table full scan or index full scan?

I have a 30,000,000 records table, counts the record number to need for 40 seconds.
The table has a primary key on column id;

perf=# explain select count(*) from test;
...
-----------------------------------------
Aggregate (cost=603702.80..603702.81 rows=1 width=0)
  -> Seq scan on test (cost=0.00..527681.04 rows=30408704 width=0)
...
perf=# select count(*) from test;
count
------------
30408704

perf=#

The
postgresql database uses the table full scan.but in oracle, the similar
SQL uses the index full scanning,speed quickly many than postgresql.  

postgresql's optimizer whether to have the necessity to make the adjustment?
 

postgresql version:8.3.7

OS : ubuntu 9

kernel:2.6.28-15-generic x86_64

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