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 an extremely brief overview 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. PostgreSQL is copyright (C) 1996-8 by the PostgreSQL Global Development Group, and is distributed under the terms of the Berkeley license. Postgres95 is copyright (C) 1994-5 by the Regents of the University of California. Permission to use, copy, modify, and distribute this software and its documentation for any purpose, without fee, and without a written agreement is hereby granted, provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies. In no event shall the University of California be liable to any party for direct, indirect, special, incidental, or consequential damages, including lost profits, arising out of the use of this software and its documentation, even if the University of California has been advised of the possibility of such damage. The University of California specifically disclaims any warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. The software provided hereunder is on an "as-is" basis, and the University of California has no obligations to provide maintainance, support, updates, enhancements, or modifications. UNIX is a trademark of X/Open, Ltd. Sun4, SPARC, SunOS and Solaris are trademarks of Sun Microsystems, Inc. DEC, DECstation, Alpha AXP and ULTRIX are trademarks of Digital Equipment Corp. PA-RISC and HP-UX are trademarks of Hewlett-Packard Co. OSF/1 is a trademark of the Open Software Foundation.