To show you queries to optimize, you need data. So far, randomly generated data for a small number of tables has been good enough for that, but that has its limits and they were reached in the previous chapter, Chapter 9, Database Indexing.
Obtaining sample data of a significant size for benchmarking purposes is a never-ending struggle, because the pace of hardware progress means yesteryear's massive test database can be today's trivial workload. A listing that's kept up to date with useful new sources for test databases is at http://wiki.postgresql.org/wiki/Sample_Databases and some of these are part of a PostgreSQL sample database project, which has several examples that are worth checking out.
