Is there such a thing as a 'background database job'?

In a recent discussion with an academician friend of mine regarding how
to improve performance on a system, he came up with the idea of taking what
is now a monthly purge/cleanup job that takes about 24 hours (and growing) 
and splitting it up into a series of smaller tasks.

That part's fairly easy, but his next idea was to run those tasks more or
less continuously in the background.

As I understand most database back-ends, including Postgresql, there 
really isn't a way to run queries for an application in the background
(ie, at a lower priority), especially for an application that does updates.

I suppose I could 'nice' the program itself, but would that have any
positive impact?
--
Mike Nolan