Having covered how to access PostgreSQL using client tools, it's time to focus on queries and query optimization. First, we will cover a few facts before looking at the decisions the optimizer has to take. Finally, we will cover a few optimization strategies and a few rules of thumb to be followed.

We need to realize that query optimization does not always mean rewriting queries. It might involve changing table definitions, creating indexes, and sometimes even dropping indexes. It's also necessary to understand what information the optimizer needs to arrive at optimal plans. Let's cover a few facts about how databases are used and how do they work.