
  ->  Nested Loop  (cost=20999.69 rows=265624 width=360)
        ->  Nested Loop  (cost=906.51 rows=524 width=336)
              ->  Nested Loop  (cost=35.26 rows=425 width=224)
                    ->  Index Scan using indstudy_oid on study
(cost=2.05 rows=2 width=200)
                    ->  Index Scan using indstudy_series_parent on
relstudy_series000 r0  (cost=16.61 rows=263398 width=24)
              ->  Index Scan using indseries_oid on series  (cost=2.05
rows=263399 width=112)
        ->  Index Scan using indseries_image_parent on
relseries_image000 r1  (cost=38.35 rows=4780608 width=24)
  ->  Index Scan using indimage_oid on image  (cost=2.05 rows=4770811
width=168)

EXPLAIN
----------------------------------------------
So we decided to dump that database.
we redumped the database on an other Server and created the indexes.
And now :
the 2 Queries have quite the same runtime behavior.

pacs=# explain SELECT study.*,series.*,image.* FROM
pacs-# study,series,image,relstudy_series000 r0, relseries_image000 r1
WHERE
pacs-# (study.chiliOID='0xaec1c13f.8271.1000122247.1') AND
pacs-# (study.chiliOID=r0.parentOID AND series.chiliOID=r0.childOID) and
pacs-# (series.chiliOID=r1.parentOID and image.chiliOID = r1.childOID) ;
NOTICE:  QUERY PLAN:

Nested Loop  (cost=579.92 rows=446 width=528)
  ->  Nested Loop  (cost=61.50 rows=255 width=360)
        ->  Nested Loop  (cost=32.80 rows=14 width=336)
              ->  Nested Loop  (cost=6.15 rows=13 width=224)
                    ->  Index Scan using indstudy_oid on study
(cost=2.05 rows=2 width=200)
                    ->  Index Scan using indstudy_series_parent on
relstudy_series000 r0  (cost=2.05 rows=262709 width=24)
              ->  Index Scan using indseries_oid on series  (cost=2.05
rows=262701 width=112)
        ->  Index Scan using indseries_image_parent on
relseries_image000 r1  (cost=2.05 rows=4768420 width=24)
  ->  Index Scan using indimage_oid on image  (cost=2.03 rows=8333843
width=168)

EXPLAIN
pacs=# explain SELECT study.*,series.*,image.* FROM study, series,
image, relstudy_series000 r0, relseries_image000 r1 WHERE (study.chiliO
ID='0xaec1c13f.8271.1000122247.1') AND (study.chiliOID=r0.parentOID AND
series.chiliOID=r0.childOID) and (series.chiliOID=r1.parentOID and
 image.chiliOID = r1.childOID) AND image.imageType ~* 'LOCALIZER';
NOTICE:  QUERY PLAN:

Nested Loop  (cost=579.92 rows=1 width=528)
  ->  Nested Loop  (cost=61.50 rows=255 width=360)
        ->  Nested Loop  (cost=32.80 rows=14 width=336)
              ->  Nested Loop  (cost=6.15 rows=13 width=224)
                    ->  Index Scan using indstudy_oid on study
(cost=2.05 rows=2 width=200)
                    ->  Index Scan using indstudy_series_parent on
relstudy_series000 r0  (cost=2.05 rows=262709 width=24)
              ->  Index Scan using indseries_oid on series  (cost=2.05
rows=262701 width=112)
        ->  Index Scan using indseries_image_parent on
relseries_image000 r1  (cost=2.05 rows=4768420 width=24)
  ->  Index Scan using indimage_oid on image  (cost=2.03 rows=2
width=168)

'#########################

We deleted and created the indexes on the first database. made a vacuum
analyze. nothing happens.
Both postgresinstallations are made with default options.
Whats wrong?
Have anyone of You experienced the same occasion of that.?
What can I do , to get the behavior of the second database, to the
first?
Why is the behavior so different??

Greetings and thanks in advance

David