Performance Concepts

There are two aspects of database performance tuning. One is improving the database's use of the CPU, memory, and disk drives in the computer. The second is optimizing the queries sent to the database. This article talks about the hardware aspects of performance tuning. The optimization of queries is done using SQL commands like CREATE INDEX, VACUUM, VACUUM FULL, ANALYZE, CLUSTER, and EXPLAIN. These are discussed in my book, PostgreSQL: Introduction and Concepts at http://www.postgresql.org/docs/awbook.html.

To understand hardware performance issues, it is important to understand what is happening inside the computer. For simplicity, a computer can be thought of as a central processing unit (CPU) surrounded by storage. On the same chip with the CPU are several CPU registers which store intermediate results and various pointers and counters. Surrounding this is the CPU cache which holds the most recently accessed information. Beyond the CPU cache is a large amount of random-access main memory (RAM) which holds executing programs and data. Beyond this main memory are disk drives, which store even larger amounts of information. Disk drives are the only permanent storage area, so anything to be kept when the computer is turned off must be placed there. In summary, here are the storage areas surrounding the CPU:

Storage Area	Measured in
CPU registers	bytes
CPU cache	kilobytes
RAM	megabytes
disk drives	gigabytes

You can see that storage areas increase in size as they get farther from the CPU. Ideally, a huge amount of permanent memory could be placed right next to the CPU, but this would be too slow and expensive. In practice, the most frequently used information is stored next to the CPU, and less frequently accessed information is stored farther away and brought to the CPU as needed.