Poor performance of group by query

Anyone have any ideas why this query would be so slow?

stats=# explain analyze SELECT work_units, min(raw_rank) AS rank FROM Trank_work_overall GROUP BY work_units;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=1050.12..1085.98 rows=14347 width=16) (actual time=163149.981..163227.758 rows=17849 loops=1)
-> Seq Scan on trank_work_overall (cost=0.00..804.41 rows=49141 width=16) (actual time=0.071..328.682 rows=49091 loops=1)
Total runtime: 163296.212 ms

(3 rows)

stats=# \d Trank_work_overall
Table "pg_temp_1.trank_work_overall"
Column | Type | Modifiers
------------+--------+-----------
raw_rank | bigint |
work_units | bigint |

stats=#

FreeBSD fritz.distributed.net 5.2.1-RELEASE FreeBSD 5.2.1-RELEASE #1:
Wed Apr 7 18:42:52 CDT 2004
root(at)fritz(dot)distributed(dot)net:/usr/obj/usr/src/sys/FRITZ amd64

The machine is a dual opteron with 4G of memory. The query in question
was not hitting the disk at all. PostgreSQL 7.4.2 compiled with -O3.

Also, if I set enable_hashagg = false, it runs in less than a second.
--
Jim C. Nasby, Database Consultant jim(at)nasby(dot)net
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