It sounds like you need to run vacuum analyze:
openacs4=# explain analyze update acs_objects set last_modified = now() where object_id = -3;
NOTICE: QUERY PLAN:
Index Scan using acs_objects_pk on acs_objects (cost=0.00..3.09 rows=1 width=86) (actual time=0.17..0.24 rows=1 loops=1)
Total runtime: 31.36 msec
EXPLAIN
openacs4=# select version();
version
------------------------------------------------------------------
PostgreSQL 7.2.1 on sparc-sun-solaris2.8, compiled by GCC 2.95.3
(1 row)
openacs4=#