Speeding up subselect ?

Hi there,

i have the following query in a pl/pgsql procedure:

    update lr_object_usage
    set status = (case status
        when ''OPEN_SUCC'' then ''CLOSED_SUCC''
        when ''OPEN_FAIL'' then ''CLOSED_FAIL'' end)
    where lr_object_usage_id in (select lr_object_usage_id from
lr_locked_objects where context = p_exam_usage_id);

the problem is the result of the subselect is obviously calculated for
every row of lr_object_usage (30K rows) -> the update takes very long.
Here is the explain output with p_exam_usage_id being 38191. Two simple
selects with the result of the first one being used in the second one
vs. the subselect. Divided into two selects

Can I somehow tell the planer not to requery the subselect for every row
?

Many TIA,

peter

--
------------------------------------------------------------------

openacs=# select lr_object_usage_id from lr_locked_objects where context
= 38191;
 lr_object_usage_id
--------------------
              38192
              38193
              38194
              38195
(4 rows)

openacs=# explain analyze  select lr_object_usage_id from
lr_locked_objects where context = 38191;
NOTICE:  QUERY PLAN:

Seq Scan on lr_locked_objects  (cost=0.00..1.04 rows=1 width=4) (actual
time=0.06..0.09 rows=4 loops=1)
Total runtime: 0.18 msec

EXPLAIN

openacs=# select * from lr_object_usage where lr_object_usage_id in
(38192,38193,38194,38195);
 lr_object_usage_id | lr_object_id |          access_time          |
user_id | status | context
--------------------+--------------+-------------------------------+----
-----+--------+---------
              38192 |        13496 | 2002-07-08 17:47:01.001332+02 |
2434 |        |   38191
              38193 |        13503 | 2002-07-08 17:47:01.001332+02 |
2434 |        |   38191
              38194 |        13434 | 2002-07-08 17:47:01.001332+02 |
2434 |        |   38191
(3 rows)

openacs=# explain analyze select * from lr_object_usage where
lr_object_usage_id in (38192,38193,38194,38195);
NOTICE:  QUERY PLAN:

Index Scan using lr_object_usage_lr_object_usage,
lr_object_usage_lr_object_usage, lr_object_usage_lr_object_usage,
lr_object_usage_lr_object_usage on lr_object_usage  (cost=0.00..12.18
rows=4 width=39) (actual time=0.06..0.20 rows=3 loops=1)
Total runtime: 0.38 msec

EXPLAIN

Doing the same thing with the subselect:

openacs=# explain analyze select * from lr_object_usage where
lr_object_usage_id in (select lr_object_usage_id from lr_locked_objects
where context = 38191);
NOTICE:  QUERY PLAN:

Seq Scan on lr_object_usage  (cost=0.00..17860.59 rows=16514 width=39)
(actual time=2640.91..2646.47 rows=3 loops=1)
  SubPlan
    ->  Seq Scan on lr_locked_objects  (cost=0.00..1.04 rows=1 width=4)
(actual time=0.03..0.06 rows=4 loops=33080)
Total runtime: 2646.60 msec

EXPLAIN