How to run recurring tasks on Postgresql without an external cron-like tool?
Even if you were running the soon-to-be-released (at time of writing) PostgreSQL 10 or the current PostgreSQL 9.6 not an ancient release like 8.3, there's still no built-in task scheduler.

Something like PgAgent or external cron jobs is required, there is no convenient workaround.

The background workers feature introduced in 9.3 should hopefully permit a tool like PgAgent to be moved into the PostgreSQL core in a later release, but it hasn't been done yet. Even on 9.3 you still have to run cron or pgagent.

A few people are working on background worker based schedulers, and there are some patches coming that should provide facilities to help with that. But as of PostgreSQL 10 there's still no good quality, widely adopted scheduler, and most people use cron / ms task scheduler / etc.

Please take a look at the version policy, too; you're running an obsolete and unsupported release.