Keeping historic records as well as current overview - performance

Question

I'm using a small collection of webscrapers to get the current GPS location of various devices. I also want to keep historic records. What's the best way of doing this without storing the data twice? For now i have two tables, both looking like this:
Column | Type | Modifiers | Storage | Description
---------+-----------------------------+---------------+----------+-------------
vehicle | character varying(20) | | extended |
course | real | | plain |
speed | real | | plain |
fix | smallint | | plain |
lat | real | | plain |
lon | real | | plain |
time | timestamp without time zone | default now() | plain |
One is named gps, and another is named gps_log. The function that updates these two does two things: first it performs an INSERT on gps_log, and afterwards it does an UPDATE OR INSERT (a user-defined function) on gps. However, this results in what seems to me as a pointless case of double-storing for other purposes than having easy SELECTable access to the current data.
Is there a simple way of only using gps_log and having a function select only the newest entry for each vehicle? Keep in mind that gps_log currently has 1397150 rows increasing with roughly 150 rows every 15 minutes, so performance is likely to be an issue.
Using PostgreSQL 8.4 via Perl DBI.

Answer 1

If SELECT performance is paramount, your current solution with redundant storage might not be such a bad idea.
If you get rid of the redundant table, you can help SELECT performance with a multi-column index like:
CREATE INDEX gps_log_vehicle_time ON gps_log (vehicle, time DESC);
Assuming that vehicle is your primary key.
Would make this corresponding query pretty fast:
SELECT *
FROM gps_log
WHERE vehicle = 'foo'
ORDER BY time DESC
LIMIT 1;
To SELECT the last entry for multiple or all rows, use this related technique.
Total storage size would probably grow, though, because the index will be bigger that the redundant table (+ index) if you have many rows per vehicle.
It might help storage and performance to add a serial column as a surrogate primary key instead of vehicle. Especially if you have foreign keys pointing to it.
Aside: don't use time as column name. It's a type name in PostgreSQL and a reserved word in every SQL standard. It is also misleading to name a timestamp column time.