User-defined objects
New types of almost all objects inside the database can be created, including:
· Casts
· Conversions
· Data types
Domains
· Functions, including aggregate functions and window functions
· Indexes including custom indexes for custom types
Operators (existing ones can be overloaded)
Procedural languages
Inheritance
Tables can be set to inherit their characteristics from a "parent" table. Data in child tables will appear to exist in the parent tables, unless data is selected from the parent table using the ONLY keyword, i.e. SELECT * FROM ONLY parent_table. Adding a column in the parent table will cause that column to appear in the child table.
Inheritance can be used to implement table partitioning, using either triggers or rules to direct inserts to the parent table into the proper child tables.
As of 2010 this feature is not fully supported yet—in particular, table constraints are not currently inheritable. All check constraints and not-null constraints on a parent table are automatically inherited by its children. Other types of constraints (unique, primary key, and foreign key constraints) are not inherited.
Inheritance provides a way to map the features of generalization hierarchies depicted in Entity Relationship Diagrams (ERD) directly into the PostgreSQL database.
Replication
Binary replication
PostgreSQL, beginning from version 9.0, includes built-in binary replication, based on shipping the changes (write-ahead logs) to slave systems asynchronously.
Version 9.0 also introduced the ability to run read-only queries against these replicated slaves, where earlier versions would only allow that after promoting them to be a new master. This allows splitting read traffic among multiple nodes efficiently. Earlier replication software that allowed similar read scaling normally relied on adding replication triggers to the master, introducing additional load onto it.
Beginning from version 9.1, PostgreSQL also includes built-in synchronous replication that ensures that, for each write transaction, the master waits until at least one slave node has written the data to its transaction log. Unlike other database systems, the durability of a transaction (whether it's asynchronous or synchronous) can be specified per-database, per-user, per-session or even per-transaction. This can be useful for workloads that don't require such guarantees, and may not be wanted for all data as it will have some negative effect on performance due to the requirement of the confirmation of the transaction reaching the synchronous standby.
There can be a mixture of synchronous and asynchronous standby servers. A list of synchronous standby servers can be specified in the configuration which determines which servers are candidates for synchronous replication. The first in the list which is currently connected and actively streaming is the one that will be used as the current synchronous server. When this fails, it falls to the next in line.
Synchronous multi-master replication is currently not included in the PostgreSQL core. Postgres-XC which is based on PostgreSQL provides scalable synchronous multi-master replication and is licensed under the BSD license.
The community has also written some tools to make managing replication clusters easier, such as repmgr.
Trigger-based replication
There are also several asynchronous trigger-based replication packages for PostgreSQL. These remain useful even after introduction of the expanded core capabilities, for situations where binary replication of an entire database cluster isn't the appropriate approach:
Slony-I
Londiste, part of SkyTools (developed by Skype)
Bucardo multi-master replication (developed by Backcountry.com)
Asynchronous notifications
PostgreSQL provides an asynchronous messaging system that is accessed through the NOTIFY, LISTEN and UNLISTEN commands. A session can issue a NOTIFY command, along with the user-specified channel and an optional payload, to mark a particular event occurring. Other sessions are able to detect these events by issuing a LISTEN command, which can listen to a particular channel. This functionality can be used for a wide variety of purposes, such as letting other sessions know when a table has updated or for separate applications to detect when a particular action has been performed. Such a system prevents the need for continuous polling by applications to see if anything has yet changed, and reducing unnecessary overhead. Notifications are fully transactional, in that messages aren't sent until the transaction they were sent from is committed. This eliminates the problem of messages being sent for an action being performed which is then rolled back.
Many of the connectors for PostgreSQL provide support for this notification system (including libpq, JDBC, Npgsql, psycopg and node.js) so it can be used by external applications.
Security
Internal
Security within the database is managed on a per-role-basis. A role is generally regarded to be a user (a role that can log in), or a group (a role which other roles are members of). Permissions can be granted or revoked on any object down to the column level, and can also allow/prevent the creation of new objects at the database, schema or table levels.
The sepgsql extension (provided with PostgreSQL as of version 9.1) provides an additional layer of security by integrating with SELinux. This utilises PostgreSQL's SECURITY LABEL feature.
External
PostgreSQL natively supports a broad number of authentication mechanisms including:
· trust (no enforcement)
· password (either MD5 or plain-text)
GSSAPI
SSPI
Kerberos
ident (maps O/S user name as provided by an ident server to database user name)
· peer (maps local user name to database user name)
LDAP
RADIUS
· certificate
PAM
The GSSAPI, SSPI, Kerberos, peer, ident and certificate methods can also use a specified "map" file that lists which users matched by that authentication system are allowed to connect as a specific database user.
These methods are specified in the cluster's host-based authentication configuration file (pg_hba.conf), which determines what connections are allowed. This allows control over which user can connect to which database, where they can connect from (IP address/IP address range/domain socket), which authentication system will be enforced, and whether the connection must use SSL.
Foreign data wrappers
As of version 9.1, PostgreSQL can link to other systems to retrieve data via foreign data wrappers (FDWs). These can take the form of any data source, such as a file system, another RDBMS, or a web service. This means regular database queries can use these data sources like regular tables, and even join multiple data sources together.
Other features
Referential integrity constraints including foreign key constraints, column constraints, and row checks
· Views. Although views are by default not updatable, they can be made so using triggers (from 9.1) or (in prior versions) by creating "insert", "update", and/or "delete" Query Re-Write Rules on the view.
Inner, outer (full, left and right), and cross joins
Sub-selects
o Correlated sub-queries
Transactions
o Supports most of the major features of SQL:2008 standard
Encrypted connections via SSL
o Binary and textual large-object storage
o Online backup
o In-place upgrades with pg_upgrade (available for upgrading to new major versions from 8.3 upwards)
o Domains
Tablespaces
Savepoints
o Point-in-time recovery, implemented using write-ahead logging
Two-phase commit
o 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
o Common table expressions and writable common table expressions
Embedded SQL is implemented using preprocessor. SQL code is first written embedded into C code. Then code is run through ECPG preprocessor, which replaces SQL with calls to code library. Then code can be compiled using a C compiler. Embedding works also with C++ but it does not recognize all C++ constructs.
Full text search
Add-ons
· MADlib - an open source analytics library for PostgreSQL providing mathematical, statistical and machine-learning methods for structured and unstructured data.
MySQL migration wizard - included with EnterpriseDB's PostgreSQL installer. (source code also available)[26]
Performance Wizard - included with EnterpriseDB's PostgreSQL installer. (source code also available)[26]
· pgRouting - extended PostGIS to provide geospatial routing functionality (GNU GPL)
PostGIS - an extremely popular add-on which provides support for geographic objects GNU GPL.
· Postgres Enterprise Manager - a non-free tool consisting of a service, multiple agents, and a GUI which provides remote monitoring, management, reporting, capacity planning and tuning.
ST-Links SpatialKit - Extension for directly connecting to spatial databases.
New features in 9.3
In order of commit:
· Support for LATERAL subqueries
· Shared memory changes to eliminate need for manual adjustment of kernel's System V shared memory limits to match any increase in PostgreSQL shared_buffer setting.
· 64-bit large objects to support storage of individual pieces of data up to 4 TB
· Updatable views
· Streaming-only cascading replication with automatic timeline following
· Custom background worker processes
· SP-GiST indexes for range types
· FREEZE mode on COPY for faster loading of initial table data
· New lock modes for foreign key references and UPDATEs, reducing deadlocks and improves concurrency
· Recursive views
· postgres_fdw foreign data wrapper to query other PostgreSQL instances with qual pushdown
· pg_xlogdump program to output a human-readable rendering of the write-ahead log
· COPY from/to STDIN/STDOUT
· Materialized Views
· Writable Foreign Tables
· Checksums for I/O reliability (early disk failure/corruption detection)
· Parallel pg_dump option
· New JSON processing functions and parser API
Indexing of regular-expression searches in contrib/pg_trgm
Interfaces
PostgreSQL has several forms of interface available and is also widely supported among programming language libraries. These include:
Built-in
· libpq - PostgreSQL's official C application interface
ECPG - An embedded C system
External
libpqxx - C++ interface
· PostgresDAC - PostgresDAC (for Embarcadero RadStudio/Delphi/CBuilder XE-XE3)
· DBD::Pg - Perl DBI driver
JDBC - JDBC interface
· Lua - Lua interface
Npgsql - .NET data provider
node-postgres - Node.js interface
pgoledb - OLEDB interface
psqlODBC - ODBC interface
psycopg - Python interface (also used by HTSQL)
pgtclng - Tcl interface
Database administration
Open source front-ends and tools
psql
The primary front-end for PostgreSQL is the psql command-line program, which can be used to enter SQL queries directly, or execute them from a file. In addition, psql provides a number of meta-commands and various shell-like features to facilitate writing scripts and automating a wide variety of tasks; for example tab completion of object names and SQL syntax
pgAdmin
The pgAdmin package is a free and open source graphical user interface administration tool for PostgreSQL, which is supported on many computer platforms.[50] The program is available in more than a dozen languages. The first prototype, named pgManager, was written for PostgreSQL 6.3.2 from 1998, and rewritten and released as pgAdmin under the GPL License in later months. The second incarnation (named pgAdmin II) was a complete rewrite, first released on January 16, 2002. The third version, pgAdmin III, was originally released under the Artistic License and then released under the same license as PostgreSQL. Unlike prior versions that were written in Visual Basic, pgAdmin III is written in C++, using the wxWidgets framework allowing it to run on most common operating systems.
phpPgAdmin
phpPgAdmin is a web-based administration tool for PostgreSQL written in PHP and based on the popular phpMyAdmin interface originally written for MySQL administration.
LibreOffice/OpenOffice.org Base
LibreOffice/OpenOffice.org Base can be used as a front-end for PostgreSQL.
pgFouine
The pgFouine PostgreSQL log analyzer generates detailed reports from a PostgreSQL log file and provides VACUUM analysis.
Proprietary front-ends and tools
A number of companies offer proprietary tools for PostgreSQL. They often consist of a universal core that is adapted for various specific database products. These tools mostly share the administration features with the open source tools but offer improvements in data modeling, importing, exporting or reporting.
Benchmarks and performance
Many informal performance studies of PostgreSQL have been done. Performance improvements aimed at improving scalability started heavily with version 8.1. Simple benchmarks between version 8.0 and version 8.4 showed that the latter was more than 10 times faster on read-only workloads and at least 7.5 times faster on both read and write workloads.
The first industry-standard and peer-validated benchmark was completed in June 2007 using the Sun Java System Application Server (proprietary version of GlassFish) 9.0 Platform Edition, UltraSPARC T1-based Sun Fire server and Postgres 8.2. This result of 778.14 SPECjAppServer2004 JOPS@Standard compares favourably with the 874 JOPS@Standard with Oracle 10 on an Itanium-based HP-UX system.
In August 2007, Sun submitted an improved benchmark score of 813.73 SPECjAppServer2004 JOPS@Standard. With the system under test at a reduced price, the price/performance improved from $US 84.98/JOPS to $US 70.57/JOPS.
The default configuration of PostgreSQL uses only a small amount of dedicated memory for performance-critical purposes such as caching database blocks and sorting. This limitation is primarily because older operating systems required kernel changes to allow allocating large blocks of shared memory.
PostgreSQL.org provides advice on basic recommended performance practice in a wiki.
In April 2012, Robert Haas of EnterpriseDB demonstrated PostgreSQL 9.2's linear CPU scalability using a server with 64 cores.
Prominent users
Yahoo! for web user behavioral analysis, storing two petabytes and claimed to be the largest data warehouse using a heavily modified version of PostgreSQL with an entirely different column-based storage engine and different query processing layer. While for performance, storage, and query purposes the database bears little resemblance to PostgreSQL, the front-end maintains compatibility so that Yahoo can use many off-the-shelf tools already written to interact with PostgreSQL.
In 2009, social networking website MySpace used Aster Data Systems's nCluster database for data warehousing, which was built on unmodified PostgreSQL.
State Farm uses PostgreSQL on their Aster Data Systems's nCluster Analytics server.
Geni.com uses PostgreSQL for their main genealogy database.
OpenStreetMap, a collaborative project to create a free editable map of the world.
Afilias, domain registries for .org, .info and others.
Sony Online multiplayer online games.
BASF, shopping platform for their agribusiness portal.
Reddit social news website.
· Skype VoIP application, central business databases.
Sun xVM, Sun's virtualization and datacenter automation suite.
MusicBrainz, open online music encyclopedia.
International Space Station for collecting telemetry data in orbit and replicating it to the ground.
MyYearbook social networking site.
Instagram, a popular mobile photo sharing service
Disqus, an online discussion and commenting service
Proprietary derivatives and support
Although the license allowed proprietary products based on Postgres, the code did not develop in the proprietary space at first. The first main offshoot originated when Paula Hawthorn (an original Ingres team member who moved from Ingres) and Michael Stonebraker formed Illustra Information Technologies to make a proprietary product based on Postgres.
In 2000, former Red Hat investors created the company Great Bridge to make a proprietary product based on PostgreSQL and compete against proprietary database vendors. Great Bridge sponsored several PostgreSQL developers and donated many resources back to the community, but by late 2001 closed due to tough competition from companies like Red Hat and to poor market conditions.
In 2001, Command Prompt, Inc. released Mammoth PostgreSQL, a proprietary product based on PostgreSQL. In 2008, Command Prompt, Inc. released the source under the original license. Command Prompt, Inc. continues to support the PostgreSQL community actively 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 database vendor Pervasive Software, known for its Btrieve product which was ubiquitous on the Novell NetWare platform. Pervasive announced commercial support and community participation and achieved some success. In July 2006, Pervasive left the PostgreSQL support market.
In mid-2005 two other companies announced plans to make proprietary products based on PostgreSQL with focus on separate niche markets. EnterpriseDB added functionality to allow applications written to work with Oracle to be more readily run with PostgreSQL. Greenplum contributed enhancements directed at data warehouse and business intelligence applications, including the BizGres project.
In October 2005, John Loiacono, executive vice president of software at Sun Microsystems, commented: "We're not going to OEM Microsoft but we are looking at PostgreSQL right now," although no specifics were released at that time. By November 2005, Sun had announced support for PostgreSQL. By June 2006, Sun Solaris 10 (6/06 release) shipped with PostgreSQL.
In August 2007, EnterpriseDB announced EnterpriseDB Postgres, a pre-configured distribution of PostgreSQL including many contrib modules and add-on components. EnterpriseDB Postgres was renamed to Postgres Plus in March 2008. Postgres Plus is available in two versions: Postgres Plus Solution Pack
(comprising PostgreSQL delivered in a GUI one-click install plus Solution Pack components that include; Postgres Enterprise Manager, Update Monitor, xDB Replication Server, SQL Profiler, SQL Protect, Migration Toolkit and PL/Secure), and Postgres Plus Advanced Server which has all the features of Postgres Plus Solutions Pack plus Oracle compatibility, performance features not available in PostgreSQL, as well as advanced security features not available in PostgreSQL. Both versions are available for download at no cost and are fully supported. The Solution Pack components and Advanced Server are restricted by a "limited use" license for evaluation purposes only unless purchased though a subscription. In 2011, EnterpriseDB announced Postgres Plus Cloud Database, which easily provisions PostgreSQL and Postgres Plus Advanced Server databases (with Oracle compatibility) in single instances, high availability clusters, or development sandboxes for Database-as-a-Service environments.
In 2011, 2ndQuadrant became a Platinum Sponsor of PostgreSQL, in recognition of their long-standing contributions and developer sponsorship. 2ndQuadrant employ one of the largest teams of PostgreSQL contributors and provide professional support for open source PostgreSQL.
In January 2012 EnterpriseDB released a cloud version of both PostgreSQL and their own proprietary Postgres Plus Advanced Server with automated provisioning for failover, replication, load-balancing, and scaling. This runs on Amazon Web Services.
Many other companies have used PostgreSQL as the base for their proprietary database projects. e.g. Truviso, Netezza, ParAccel. In many cases the products have been enhanced so much that the software has been forked, though with some features cherry-picked from later releases