Ask HN: Learning to tune a database

I've been wanting to expand my knowledge around databases and sql/tuning. I've worked mostly with Oracle and would like to learn Postgres.

My problem is: With a programming language, I can do the 'Hello world' app and progress from there. With a database I need a different problem to solve or a different dataset to work with. I'm not going to learn much about tuning if I release a little used webapp :)

I bet there are plently of datasets around that I could load and mash up but I'm not too such on where to start.

Any suggestions?