select max(id) from aTable is very slow

Hi,

we have a table with about 6.000.000 rows. There is an index on a
column with the name id which is an integer and serves as primary key.

When we execute select max(id) from theTable; it takes about 10
seconds. Explain analyze returns:

------------------------------------------------------------------------
--------------------------------------------------------
Aggregate (cost=153635.15..153635.15 rows=1 width=4) (actual
time=9738.263..9738.264 rows=1 loops=1)
-> Seq Scan on job_property (cost=0.00..137667.32 rows=6387132
width=4) (actual time=0.102..7303.649 rows=6387132 loops=1)
Total runtime: 9738.362 ms
(3 rows)

I recreated the index on column id and ran vacuum analyze job_property
but this did not help. I tried to force index usage with SET
ENABLE_SEQSCAN TO OFF; but the explain analyze still looks like the
query is done using a seqscan.

Is the speed more or less normal for a 'dual G5 with 2 GHZ and 4 GB of
Ram and a SATA hd' or do i miss something?

regards David