Sort causes system to freeze

Maybe this is an obviously dumb thing to do, but it looked reasonable to me.  The problem is, the seemingly simple sort
belowcauses a fairly powerful computer to completely freeze for 5-10 minutes.  During the sort, you can't login, you
can'tuse any shell sessions you already have open, the Apache server barely works, and even if you do "nice -20 top"
beforeyou start the sort, the top(1) command comes to a halt while the sort is proceeding!  As nearly as I can tell,
thesort operation is causing a swap storm of some sort -- nothing else in my many years of UNIX/Linux experience can
causea "nice -20" process to freeze. 

The sort operation never finishes -- it's always killed by the system.  Once it dies, everything returns to normal.

This is 8.3.0.  (Yes, I'll upgrade soon.)  Is this a known bug, or do I have to rewrite this query somehow?  Maybe add
indexesto all four columns being sorted? 

Thanks!
Craig


=> explain select * from plus order by supplier_id, compound_id, units, price;
                              QUERY PLAN
-----------------------------------------------------------------------
 Sort  (cost=5517200.48..5587870.73 rows=28268100 width=65)
   Sort Key: supplier_id, compound_id, units, price
   ->  Seq Scan on plus  (cost=0.00..859211.00 rows=28268100 width=65)

=> \d plus       Table "emol_warehouse_1.plus"
    Column     |     Type      | Modifiers
---------------+---------------+-----------
 supplier_id   | integer       |
 supplier_name | text          |
 compound_id   | text          |
 amount        | text          |
 units         | text          |
 price         | numeric(12,2) |
 currency      | text          |
 description   | text          |
 sku           | text          |
Indexes:
    "i_plus_compound_id" btree (supplier_id, compound_id)
    "i_plus_supplier_id" btree (supplier_id)


max_connections = 1000
shared_buffers = 2000MB
work_mem = 256MB
max_fsm_pages = 1000000
max_fsm_relations = 5000
synchronous_commit = off
#wal_sync_method = fdatasync
wal_buffers = 256kB
checkpoint_segments = 30
effective_cache_size = 4GB

Machine: Dell, 8x64-bit CPUs, 8GB ram, Perc6i battery-backed RAID controller, 8 disks as RAID10