Heavy reports with a lot of calculations


I am working on reports now where we are using MySQL (InnoDB) and Rails 2.3.4.

Tables are big and there are a lot of calculation with several big tables.
Please give me some suggestions whether there any gem, plugin, articles, or techniques that allows me to create heavy reports.

I am even going to use cron job that allows me to calculate some middle data in order to provide heavy reports more faster to end users.

Also I am interesting whether anybody has an experience with JasperReport. What are benefits and disadvantages of using it?