There are numerous references on database performance tuning. Most are directed to other specific databases, such as Oracle, DB/2 Informix, and the likes. They may include recommendations that are specific to the respective products, but often also include principles that are common to many database systems including PostgreSQL.
[ Oracle Performance Tuning], by Mark Gurry and Peter Corrigan, O'Reilly.
It is certainly quite specific, in its details, to Oracle, but if you look beyond to the principles, a lot of its ideas are applicable to other database systems.
[ Database Tuning: Principles, Experiments, and Troubleshooting Techniques] by Phillipe Bonnet, Dennis Elliott Shasta, Morgan Kaufmann.
This book presents a broad set of principles for tuning database performance, including experimental tests for evaluating the effects of changes.
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