Partitioning / Strange optimizer behaviour

We have an optimizer problem regarding partitioned tables on 8.4.11.

We started partitioning a large table containing approx. 1 billion records.

So far, there is only the master table, called edifactmsgpart (which is empty) and 1 partition,
called edifactmsgpart_pact.
There is a bigint column called emg_id with a btree-index on it.

\d edifactmsgpart_pact
...
... "emp_emg_ept_i_pact" btree (emg_id, ept_id)
...

gdw=> select relname, reltuples from pg_class where relname in( 'edifactmsgpart',
'edifactmsgpart_pact' );
relname | reltuples
---------------------+-------------
edifactmsgpart_pact | 1.03102e+09
edifactmsgpart | 0

a select on the big partition yields a decent plan and performs as expected, lasting only a fraction
of a second.

gdw=> explain select min( emg_id ) from edifactmsgpart_pact;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Result (cost=2.05..2.06 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.00..2.05 rows=1 width=8)
-> Index Scan using emp_emg_ept_i_pact on edifactmsgpart_pact (cost=0.00..2109171123.79
rows=1031020672 width=8)
Filter: (emg_id IS NOT NULL)

gdw=> select min( emg_id ) from edifactmsgpart_pact;
min
-----------
500008178

=>>> very fast.

a select on the partitioned table, however, yields a... shall we call it "sub-optimal" plan:

gdw=> explain select min( emg_id ) from edifactmsgpart;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------
Aggregate (cost=23521692.03..23521692.04 rows=1 width=8)
-> Append (cost=0.00..20944139.42 rows=1031021042 width=8)
-> Seq Scan on edifactmsgpart (cost=0.00..13.70 rows=370 width=8)
-> Seq Scan on edifactmsgpart_pact edifactmsgpart (cost=0.00..20944125.72 rows=1031020672
width=8)

I would expect this to run half an hour or so, completely overloading the server...

Any Ideas?

Kind regards
Marc