efficient data reduction (and deduping)

Hi folks,

I have a system that racks up about 40M log lines per day. I'm able to COPY
the log files into a PostgreSQL table that looks like this:

CREATE TABLE activity_unlogged
(
user_id character(24) NOT NULL,
client_ip inet,
hr_timestamp timestamp without time zone,
locale character varying,
log_id character(36),
method character varying(6),
server_ip inet,
uri character varying,
user_agent character varying
)

Now, I want to reduce that data to get the last activity that was performed
by each user in any given hour. It should fit into a table like this:

CREATE TABLE hourly_activity
(
activity_hour timestamp without time zone NOT NULL,
user_id character(24) NOT NULL,
client_ip inet,
hr_timestamp timestamp without time zone,
locale character varying,
log_id character(36),
method character varying(6),
server_ip inet,
uri character varying,
user_agent character varying,
CONSTRAINT hourly_activity_pkey PRIMARY KEY (activity_hour , user_id )
)

where activity_hour is date_trunc('hour', hr_timestamp); (N.B. the primary
key constraint)

I am attempting to do that with the following:

INSERT INTO hourly_activity
SELECT DISTINCT date_trunc('hour', hr_timestamp) AS activity_hour,
activity_unlogged.user_id,
client_ip, hr_timestamp, locale, log_id, method,
server_ip, uri, user_agent
FROM activity_unlogged,
(SELECT user_id, MAX(hr_timestamp) AS last_timestamp
FROM activity_unlogged GROUP BY user_id, date_trunc('hour',
hr_timestamp)) AS last_activity
WHERE activity_unlogged.user_id = last_activity.user_id AND
activity_unlogged.hr_timestamp = last_activity.last_timestamp;

I have two problems:

1. It's incredibly slow (like: hours). I assume this is because I am
scanning through a huge unindexed table twice. I imagine there is a more
efficient way to do this, but I can't think of what it is. If I were doing
this in a procedural programming language, it might look something like:
for row in activity_unlogged:
if (date_trunc('hour', hr_timestamp), user_id) in
hourly_activity[(activity_hour, user_id)]:
if hr_timestamp > hourly_activity[(date_trunc('hour',
hr_timestamp), user_id)][hr_timestamp]:
hourly_activity <- row # UPDATE
else:
hourly_activity <- row # INSERT
I suspect some implementation of this (hopefully my pseudocode is at
least somewhat comprehensible) would be very slow as well, but at least it
would only go through activity_unlogged once. (Then again, it would have
to rescan hourly_activity each time, so it really wouldn't be any faster
at all, would it?) I feel like there must be a more efficient way to do
this in SQL though I can't put my finger on it.
2. Turns out (hr_timestamp, user_id) is not unique. So selecting WHERE
activity_unlogged.user_id = last_activity.user_id AND
activity_unlogged.hr_timestamp = last_activity.last_timestamp leads to
multiple records leading to a primary key collision. In such cases, I don't
really care which of the two rows are picked, I just want to make sure that
no more than one row is inserted per user per hour. In fact, though I would
prefer to get the last row for each hour, I could probably get much the
same effect if I just limited it to one per hour. Though I don't know if
that really helps at all.