Finding overlapping records

Hello,

I had a question that I meant to bring up during the meeting but forgot
until the end..

I have these tables representing DHCP leases:

CREATE TABLE ips (
ip serial PRIMARY KEY,
address inet NOT NULL
);

CREATE TABLE leases (
lease serial PRIMARY KEY,
ip integer NOT NULL REFERENCES ips (ip) ON UPDATE CASCADE ON DELETE RESTRICT,
mac macaddr NOT NULL,
starts timestamp NOT NULL,
ends timestamp NOT NULL,
server integer NOT NULL,
UNIQUE (ip, mac, starts, ends)
);

To optimize the number of records, I would like to make it so when I insert a
new lease, it checks to see if there's a relevant overlapping lease for that
mac/ip combination. If so, I want to update the end time of the old record
with the end time of the new record instead of doing the insert.

Currently, I'm accomplishing this with a trigger that calls this function:

CREATE FUNCTION optimize_lease_fx() RETURNS trigger AS $optimize_lease_fx$
DECLARE
id integer;
BEGIN
SELECT lease INTO id FROM leases WHERE mac = NEW.mac and
ip = NEW.ip and NEW.starts between starts AND ends AND
NEW.ends >= ends;
IF NOT FOUND THEN
RETURN NEW;
ELSE
EXECUTE 'UPDATE leases SET ends = '
|| quote_literal(NEW.ends)
|| ' where lease = '
|| quote_literal(id);
RETURN NULL;
END IF;
END
$optimize_lease_fx$ LANGUAGE plpgsql;

It works, but I'm wondering if there's a better way to do this. I'll be
flushing data when it's older than say.. 3 months. Also, keep in mind that
some pre-optimized data may look like this (below). The new rows will
overlap with each other and/or existing rows.

lease | ip | mac | starts | ends | server
--------+-------+-------------------+---------------------+---------------------+---------
255646 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 04:21:02 | 2009-12-10 06:21:02 | 400001
256948 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:13:38 | 2009-12-10 08:13:38 | 400001
256951 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:13:42 | 2009-12-10 08:13:42 | 400001
256954 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:13:50 | 2009-12-10 08:13:50 | 400001
257073 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:17:34 | 2009-12-10 08:17:34 | 400001
257077 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:17:38 | 2009-12-10 08:17:38 | 400001
257084 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:17:47 | 2009-12-10 08:17:47 | 400001
257157 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:19:33 | 2009-12-10 08:19:33 | 400001
257158 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:19:37 | 2009-12-10 08:19:37 | 400001
257168 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:19:44 | 2009-12-10 08:19:44 | 400001
257212 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:20:31 | 2009-12-10 08:20:31 | 400001
257215 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:20:34 | 2009-12-10 08:20:34 | 400001
257217 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:20:43 | 2009-12-10 08:20:43 | 400001
257230 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:20:59 | 2009-12-10 08:20:59 | 400001
257234 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:21:02 | 2009-12-10 08:21:02 | 400001
257236 | 22625 | 00:0d:36:e8:4b:d2 | 2009-12-10 06:21:03 | 2009-12-10 08:21:03 | 400001

Thanks,
Frank