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2.2 基础概念

PostgreSQL是一个关系型数据库管理系统（RDBMS）。也就是说，它是用来管理关系型数据的。关系本质是表的数学术语。当今，在表中存储数据已是一个众所周知的概念，以至于它看起来就是这个样子的，但实际上，还有许多其他组织数据库的方法。在类UNIX系统上的文件和目录构成了一个分级数据库的示例。更先进些的还有面向对象的数据库。

每个表是行的集合。表的每一行具有相同的列，每个列有各自的数据类型。虽然每一行的列顺序是一定的，但有一点，SQL并不能确保表中行的顺序（当然，可以对行显式的进行排序显示）。

表存在数据库中，多个由同一PostgreSQL服务实例管理的数据库共同组成数据库群集。

2.2. Concepts

PostgreSQL is a relational database management system (RDBMS). That means it is a system for managing data stored in relations. Relation is essentially a mathematical term for table. The notion of storing data in tables is so commonplace today that it might seem inherently obvious, but there are a number of other ways of organizing databases. Files and directories on Unix-like operating  systems form an example of a hierarchical database. A more modern development is the object-oriented database.

Each table is a named collection of rows. Each row of a given table has the same set of named columns, and each column is of a specific data type. Whereas columns have a fixed order in each row,it is important to remember that SQL does not guarantee the order of the rows within the table in any way (although they can be explicitly sorted for display).

Tables are grouped into databases, and a collection of databases managed by a single PostgreSQL server instance constitutes a database cluster.