This document is the programmer's manual for the PostgreSQL database management system, originally developed at the University of California at Berkeley. PostgreSQL is based on Postgres release 4.2. The Postgres project, led by Professor Michael Stonebraker, has been sponsored by the Defense Advanced Research Projects Agency (DARPA), the Army Research Office (ARO), the National Science Foundation (NSF), and ESL, Inc. The first part of this manual explains the Postgres approach to extensibility and describe how users can extend Postgres by adding user-defined types, operators, aggregates, and both query language and programming language functions. After a discussion of the Postgres rule system, we discuss the trigger and SPI interfaces. The manual concludes with a detailed description of the programming interfaces and support libraries for various languages. We assume proficiency with Unix and C programming. &info; ¬ation; &y2k; &legal;