PostgreSQL

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PostgreSQL
Developer:	PostgreSQL Global Development Group
Latest release:	8.2.3 / February 8, 2007
OS:	Cross-platform
Use:	ORDBMS
License:	BSD
Website:	www.postgresql.org

PostgreSQL is a free software object-relational database management system (ORDBMS), released under a BSD-style license. It offers an alternative to other database systems. Similar to other free software projects such as Apache, GNU/Linux, and MediaWiki, PostgreSQL is not controlled by any single company, but relies on a global community of developers and companies to develop it.

PostgreSQL's unusual-looking name makes some readers pause when trying to pronounce it, especially those who pronounce SQL as "sequel". PostgreSQL's developers pronounce it /poːst ɡɹɛs kjuː ɛl/. (Audio sample, 5.6k MP3). It is also common to hear it abbreviated as simply "postgres", which was its original name. The name refers to the project's origins as a "post-Ingres" database, the original authors having also developed the Ingres database.

Contents 1 Features 1.1 Functions 1.2 Indices 1.3 Triggers 1.4 MVCC 1.5 Rules 1.6 Data types 1.7 User-defined objects 1.8 Inheritance 1.9 Other features 1.10 Add-ons 2 History 3 Prominent users 4 See also 5 References 5.1 Notes 6 External links

[edit] Features

[edit] Functions

Functions allow blocks of code to be executed by the server. Although these blocks can be written in SQL, the lack of basic programming operations, such as branching and looping, has driven the adoption of other languages inside of functions. Some of the languages can even execute inside of triggers. Functions in PostgreSQL can be written in the following languages:

• A built-in language called PL/pgSQL resembles Oracle's procedural language PL/SQL.
• Scripting languages are supported through PL/Perl, plPHP, PL/Python, PL/Ruby, PL/sh, PL/Tcl and PL/Scheme.
• Compiled languages C, C++, or Java (via PL/Java).
• The statistical language R through PL/R.

PostgreSQL supports row-returning functions, where the output of the function is a set of values which can be treated much like a table within queries.

Functions can be defined to execute with the privileges of either the caller or the user who defined the function. Functions are sometimes referred to as stored procedures, although there is a slight technical distinction between the two.

[edit] Indices

User-defined index methods can be created, or the built-in B-tree, hash table and GiST indices can be used. Indexes in PostgreSQL also support the following features:

• PostgreSQL is capable of scanning indexes backwards when needed; you never need a separate index to support ORDER BY field DESC.
• Expression indexes can be created with an index of the result of an expression or function, instead of simply the value of a column.
• Partial indexes, which only index part of a table, can be created by adding a WHERE clause to the end of the CREATE INDEX statement. This allows a smaller index to be created.
• Bitmap index scans are supported as of version 8.1. This involves reading multiple indexes and generating a bitmap that expresses their intersection with the tuples that match the selection criteria. This provides a way of composing indexes together; on a table with 20 columns, there are, in principle, 20! indexes that could be defined - which is far too many to actually use. If you create one index on each column, bitmap scans can compose arbitrary combinations of those indexes at query time for each column that seems worth considering as a constraint.

[edit] Triggers

Triggers are fully supported and can be attached to tables but not to views. Views can have rules, though. Multiple triggers are fired in alphabetical order. In addition to calling functions written in the native PL/PgSQL, triggers can also invoke functions written in other languages like PL/Perl.

[edit] MVCC

PostgreSQL manages concurrency through a system known as Multi-Version Concurrency Control (MVCC), which gives each user a "snapshot" of the database, allowing changes to be made without being visible to other users until a transaction is committed. This largely eliminates the need for read locks, and ensures the database maintains the ACID principles in an efficient manner.

[edit] Rules

Rules allow the "query tree" of an incoming query to be rewritten. One common usage is to implement updatable views.

[edit] Data types

A wide variety of native data types are supported, including:

• Arbitrary precision numerics
• Unlimited length text
• Geometric primitives
• IP and IPv6 addresses
• CIDR blocks, and MAC address data types
• Arrays

In addition, users can create their own data types which can usually be made fully indexable via PostgreSQL's GiST infrastructure.

Examples of these are the Geographic information system (GIS) data types from the PostGIS project for PostgreSQL.

[edit] User-defined objects

New types of almost all objects inside the database can be created, including:

• Indices
• Operators (existing ones can be overloaded)
• Aggregate functions
• Domains
• Casts
• Conversions

[edit] Inheritance

Tables can be set to inherit their characteristics from a "parent" table. Data is shared between "parent" and "child(ren)" tables. Tuples inserted or deleted in the "child" table will respectively be inserted or deleted in the "parent" table. Also adding a column in the parent table will cause that column to appear in the child table as well. This feature is not fully supported yet -- in particular, table constraints are not currently inheritable. This means that attempting to insert the id of a row from a child table into table that has a foreign key constraint referencing a parent table will fail because Postgres doesn't recognize that the id from the child table is also a valid id in the parent table.

Inheritance provides a way to map the features of generalization hierarchies depicted in Entity Relationship Diagrams (ERD) directly into the PostgreSQL database.

[edit] Other features

• Referential integrity constraints including foreign key constraints, column constraints, and row checks
• Views While updateable views have not been implemented, the same functionality can be achieved using the rules system.
• Full, inner, and outer (left and right) joins
• Sub-selects
• Transactions
• Supports most of the major features of SQL:2003 standard [1] unsupported supported <-- lead to documentation for the next release of PostgreSQL, follow this link to find manuals for already released versions of PostgreSQL
• Encrypted connections via SSL
• Binary and textual large-object storage
• Online backup
• Domains
• Tablespaces
• Savepoints
• Point-in-time recovery
• Two-phase commit
• TOAST (The Oversized-Attribute Storage Technique) is used to transparently store large table attributes (such as big MIME attachments or XML messages) in a separate area, with automatic compression.
• Regular expressions [2]

[edit] Add-ons

• Geographic objects via PostGIS. GPL.
• Full text search via Tsearch2 and OpenFTS. GPL.
• Several asynchronous master/slave replication packages, including Slony-I (BSD license) and Mammoth Replicator.
• XML/XSLT support via XPath Extensions in the contrib section. GPL.

[edit] History

PostgreSQL has had a lengthy evolution, starting with the Ingres project at UC Berkeley. The project leader, Michael Stonebraker, had left Berkeley to commercialize Ingres in 1982, but eventually returned to academia. After returning to Berkeley in 1985, Stonebraker started a post-Ingres project to address the problems with contemporary database systems that had become increasingly clear during the early 1980s. While they share many of the same ideas, the code bases of PostgreSQL and Ingres started (and remain) completely separated.

The resulting project, named POSTGRES, aimed to introduce the minimum number of features needed to add complete support for types. These features included the ability to define types, but also the ability to fully describe relationships – something used widely before this time but maintained entirely by the user. In POSTGRES the database "understood" relationships, and could retrieve information in related tables in a natural way using rules.

Starting in 1986 the team released a number of papers describing the basis of the system, and by 1988 the project had a prototype version up and running. The team released version 1 to a small number of users in June 1989, followed by version 2 with a re-written rules system in June 1990. 1991's version 3 re-wrote the rules system again, but also added support for multiple storage managers and for an improved query engine. By 1993 a huge number of users existed and began to overwhelm the project with requests for support and features. After releasing a Version 4 — primarily as a cleanup — the project ended.

Although the POSTGRES project had officially ended, the BSD license (under which Berkeley had released POSTGRES) enabled Open Source developers to obtain copies and to develop the system further. In 1994 two UC Berkeley graduate students, Andrew Yu and Jolly Chen, added a SQL language interpreter to replace the earlier Ingres-based QUEL system, creating Postgres95. The code was subsequently released to the web to find its own way in the world.

In July 1996, Marc Fournier at Hub.Org Networking Services provided the first non-university development server for the open source development effort. Along with Bruce Momjian and Vadim B. Mikheev, work began to stabilize the code inherited from UC Berkeley, with the first open source version released on August 1st 1996.

1996 saw a re-naming of the project: in order to reflect the database's new SQL query language, Postgres95 became PostgreSQL. The first PostgreSQL release formed version 6.0 in January 1997. Since then, a group of database developers and volunteers from around the world, coordinating via the Internet, have maintained the software.

Although the license allowed for the commercialization of Postgres, the Postgres code did not develop commercially with the same rapidity as Ingres — somewhat surprisingly considering the advantages Postgres offered. The main offshoot originated when Paula Hawthorn (an original Ingres team member who moved from Ingres) and Michael Stonebraker formed Illustra Information Technologies to commercialize Postgres.

In 2000, former Red Hat investors put together a company known as Great Bridge to commercialize PostgreSQL and compete against commercial database vendors. Great Bridge sponsored several PostgreSQL developers and donated many resources back to the community, however by late 2001 the company closed its doors citing tough competition from companies like Red Hat as well as poor market conditions.

In 2001, Command Prompt, Inc. released Mammoth PostgreSQL, the oldest surviving commercial PostgreSQL distribution. They continue to actively support the PostgreSQL community through developer sponsorships and projects including PL/Perl, PL/php, and hosting of community projects such as the PostgreSQL Build Farm.

In January 2005, PostgreSQL received backing by another database vendor. Pervasive Software, well known for their Btrieve product which was ubiquitous on the Novell NetWare platform, announced commercial support & community participation. While they achieved success for a time, in July 2006, Pervasive left the PostgreSQL support market.^[1]

In mid-2005 two other companies announced plans to commercialize PostgreSQL with focus on separate niche markets. EnterpriseDB announced plans to focus on adding functionality to allow applications written to work with Oracle to be more readily run atop PostgreSQL. Greenplum contributed enhancements directed at data warehouse and business intelligence applications, notably including the BizGres project.

In October 2005, John Loiacono, executive vice-president of software at Sun Microsystems, commented that "We're not going to OEM Microsoft but we are looking at PostgreSQL right now,"^[2] although no specifics were released at that time. By November 2005, Sun Microsystems had announced support for PostgreSQL.^[3] As of June 2006, Sun Solaris 10 6/06 ships PostgreSQL.

As for the PostgreSQL project itself, it continues to make yearly major releases and minor "bugfix" releases, all available under the BSD license, based on contributions from both commercial vendors, support companies, and open source hackers at large.

[edit] Prominent users

• .org domain registry [3]
• The American Chemical Society
• Chicagocrime.org (Adrian Holovaty's blog)
• Skype (PostgreSQL Users)
• Penny Arcade (PostgreSQL Users)
• IMDB (PostgreSQL Users)
• Sony Online (ComputerWorld Article) (March 20, 2006)
• U.S. Department of Labor (PostgreSQL Users)
• whitepages.com
• Wisconsin Circuit Court Access with 6 * 180GB DBs replicated in real time
• OpenACS and dotLRN

[edit] See also

• List of relational database management systems
• Comparison of relational database management systems

[edit] References

• Matthew, Neil; Stones, Richard. Beginning Databases with PostgreSQL, Second Edition. ISBN 1-59059-478-9. 
• Gilmore, W. Jason; Treat, Robert. Beginning PHP and PostgreSQL 8: From Novice to Professional. ISBN 1-59059-547-5. 
• Worsley, John C.; Drake, Joshua D.. Practical PostgreSQL. ISBN 1-56592-846-6. 
• Douglas, Korry. PostgreSQL. ISBN 0-672-32756-2. 

[edit] Notes

[edit] External links

• PostgreSQL FAQ (Frequently Asked Questions)
• PostgreSQL Website
• PostgreSQL Documentation
• PostgreSQL Universe Comprehensive (Link-)Directory
• PostgreSQL at the Open Directory Project (suggest site)
• Planet PostgreSQL, blog aggregator
• PostgreSQL Performance Tuning
• Tuning PostgreSQL for performance
• Annotated POSTGRESQL.CONF Guide for PostgreSQL
• SourceForge PostgreSQL-related projects
• PgFoundry PostgreSQL-related projects
• Open Source Database Network
• Database Journal articles on PostgreSQL
• A PostgreSQL Tutorial
• Linux Productivity Magazine: a complete issue on PostgreSQL
• a rebuttal to the FUD (fear, uncertainty, and doubt) surrounding much of the criticism against PostgreSQL.
• PostgreSQL gotchas, documented but counterintuitive behavior

Topics in database management systems (DBMS) ( view • talk • edit )
Concepts Database • Database model • Relational database • Relational model • Relational algebra • Primary key, Foreign key, Surrogate key, Superkey, Candidate key • Database normalization • Referential integrity • Relational DBMS • Distributed DBMS • ACID
Objects Trigger • View • Table • Cursor • Log • Transaction • Index • Stored procedure • Partition	Topics in SQL Select • Insert • Update • Merge • Delete • Join • Union • Create • Drop
Implementations of database management systems
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