is a
Free Object-relational Database server (
Database Management System ), released under a flexible
BSD-style License . It offers an alternative to other open-source database systems (such as
MySQL and
Firebird ), as well as to
Proprietary systems such as
Oracle ,
Sybase , IBM's
DB2 and
Microsoft SQL Server . Similar to other open-source projects such as
Apache ,
Linux , and even
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 gives some readers pause in trying to pronounce it, especially those who pronounce
SQL as "sequel". PostgreSQL's developers pronounce it "post-gress-Q-L". (
Audio sample , 5.6k MP3). It is also common to hear it abbreviated as simply "postgres."
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:
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.
User-defined indexes can be created, or the built-in B-tree, hash 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.
- can be created which index the result of an expression or function, instead of simply the value of a column.
- , 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.
- As of version 8.1, bitmap index scans are supported. This involves reading multiple indexes and generating a bitmap that expresses their intersection with the Tuple s 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.
Triggers are fully supported and can be attached to tables and to views. 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.
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.
Rules allow the "query tree" of an incoming query to be rewritten. One common usage is to implement updatable views.
A wide variety of native
Data Types are supported, including:
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.
New types of almost all objects inside the database can be created, including:
- Indices
- Operators (and existing ones can be Overloaded )
- Aggregates
- Domains
- Casts
- Conversions
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.
Inheritance provides a way to map the features of generalization hierarchies depicted in Entity Relationship Diagrams (ERD) directly into the PostgreSQL database.
- Geographic objects via PostGIS . GPL license.
- . GPL license.
- Several asynchronous master/slave replication packages, including Slony-I (BSD license) and Mammoth Replicator (closed source, max 50 slaves, US $1,000 for one master and one slave).
- XML/XSLT support via XPath Extensions . GPL license.
PostgreSQL has had a lengthy evolution, starting with the
Ingres project at
UC Berkeley . The project lead,
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. The code bases of Postgres and Ingres started (and remain) completely separated.
The resulting project, named , 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 . 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 2001,
Command Prompt, Inc. released Mammoth PostgreSQL, the oldest surviving commercial PostgreSQL distribution. They 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 and
Business Intelligence applications, notably including the
BizGres project.
In October 2005, John Loiacono, executive vice-president of software at Microsoft but we are looking at PostgreSQL right now," although no specifics have yet been released.
In November 2005, Sun Microsystems
announced support for PostgreSQL .
- "Beginning Databases with PostgreSQL, Second Edition" (Neil Matthew, Richard Stones). ISBN　 1590594789
- "Beginning PHP and PostgreSQL 8: From Novice to Professional" (W. Jason Gilmore, Robert Treat). ISBN　 1590595475
- "Practical PostgreSQL" (John C. Worsley, Joshua D. Drake). ISBN 1565928466 (also available online )
- "PostgreSQL" (Korry Douglas). ISBN 0672327562
