Longer and longer updates

Hi all-

In doing some stress testing I noticed that our database was slowing
down significantly after doing the same update repeatedly on a table
with no indexes, so I tried this:

1) created an empty database

2) made a simple table with a single row:

create table test (key int4, data int4);
insert into test values (1, 0);

3) executed this statement tons of times:

update test set data=1234 where key=1

Here are the results -- it's pretty discouraging, I hope I'm making some
simple mistake, or maybe this is expected behavior for some reason?

After this many updates ...it took this long for 1000 more updates
----------------------- ------------------------------------------
0 10880 ms
5,000 10549 ms
10,000 17380 ms
15,000 20040 ms
20,000 20060 ms
25,000 20589 ms
30,000 30749 ms
35,000 30350 ms
40,000 30910 ms
45,000 37570 ms
50,000 40379 ms

This seems to be independent of starting and stopping my client and the
postmaster, running vacuum, praying, etc. I'm on RedHat6.2
running with the 7.1beta4 rpms.

Anyone know what's going on here?

Steve Wilmarth