PostgreSQL, often simply Postgres, is an object-relational database management system (ORDBMS) with an emphasis on extensibility and standards-compliance. As a database server, its primary function is to store data securely, supporting best practices, and to allow for retrieval at the request of other software applications. It can handle workloads ranging from small single-machine applications to large Internet-facing applications with many concurrent users.
PostgreSQL implements the majority of the core SQL:2011 standard, is ACID-compliant and transactional (including most DDL statements) avoiding locking issues using multi-version concurrency control (MVCC), provides immunity to dirty reads and full serializability; handles complex SQL queries using many indexing methods that are not available in other databases; has updateable views and materialized views, triggers, foreign keys; supports functions and stored procedures, and other expandability, and has a large number of extensions written by third parties. In addition to the possibility of working with the major proprietary and open source databases, PostgreSQL supports migration from them, by its extensive standard SQL support and available migration tools. Proprietary extensions in databases such as Oracle can be emulated by built-in and third-party open source compatibility extensions. Recent versions also provide replication of the database itself for availability and scalability.
PostgreSQL is cross-platform and runs on many operating systems including Linux, FreeBSD, OS X, Solaris, and Microsoft Windows. On OS X, PostgreSQL has been the default database starting with Mac OS X 10.7 Lion Server, and PostgreSQL client tools are bundled with in the desktop edition. The vast majority of Linux distributions have it available in supplied packages.
PostgreSQL is developed by the PostgreSQL Global Development Group, a diverse group of many companies and individual contributors. It is free and open-source software, released under the terms of the PostgreSQL License, a permissive free-software license.