While in many cases you might still want to save full logs, the pg_stat_statements feature added in PostgreSQL 8.4 can substitute as a way to analyze queries without needing an intermediate logging facility. The feature is documented at http://www.postgresql.org/docs/current/static/pgstatstatements.html.
This is a contrib module, not necessarily installed by default. See the discussion of contrib modules in Chapter 1, PostgreSQL Versions. Like pg_buffercache, it needs to be installed in each database you use it against. Here's an example of installing it on a system with the RedHat Linux directory layout for PostgreSQL:
$ psql -d pgbench -f /usr/share/postgresql/contrib/contrib/pg_stat_statements.sql
You enable this using ...
