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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly. You can read PostgreSQL as Postgres-XC except for version number, which is specific to each product.
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.
Note: XCONLY: The following description applies only to Postgres-XC.
In Postgres-XC these databases can be distributed into more than one Datanodes. This will not affect how tables and rows are seen from applications point of view, but will be important for Database Administrator (DBA). Table distribution will be described later.