PostgreSQL is a fully featured object-relational database management system. It supports a large part of the SQL standard and is designed to be extensible by users in many aspects.
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Some of the features are: ACID transactions, foreign keys, views, sequences, sub queries, triggers, user-defined types and functions, outer joins, multi version concurrency control. Graphical user interfaces and bindings for many programming languages are available as well.
This tutorial is how to installing PostgreSQL on Ubuntu Linux.