The "vacuum" command must be run periodically to reclaim space in versions of PostgreSQL before 7.4. The "vacuum analyze" form additionally collects statistics on the disbursion of columns in the database, which the optimizer uses when it calculates just how to execute queries. The availability of this data can make a tremendous difference in the execution speed of queries. This command can also be run from cron, but it probably makes more sense to run this command as part of your nightly backup procedure - if "vacuum" is going to screw up the database, you'd prefer it to happen immediately after (not before!) you've made a backup! The "vacuum" command is very reliable, but conservatism is the key to good system management. So, if you're using the export procedure described above, you don't need to do this extra step.
Edit your crontab:
We'll set vacuum up to run nightly at 1 AM. Add the following line:
0 1 * * * /usr/local/pgsql/bin/vacuumdb $OPENACS_SERVICE_NAME